燃烧当量比对燃烧特性的影响

Co-combustion of NH3 (ammonia) and hydrochar is a promising strategy to reduce fossil fuel usage and CO2 emissions. In this study, reactive molecular dynamics simulations (ReaxFF MD) were performed to investigate the effects of oxygen equivalence ratio (λ), ammonia co-combustion ratio, and combustion atmosphere on the combustion characteristics of the NH3/hydrochar mixture. Firstly, Increasing the oxygen equivalence ratio from 0.5 to 1.5 accelerated the reaction and enhanced fuel conversion: the total molecule count rose by ∼56 % (from 2449 to 3810), and heavy coke species (C40+ compounds) were completely eliminated at λ ≥ 1.0. A higher λ facilitated the conversion of H2 into H2O and N2 into NO/NO2, increasing H2O, NO, and NO2 yields while suppressing H2 and N2. At λ = 0.5, no CO2 or NO2 formed; at λ = 1.5, small amounts appeared, with CO produced being several-fold more abundant than CO2. Correspondingly, the peak NH2 intermediate count increased from 181 to 233 as λ rose from 0.5 to 1.5, whereas N2Hx intermediates (N2H2, N2H3, N2H4) declined - an oxygen-driven shift favoring NO formation over N2. Secondly, increasing the ammonia co-combustion ratio from 62.5 % to 85 % led to higher final yields of CO, NO, and N2, with a corresponding drop in residual unconverted carbon (UC). A greater NH3 proportion promoted the conversion of hydrochar into small molecules (C1-C4 gases), thereby reducing tar and coke formation (fewer tar/coke species at 85 % NH3 than at 62.5 %). Mechanistic analysis showed that a hydrochar fragment (C49H40O5) can be stepwise broken down by NH3-derived radicals (e.g. NH2, OH, H) into intermediate species such as C49H38O5 and C49H37O5, ultimately producing CO. Finally, the combustion atmosphere had a notable impact on reaction heat release. Under both air (O2/N2) and pure O2 conditions, the co-combustion process began with an endothermic phase followed by exothermic heat release. The peak heat absorption at λ = 1.0 reached 69.18 Ha under pure O2, much higher than the 16.78 Ha under air, indicating a more intense initial reaction in an oxygen-rich environment. By the end of 500 ps, however, the net energy released in air (20.62 Ha) slightly exceeded that in pure O2 (14.10 Ha). These results demonstrate that pure oxygen accelerates fuel consumption and product formation, whereas air yields a greater overall energy release - providing quantitative insight for optimizing practical NH3-hydrochar co-combustion.

No abstract available

Although their ease of transport, storage, and use makes hydrocarbon fuels dominant in commercial energy systems, the emission of harmful gases, including greenhouse gases, is a fatal disadvantage. Despite ongoing research to improve thermal efficiency and reduce the emissions of internal combustion engines using conventional hydrocarbon fuels, achieving net-zero carbon requires decarbonizing fuels rather than reducing the use of internal combustion engines. Hence, transitioning away from hydrocarbon fuels and evolving internal combustion engines into clean engines using carbon-free fuels, such as hydrogen, is necessary. This study designs a 2.0 L research engine and numerically analyzes its combustion characteristics and spray behavior by varying the spray angle and equivalence ratio. When comparing the turbulence kinetic energy at a 45-degree spray angle with that at 30 degrees and 60 degrees, on average, there was a difference of approximately 37.54 m2/s2 and 26.21 m2/s2, respectively. However, misfires occur in the lean condition. Although hydrogen has a wide flammability range, poor mixture formation under lean conditions can result in misfires. The 60-degree spray angle resulted in the highest combustion temperatures and pressures for all equivalence ratio conditions, consequently leading to the highest emissions of nitrogen oxides. Specifically, at a lambda value of 2.5, the 60-degree spray angle emitted approximately 29 ppm, 0 ppm, and 161 ppm of nitrogen oxides for each respective spray angle.

No abstract available

The adoption of zero-carbon fuel like ammonia will play a key role in the achievement of carbon neutrality targets. This work reports a comparative study on the effect of premixed equivalence ratio (varied from 0.8 to 1.25) on combustion and emission characteristics of ammonia fueled engine operating initially under (a) compression ignition (CI) mode (ignited by dodecane pilot injection), and then converted to (b) spark ignition (SI) mode (by replacing fuel injector with spark plug). The experiments were performed in a single-cylinder engine (compression ratio = 16.4:1) and the ammonia energy fraction was maintained at 95% during the CI mode and 100% ammonia in the SI mode. The results indicated that neat ammonia operation could be achieved satisfactorily by converting the existing CI engine configuration to operate in spark ignition mode with a stable engine operation with lower COVIMEP (less than 3%) at near stoichiometric to rich conditions. The power output and indicated thermal efficiency are lower in SI mode than in CI mode, certainly due to the occurrence of multiple auto-ignition sites. Moreover, the unburned ammonia emissions were observed to be higher in SI mode as compared to CI mode, especially in rich conditions. As expected, the carbon-based emissions reduced significantly in the SI operating mode as it is only due to lubricant oil leakages. N2O emissions were higher at near stoichiometric-rich conditions but remain below 100 ppm.

No abstract available

The adoption of zero-carbon fuel like ammonia will play a key role in the achievement of carbon neutrality targets. This work reports a comparative study on the effect of premixed equivalence ratio (varied from 0.8 to 1.25) on combustion and emission characteristics of ammonia fueled engine operating initially under (a) compression ignition (CI) mode (ignited by dodecane pilot injection), and then converted to (b) spark ignition (SI) mode (by replacing fuel injector with spark plug). The experiments were performed in a single-cylinder engine (compression ratio = 16.4:1) and the ammonia energy fraction was maintained at 95% during the CI mode and 100% ammonia in the SI mode. Fourier Transform Infra-Red (FTIR) spectrometer was used for measuring the exhaust emissions, which included unburned ammonia (NH3), nitrogen oxide (NOx), nitrous oxide (N2O) and carbon dioxide (CO2). The results of this study indicated that neat ammonia operation could be achieved satisfactorily by converting the existing CI engine configuration to operate in spark ignition mode, and the stable engine operation with lower COVIMEP (less than 3%) was observed to be possible at near-stoichiometric to rich conditions. The power output and indicated thermal efficiency are lower in SI mode than in CI mode, certainly due to the occurrence of multiple auto-ignition sites. Moreover, the unburned ammonia emissions were observed to be higher in SI mode as compared to CI mode, especially in rich conditions. As expected, the carbon-based emissions reduced significantly in the SI operating mode as it is only due to lubricant oil leakages. N2O emissions were higher at near stoichiometric-rich conditions (Φpremix 1.05 to 1.25) emissions but negligible in SI mode, except at the highest equivalence ratio (1.25). As N2O has very high global warming potential (265 times CO2 after 100 years) CO2–equivalent impact was evaluated by considering both N2O and CO2 emissions.

No abstract available

No abstract available

Ammonia, as a clean energy carrier, has the potential to be used as a hydrogen energy source. Methaneammonia co-combustion technology involves mixing ammonia (NH3) with methane (CH4) in a specific ratio and then burning the mixture, combining the advantages of both to enhance combustion performance as a clean energy technology. This study employs a combination of experimental and computational methods to measure and simulate the flame stretch extinction limit and flame structure of non-premixed counterflow flames containing ammonia/methane/nitrogen mixtures with ammonia concentrations of 10%, 20%, 30%, and 40%, as well as air, to assess their potential as clean fuels. The results show that as the ammonia concentration increases, the flame stretch extinction limit of the non-premixed counterflow flame decreases, the maximum flame temperature decreases, and flame stability decreases. At the same time, CO2 emissions are significantly reduced, achieving a certain emission reduction effect.

As a carbon-free fuel and hydrogen carrier, ammonia has gained increasing attention in the combustion community to support the global decarbonisation efforts. However, it has unfavorable flame properties and emissions characteristics such as low flammability and intrinsic trade-off between nitric oxide emissions and slip ammonia that hinders its widespread integration to the heat and power sectors. As a promising technology, non-thermal plasma has already presented the potential to assist swirling ammonia flames with significant improvements in NOX abatement and flammability, while eliminating the need for blending with other reactive fuels such as hydrogen and methane. For this study, a plasma burner was designed to explore plasma discharge interaction with a highly swirling premixed ammonia flame over a range of equivalence ratios, and its effect on NO emissions. The results revealed that average plasma power varied with the equivalence ratio and the reduced electric field affected the NO emissions characteristics of premixed ammonia flames.

The global rise in carbon emissions presents a rising challenge for current and future generations. In the pursuit of zero carbon emissions, ammonia (NH3) has emerged as an attractive alternative energy source. Ammonia offers a carbon-free fuel option with a higher energy density than liquid hydrogen while maintaining ease of transport and storage. However, ammonia still has its drawbacks, such as a high autoignition temperature, slow burning velocity, and low heating value, that demand further investigation of its combustion characteristics. This experiment was done to study the effect of nozzle shape and equivalence ratio (ɸ) on the combustion of an ammonia/oxygen/argon mixture using a constant volume combustor equipped with a sub-chamber. The fuels were premixed for 10 minutes and conditioned to an initial pressure of 0.2 MPa and an initial mixture temperature of 423 K. The results show that the different nozzle shapes each have their advantages in terms of pressure and jet speed. Overall, the lean mixtures (ɸ0.6 and ɸ0.8) consistently performed better compared to the stoichiometric mixtures (ɸ1.0) in all categories investigated in this study. The round nozzle generates higher pressure, while the special shape nozzle enhances jet speed, highlighting trade-offs between the two.

The diffusion porous media combustion is one possible way to eliminate the drawbacks of the existing combustion systems. Inverse diffusion flame (IDF) has features of both premixed and non-premixed flames. To integrate the advantages of porous media combustion with IDF, inverse diffusion porous (IDP) medium burner is tested for change in flame morphology and emissions at different equivalence ratio ( ɸ ). The porous media located at the exit of IDF burner has potential to deliver minimum flame length with low emissions. Flame appearance, flame height, flame zones etc. and emissions are experimentally investigated. Methane is used as fuel. Visible flame height is captured digitally and evaluated using ImageJ software. Central plane flame temperature is measured experimentally. CO and NO X emissions are recorded with Testo-340 flue gas analyser. The use of porous media at flame base is beneficiary in terms of achieving better air-fuel mixing and radial diffusion of air-fuel mixture. This reduces flame height with porous medium at all range of ɸ . Increase in ɸ reduces CO and enhances NO X emissions. Porous media reduces CO by 75 % and NO X by 60 %. Inverse diffusion porous medium burner emits lowest emissions in rich conditions.

This study experimentally explored the effects of equivalence ratio settings on ethanol fuel combustion oscillations with a laboratory-scale combustor. A contrary flame equivalence ratio adjusting trend was selected to investigate the dynamic characteristics of an ethanol atomization burner. Research findings denote that optimizing the equivalence ratio settings can prevent the occurrence of combustion instability in ethanol burners. In the combustion chamber, the sound pressure amplitude increased from 138 Pa to 171 Pa and eventually dropped to 38 Pa, as the equivalence ratio increased from 0.45 to 0.90. However, the sound pressure amplitude increased from 35 Pa to 199 Pa and eventually dropped to 162 Pa, as the equivalence ratio decreased from 0.90 to 0.45. The oscillation frequency of the ethanol atomization burner presents a migration characteristic; this is mainly due to thermal effects associated with changes in the equivalence ratio that increase/decrease the speed of sound in burnt gases, leading to increased/decreased oscillation frequencies. The trend of the change in flame heat release rate is basically like that of sound pressure, but the time-series signal of the flame heat release rate is different from that of sound pressure. It can be concluded that the reversible change in equivalence ratio will bring significant changes to the amplitude of combustion oscillations. At the same time, the macroscopic morphology of the flame will also undergo significant changes. The flame front length decreased from 25 cm to 18 cm, and the flame frontal angle increased from 23 to 42 degrees when the equivalence ratio increased. A strange phenomenon has been observed, which is that there is also sound pressure fluctuation inside the atomized air pipeline, and it presents a special square waveform. This study explored the equivalence ratio adjusting trends on ethanol combustion instability, which will provide the theoretical basis for the design of ethanol atomization burners.

In long-distance freight transport, where decarbonization targets are difficult to achieve through electrification, the adoption of cleaner fuels is more applicable to heavy-duty engines. This study explores the effects of ammonia–hydrogen mass ratio and equivalence ratio on the combustion and emissions of heavy-duty engines under low-load conditions. The results show that the indicated thermal efficiency of the engine increased and then decreased as the hydrogen mass ratio increased. Between hydrogen mass ratios of 3.5% and 20%, the blended fuels exhibited higher thermal efficiency during rarefied combustion. As the hydrogen mass ratio increases, NOx emissions increase and N2O emissions decrease. As the equivalence ratio increases, NOx emissions first increase and then decrease, and N2O emissions decrease. Under the low-load conditions of heavy-duty engines, considering performance and emissions, it is recommended to choose an equivalence ratio between 0.6 and 0.7, with a hydrogen mass ratio of 7.5%, can achieve an indicated thermal efficiency of 37.3% or higher, with NOx emission of less than 21.12 g/kwh and N2O emission of less than 0.74 g/kwh.

This paper investigates the effect of equivalence ratio on pollutant formation characteristics of CH4O/H2/NH3 ternary fuel combustion and analyzes the pollutant formation mechanisms of CO, CO2, and NOX at the molecular level. It was found that lowering the equivalence ratio accelerates the decomposition of CH4O, H2, and NH3 in general. The fastest rate of consumption of each fuel was found at φ = 0.33, while the rates of CH4O and NH3 decomposition were similar for the φ = 0.66 and φ = 0.4. CO shows an inverted U-shaped trend with time, and peaks at φ = 0.5. The rate and amount of CO2 formation are inversely proportional to the equivalence ratio. The effect of equivalence ratio on CO2 is obvious when φ > 0.5. NO2 is the main component of NOX. When φ < 0.66, NOX shows a continuous increasing trend, while when φ ≥ 0.66, NOX shows an increasing and then stabilizing trend. Reaction path analysis showed that intermediates such as CH3 and CH4 were added to the CH4O to CH2O conversion stage as the equivalence ratio decreased with φ ≥ 0.5. New pathways, CH4O→CH3→CH2O and CH4O→CH3→CH4→CH2O, were added. At φ ≤ 0.5, new intermediates CHO2 and CH2O2 were added to the CH2O to CO2 conversion stage, and new pathways are added: CH2O→CO→CHO2→CO2, CH2O→CO→CO2, CH2O→CHO→CO→CHO2→CO2, and CH2O→CH2O2→CO2. The reduction in the number of radical reactions required for the conversion of NH3 to NO from five to two directly contributes to the large amount of NOX formation. Equivalent ratios from 1 to 0.33 corresponded to 12%, 21.4%, 34%, 46.95%, and 48.86% of NO2 remaining, respectively. This is due to the fact that as the equivalence ratio decreases, more O2 collides to form OH and some of the O2 is directly involved in the reaction forming NO2.

The main goal of automobile researchers is to develop internal combustion engines that are fuel efficient and emit zero pollutants. It can be inferred from prior research publications that lean burn conditions can significantly reduce emissions while improving engine efficiency. The lean-burn engine combustion temperatures are lower hence harmful emissions like NO are reduced. Gasoline fuels have a narrow equivalence ratio window hence it was necessary to evaluate the other alternative fuels with a wider equivalence ratio for using it in IC engines for better performance and fewer emissions. This experiment is conducted on a single-cylinder digital three-spark ignited electronic fuel injected (DTSI-EFI) single-cylinder, 4 stroke high-speed SI engine fuelled by hydrogen. The excess air ratios are changed and MBT timing was also optimized. Hydrogen has delivered the lowest emissions under lean conditions. This data gives guidelines for developing SI engines with hydrogen port fuel injection for meeting future emissions norms. This experimental attempt is to protect the environment from greenhouse gas (GHG) emissions. The highest Brake Thermal Efficiency (BTE) is recorded at the leaner condition (λ = 4) as 37.53%, the highest power output is 7.02 kW at λ=1.5. CO and THC emissions are absent in hydrogen fuel and NO emissions reduces towards lean combustion.

Methanol is a promising alternative marine fuel due to its favorable combustion characteristics and potential to reduce exhaust emissions under increasingly stringent International Maritime Organization (IMO) regulations. This study presents a three-dimensional computational fluid dynamics (CFD) analysis of a four-stroke, medium-speed marine engine operating in methanol–diesel dual-fuel (DF) mode. Simulations were performed using AVL FIRE for a MAN B&W 6H35DF engine, covering the in-cylinder process from intake valve closing to exhaust valve opening. Nine operating cases were investigated, including seven methanol–diesel DF cases with equivalence ratios (Φ) from 0.18 to 0.30, one methane–diesel DF case (Φ = 0.22), and one pure diesel baseline. A power-matched condition (IMEP ≈ 20 bar) enabled consistent comparison among fueling strategies. The results show that methanol–diesel DF operation reduces peak in-cylinder pressure, heat-release rate, turbulent kinetic energy, and wall heat losses compared with diesel operation. At low to moderate Φ, methanol DF combustion significantly suppresses nitric oxide (NO), soot, and carbon monoxide (CO emissions), while carbon dioxide (CO2) emissions increase with Φ and approach diesel levels under power-matched conditions. These results highlight methanol’s potential as a viable low-carbon fuel for marine engines.

An annular scramjet combustor features structural symmetry, uniform flow field, and minimal corner effects. Its relatively small flow path height enhances fuel mixing. This study employs numerical simulations to investigate fuel mixing and combustion characteristics in the annular scramjet combustor at an inflow Mach number of 2.35. At a fixed injection position, increasing the equivalence ratio improves fuel penetration and spanwise spread but decreases mixing efficiency. As the distance from the injector orifice to the cavity leading edge (Lj) increases from 40 to 140 times the injector diameter (d), fuel mixing efficiency improves upstream of the cavity. Three combustion modes are identified: supersonic combustion mode (mode 1), cavity partially choked mode (mode 2), and injection section partially choked mode (mode 3). The combustion mode is determined jointly by injection distance and equivalence ratio. For short injection distances (Lj/d ≤ 80), the combustion zone is mainly in the jet windward shear layer and cavity. At low equivalence ratios, heat release is insufficient to induce thermal choking. The combustion occurs in mode 1. As equivalence ratio increases, thermal choking occurs near the cavity. The combustion is characterized by mode 2. For 80 < Lj/d ≤ 100, increasing the equivalence ratio shifts the thermal choking from the cavity to the injection section. As a result, the combustion mode transitions from mode 1 to mode 2 and eventually to mode 3. For Lj/d > 100, the combustion zone consistently resides upstream of the cavity. At low equivalence ratios, the combustion mode is mode 1, while higher equivalence ratios correspond to mode 3.

The greenhouse effect issue is becoming more serious, and renewable energy is playing an increasingly important role. Among all alternative fuels, ammonia has been attracting attention as a carbon-free energy carrier for hydrogen, because of its large energy density per volume and easy storage and transportation. On the other hand, ammonia has a low combustion speed, which is an important issue for the use of ammonia as a vehicle fuel. To increase the mean flame speed of ammonia, the present study used the burned gas ejected from the sub-chamber for the compression of the mixture in the main chamber and the promotion of its HCCI combustion. Thus, the constant volume combustor with sub-chamber was used to realize the above combustion and to study the combustion characteristics of ammonia and oxygen mixture. In the experiments, initial pressure and initial temperature were unchanged and only the equivalence ratio was changed. The combustion pressure data were recorded and analyzed. As the result, the maximum combustion pressure (2.5 MPa) was obtained when the equivalence ratio was 0.4. The combustion speed was the fastest when the equivalence ratio was 0.6, and the mean flame speed was about 57.5 m/s.

The effects of cavity depth on kerosene combustion characteristics during mode transitions in a variable-cavity scramjet combustor at Mach 2.5 are investigated numerically by improved delayed detached eddy simulation coupled with a dynamic zone flamelet model. A mode transition control method based on the variable cavity for the variable-geometry scramjet combustor is proposed. Depth-increasing (from 28 to 34 mm) and depth-decreasing (from 34 to 28 mm) paths are reproduced numerically by dynamic meshing to reveal the influence of the variable cavity on the mode transition under an equivalence ratio of 0.4. Mode transition occurs because of reduced incoming flow velocity and increased participation of kerosene droplets in the reaction. A critical cavity depth for mode transitions is identified as 32 mm. The scramjet and ramjet modes assume cavity stabilization and jet wake flame stabilization modes, respectively. Mode transition hysteresis occurs during the ram-to-scram transition. The increased kerosene vaporization rate due to the intensified interaction between the cavity shear layer and the kerosene jet is the underlying reason for the flame upstream propagation. The flame upstream propagation creates a high-temperature but low-speed reaction zone, which intensifies the reactions and eventually contributes to hysteresis.

The rocket-aided ramjet engine integrates high-thrust rocket engines into the flow passage of the ramjet, effectively broadening the operating speed range and altitude capability of the ramjet. This integration enables rapid thrust modulation and stable mode transitions. To enhance the understanding of turbulent combustion organization within rocket-aided ramjet engines and to better devise combustion organization strategies, numerical simulations were conducted using the Reynolds-Averaged Navier-Stokes method to investigate the influence of key parameters, including the number of fuel injection holes, equivalence ratios, injection positions, and rocket mass flow rates, on engine combustion characteristics and performance under Mach 6 flight conditions. The results indicate that, under the research conditions of this paper, in pure ramjet mode, a configuration with 12 fuel injection holes can achieve superior overall performance. As the fuel injection equivalence ratio increases, the heat release area in the combustor expands, but combustion efficiency and specific impulse decrease. Advancing the fuel injection location enhances mixing, and simultaneously alters the core heat release region. When the rocket is ignited and its mass flow rate is set to 0.2 kg/s, the engine's thrust increases by 53%, while the specific impulse decreases by 48%. As the rocket mass flow rate increases, the engine's overall performance gradually shifts toward a rocket-dominated mode.

The choice of turbulence model significantly impacts the prediction of the supersonic combustion flow field. In this study, the transient autoignition process and steady supersonic combustion performance of methane fuel with adjustable reactivity in a scramjet combustor were numerically investigated, employing different turbulence models including standard k - ε, renormalization group (RNG) k - ε, and realizable k - ε. Within the flight Mach number range of 3.5–7.0 and activation energy coefficient range of 0.5–1.0, five distinct autoignition modes are observed: misfire, blowoff, diverging section combustion, flashback, and constant‐area section combustion. For a given combustor inlet Mach number, equivalence ratio, and fuel temperature, each turbulence model corresponds to a distinct autoignition boundary. This discrepancy reflects the variations in the interaction between supersonic flow and chemical reaction. Moreover, the disparities are also evident in the characteristics of the steady supersonic combustion flow field. Specifically, at a flight Mach number of 7.0 and an activation energy coefficient of 1.0, both the standard k - ε and realizable k - ε models exhibit similar trends in the streamwise distribution of static temperature, as well as static pressure, Mach number, and combustion efficiency. When using the RNG k - ε model, autoignition occurs closer to the fuel injector, thereby attenuating flow disturbance caused by combustion heat. Consequently, mixing between the main flow and fuel is not significantly enhanced, resulting in a lower combustion efficiency at the combustor outlet. With the activation energy coefficient reduced to 0.5, reactions occur more readily under the Ma0 = 7.0 condition. All three k - ε turbulence models predict a constant‐area section combustion mode, while they display notable differences in the reacting flow fields. These findings are valuable for analyzing how different turbulence models influence the autoignition process and for performance evaluation in supersonic combustion.

The present research work investigated the combustion characteristics of lean premixed Ammonia/Methane/Air flames in an atmospheric pressure swirl-stabilized gas turbine can combustor. The study focused on characteristics such as flame structure, flame stability, combustor liner wall heat load and emissions. Different volume % of Ammonia-Methane (0-50 % Ammonia, rest being Methane) blends were considered at an equivalence ratio = 0.6 and at Reynolds number ∼ 50000 where the flame was sustained using a 10 % Methane pilot flame. High-speed flame luminosity imaging was carried out to study the flame structure and flame stability. Infrared thermography was used to simultaneously measure both outer/inner wall temperatures and to estimate the liner wall heat load. For studying emissions, steady-state numerical modeling was carried out using the CONVERGE CFD 3.0 software where both isothermal and adiabatic cases were studied. The latter comprised the entire volume fraction of Ammonia and Methane. Particle Image Velocimetry data were used to validate the numerical model. In the study, Ammonia/Methane/Air flames were found to exhibit increased flame-turbulence interaction compared to the pure Methane-Air flame. Flame instability and flame extinction were observed in the 50 % Ammonia-50 % Methane flame in the downstream section of the combustor away from the pilot flame and along the combustor wall. Compared to the combustor wall heat in the pure Methane-Air flame, in Ammonia/Methane flames, combustor wall heat load was found to be reduced by ∼ 10 to 40 % for various cases. In addition, NOx emissions were found to be less under isothermal condition as compared to adiabatic condition because of unburnt fuel.

No abstract available

Abstract Hydrogen (H2) and ammonia (NH3) are highly promising carbon-free fuels and can mitigate the greenhouse effect threat. The laminar combustion characteristics of ethylene (C2H4) doped with H2 and NH3 were numerically calculated at large doping proportion (0–50 %), initial temperatures (Tu = 300–400 K), and initial pressures (Pu = 0.1–1.0 MPa) by using the Chemkin/Premix Code. The equivalence ratio (Φ) ranged from 0.75 to 1.5. Laminar burning velocities (LBVs), adiabatic flame temperatures (AFTs), net heat release rates (NHRRs), temperature sensitivity analysis (TSA), mole fractions of radicals of H, O, OH and intermediates of C2H2, NO, NO2, the rate of production (ROP) and the reaction pathways were studied in this research. The results showed that H2 promoted the increase of C2H4/air LBVs, AFTs and NHRRs, while NH3 had the contrary effects. R1 (H + O2 <=> H + OH) had the largest positive sensitivity coefficient more than 0.3. Through the analysis of TSA and ROP, R146 (C2H3 + H <=> C2H2 + H2) was the main reaction to product C2H2, and C2H2 could be effectively inhibited after doping NH3. Additionally, the mole fraction of NO decreased as H2 increased but increased with the increase of NH3. The peak NO2 located much closer to the nozzle inlet after doping H2 and NH3, and R392 (NO + HO2 <=> NO2 + OH) was the main reaction linked NO and NO2. The reaction pathway showed the effect of NH3 on reducing CO2 was stronger than that of H2.

Methanol is a renewable and sustainable energy source with potential to overcome energy scarcity, global warming, and environmental pollution caused by the overdependence on fossil energy. However, its application in engines faces challenges, particularly the cold start issues. Turbulent Jet Ignition (TJI) presents a promising solution due to its advantage of multipoint ignition, and thereby a comprehensive study of TJI can contribute to guide the design and optimization of methanol engines. This work establishes an experimental system, employing the high-speed schlieren imaging technique to observe the propagation of methanol jet flames, and investigates the influence on the combustion characteristics of various operating parameters, such as the initial temperature and pressure and the intervals between the two ignitions. The results indicate that the propagation of the jet flame accelerates with the equivalence ratio increasing from 0.8 to 1.2. When the equivalence ratio is 1.0 and 1.2, the higher initial pressure results in slower development of the jet flame. Additionally, a short ignition interval leads to sluggish jet flame development and prolonged combustion duration, while an excessively prolonged ignition interval makes it impossible to generate effective jet flame. This research provides valuable insights for optimizing the jet ignition operation and enhancing the efficiency of methanol engines.

Constant volume chamber (CVC) was adopted to measure the laminar burning velocity (LBV) and Markstein length Lb of hydrous ethanol/RP-3 mixed fuel at initial temperature T = 450 K, initial pressure P = 0.1 MPa, and equivalence ratio ϕ = 0.8–1.4 with hydrous ethanol blending ratio Rhydrous ethanol of 0.2, and the effects of the water content in hydrous ethanol on the LBV and Lb were studied. It is found that with the increased water ratio, the LBV of hydrous ethanol/RP-3 decreases. Kinetic analysis of chemical reactions shows that with the increased water ratio the generations of CH3 and CH2 decreased because of more OH radicals participating in the key reactions. Meanwhile, the concentrations of H, O, and OH radicals decreased too, which inhibits the LBV of hydrous ethanol/RP-3. The results also show that with the increased water ratio, the Lb of hydrous ethanol/RP-3 increased, which means that the water content in hydrous ethanol would be beneficial to improve combustion stability. The experimental data obtained in our study will bring theoretical basis and technical support for the use of bioethanol in thermodynamic machinery.

The operational optimization of industrial boilers utilizing hydrogen-enriched natural gas is constrained by two critical gaps: insufficient understanding of the coupled effects of hydrogen blending ratio, equivalence ratio, and boiler load on combustion performance—compounded by unresolved challenges of combustion instability, flashback, and elevated NOx emissions—and a lack of systematic investigations combining these parameters in industrial-scale systems (prior studies often focus on single variables like hydrogen fraction). To address this, a comprehensive computational fluid dynamics (CFD) analysis was conducted on a 2.1 MW industrial boiler, employing the Steady Laminar Flamelet Model (SLFM) with a modified k-ε turbulence model and the GRI-Mech 3.0 mechanism. Simulations covered hydrogen fractions (f(H2) = 0–25%), equivalence ratios (Φ = 0.8–1.2), and load conditions (15–100%). All NOx emissions reported herein are normalized to 3.5% O2 (mg/Nm3) for regulatory comparison. Results show that increasing the hydrogen content raises the flame temperature and NOx emissions while reducing CO and unburned hydrocarbons; a higher equivalence ratio elevates temperature and NOx, with Φ = 0.8 balancing efficiency and emission control; and reducing load significantly lowers furnace temperature and NO emissions. Notably, the boiler’s unique staged-combustion configuration (81% fuel supply to the central rich-combustion nozzle, 19% to the concentric lean-combustion nozzle) was found to mitigate NOx formation by 15–20% compared to single-inlet burner designs, and its integrated cyclone blades (generating maximum swirling velocity of 14.2 m/s at full load) enhanced fuel–air mixing, which became particularly critical for maintaining combustion stability at low loads (≤20%) and high hydrogen blending ratios (≥20%). This study provides quantitative trade-off insights between combustion efficiency and pollutant formation, offering actionable guidance for the safe, efficient operation of hydrogen-enriched natural gas in industrial boilers.

The challenges of energy security and environmental sustainability have driven the development of eco-friendly alternative fuels, among which second-generation biodiesel has gained significant attention. This study aims to analyze the combustion quality of biodiesel derived from Calophyllum inophyllum oil (CIME) under the influence of an external magnetic field on the laminar flame speed in a premixed flame system. CIME biodiesel was produced through a transesterification process following degumming and esterification stages. Experimental tests were conducted using a stainless-steel Bunsen burner with a T-junction configuration and integrated heating system, in which magnetic fields with strengths ranging from 7000 to 10000 gauss were applied directly to the flame region. The equivalence ratio (ϕ) of the fuel-air mixture was varied between 0.4 and 1.4. Laminar flame speed was calculated from flame visualization data based on the observed flame angle. The results indicate that the magnetic field significantly enhanced the laminar flame speed, with the highest value observed at ϕ = 0.8 and a magnetic field strength of 10000 gauss. This phenomenon is attributed to the increased oxygen concentration in the reaction zone, induced by the magnetic attraction of paramagnetic O₂ molecules. These findings suggest that the integration of magnetic fields into biodiesel combustion processes can potentially improve energy efficiency and reduce emissions, offering an innovative approach for advancing renewable energy systems.

Ammonia as a carbon-free alternative fuel has received much attention with the consumption of fossil fuels. In order to explore the mixed combustion of methane and ammonia, a combined porous media burner was designed with pellets embedded in annular ceramic foam. And the effects of operating parameters on combustion characteristics were investigated. The results showed that the ammonia addition increased the combustion temperature and reduced carbon dioxide emissions at the equivalence ratio of <1. And the ammonia promoted the conversion of CO2 to CO for an equivalence ratio of >1. With the increasing of the ammonia ratio, the CO selectivity increased but the CO2 selectivity decreased. In addition, the mixed combustion of ammonia and methane improved the hydrogen production. The fuel ratio of methane to ammonia (0.80: 0.20) resulted in higher syngas production and lower CO2 mole fraction. The flame propagated faster in ceramic foam with lower pore densities (20 PPI) so the preheating time was greatly reduced. Moreover, the 40 PPI ceramic foam was conducive to the stability of the flame position in the upstream zone, and the H2 mole fraction achieved 10.60 % at the inlet velocity of 14 cm/s.

Several studies have investigated the effects of equivalence ratio and blending ratio in constant power cases for ammonia-based fuels as ammonia is being investigated extensively as an alternative zero-carbon fuel in recent years. However, in many fields, such as power generation, it is common to operate practical equipment at constant Reynolds number (Re) conditions. To that extent, this study investigates the impact of Re (4000 ≤ Re ≤ 7000), equivalence ratio (0.6 ≤ Φ ≤ 1.45), and ammonia volume ratio (0.55 ≤ X NH3 ≤ 0.9) on the combustion characteristics of NH 3 /H 2 swirling flames. To supplement the experimental work, a previously developed Chemical Reactor Network (CRN) has been used to identify the reaction pathways affecting NOx emissions through reaction analysis. Sampled gas analysis (NO, NO 2 , N 2 O, NH 3 ) at the exhaust was reported for the first time at a wide range of equivalence ratios and X NH3 at different Re. At constant Re = 6000, NO and NO 2 peaked at around Φ = 0.8 – 0.9, while N 2 O became substantial at Φ ≤ 0.8 and unburnt ammonia became significant at Φ ≥ 1.05. The chemiluminescence images suggested that at Re = 6000, as X NH3 or Φ increases, the NH 2 * region expands, and reactions occur further downstream. Furthermore, with decreasing X NH3 at Re = 6000 and Φ = 0.9, the system becomes partially abundant in H 2 and H but deficient in O 2 , suppressing the generation of O and OH radicals, and ultimately also suppressing HNO generation. This leads to the N/NH system reactions becoming dominant and increased NO consumption. NO emissions increased steadily with increasing Re at Φ = 0.8, while at stoichiometry, NO emissions remained somewhat unchanged with changing Re. Findings from this study will aid further in the development of carbon-free zero-emissions systems.

This study utilizes a zero-dimensional, constant-pressure, perfectly stirred reactor (PSR) model within the Cantera framework to examine the combustion characteristics of hydrogen, methane, methanol, and propane, both singly and in hydrogen-enriched mixtures. The impact of the equivalence ratio (ϕ = 0.75, 1.0, 1.5), fuel composition, and residence duration on temperature increase, heat release, ignition delay, and emissions (NOx and CO2) is methodically assessed. The simulations are performed under steady-state settings to emulate the ignition and flame propagation processes within pre-chambers and primary combustion zones of internal combustion engines. The results demonstrate that hydrogen significantly improves combustion reactivity, decreasing ignition delay and increasing peak flame temperature, especially at short residence times. The incorporation of hydrogen into hydrocarbon fuels, such as methane and methanol, enhances ignition speed, improves thermal efficiency, and stabilizes lean combustion. Nevertheless, elevated hydrogen concentrations result in increased NOx emissions, particularly at stoichiometric equivalence ratios, due to higher flame temperatures. The examination of fuel mixtures at varying hydrogen concentrations (10–50% by mole) indicates that thermal performance is optimal under stoichiometric settings and diminishes in both fuel-lean and fuel-rich environments. A thermodynamic model was created utilizing classical combustion theory to validate the heat release estimates based on Cantera. The model computes the heat release per unit volume (MJ/m3) by utilizing stoichiometric oxygen demand, nitrogen dilution, fuel mole fraction, and higher heating values (HHVs). The thermodynamic estimates—3.61 MJ/m3 for H2–CH3OH, 3.43 MJ/m3 for H2–CH4, and 3.35 MJ/m3 for H2–C3H8—exhibit strong concordance with the Cantera results (2.82–3.02 MJ), thereby validating the physical consistency of the numerical methodology. This comparison substantiates the Cantera model for the precise simulation of hydrogen-blended combustion, endorsing its use in the design and development of advanced low-emission engines.

For the safe and efficient utilization of hydrogen-enriched natural gas combustion in industrial gas-fired boilers, the present study adopted a combination of numerical simulation and field tests to investigate its adaptability. Firstly, the combustion characteristics of hydrogen-enriched natural gas with different hydrogen blending ratios and equivalence ratios were evaluated by using the Chemkin Pro platform. Secondly, a field experimental study was carried out based on the WNS2-1.25-Q gas-fired boiler to investigate the boiler’s thermal efficiency, heat loss, and pollutant emissions after hydrogen addition. The results show that at the same equivalence ratio, with the hydrogen blending ratio increasing from 0% to 25%, the laminar flame propagation speed of the fuel increases, the extinction strain rate rises, and the combustion limit expands. The laminar flame propagation speed of premixed methane/air gas reaches the maximum value when the equivalence ratio is 1.0, and the combustion intensity of the flame is the highest at this time. In the field tests, as the hydrogen blending ratio increases from 0% to nearly 10% with the increasing excess air ratio, the boiler’s thermal efficiency decreases as well as the NOx emission. This indicates that there exists a tradeoff between the boiler thermal efficiency and NOx emission in practice.

ABSTRACT This paper introduced a machine learning-based method for reconstruction spectral information from RGB images for measurements of the equivalence ratio (Φ) of premixed air-methane flames. Digital color cameras capture color and spatial information of hydrocarbon premixed flames in the visible band. The color of the flame is the result of spectral integration of chemiluminescence in the visible wavelength band, and direct use for flame equivalence ratio measurements would result in large errors due to low spectral resolution. The mapping function was used to reconstruct the spectral characteristics of the premixed methane flame for the RGB image, which were built by three types of machine learning methods: support vector regression (SVR), random forest (RF) regression, and Levenberg-Marquardt backpropagation neural network (LM-BPNN), respectively. Finally, LM-BPNN-based reconstructed spectral features are used for the development of Φ measurement model, which measurement error below 0.01, accurately measures the equivalence ratio of premixed methane flames within the range of 0.73 to 1.47. Further analyze the two-dimensional spatial distribution of flame equivalence ratio.

Oxy-fuel combustion is a promising way to avoid process-based CO2 emissions. In this paper, the operational range of a new semi-industrial oxy-fuel combustion chamber for pulverized biomass is analyzed. This approach is used to gain a deeper understanding of the combustion setup and to examine the differences between air and oxy-fuel combustion on an industrial scale. Both analyzed parameters—flame spread and temperature distribution—have a significant influence on heat transfer in commercial boilers. The stability of various operating conditions is assessed by monitoring the CO content in the flue gas via a gas analyzer unit. For stable operation using walnut shells as fuel in an air atmosphere, an overall air-to-fuel ratio of 1.57–1.75 and a local air-to-fuel ratio of 0.75–0.95 provide the most stable conditions. A high swirl number of 0.9 is found to be critical for stability, as the increased fuel momentum entering the combustion chamber promotes a fuel jet-dominated swirl flame. For the corresponding oxy-fuel combustion with the same volume flows and three different oxygen concentrations between 27% and 33%, stable combustion behavior is also observed. Using a camera setup to analyze flame shape and spread, it is observed that the flame formed with an oxygen content of 33% most closely resembles the flame shape achieved under air combustion conditions. However, the combustion temperatures most closely match those of the air operating condition when the oxygen content is 27%. Overall, it is shown that the approach for corresponding oxy-fuel conditions features similar flame shapes to oxy-fuel combustion with flue gas recirculation in a semi-industrial combustion chamber.

This study investigates the impact of the staging factor, the ratio between the fuel injected through the pilot stage and the multipoint injection, on the flame dynamic. The BIMER combustor is an atmospheric pressure rig equipped with two co-rotating swirling air injections (a fixed amount of around 87% of the air goes inside the multipoint stage) and two fuel injection paths for staged combustion. Liquid dodecane is injected with air preheated at 437K with a global equivalence ratio of 0.6 and a thermal power around 72 kW. The change of the staging factor from 100% (pilot-only injection) toward 0% (multipoint-only injection) generates changes in the flame-shape which bifurcates from an anchored V-flame into a lifted flame. This flame shape bifurcation appears at a staging factor around 25%. Around this staging factor, one can witness multistable flames where the flame structure transits randomly between five different states. Processing microphone signals recorded in the chamber provides an understanding of the flame dynamics. The attached flame presents limited pressure fluctuations level at 270 Hz, while the lifted flame features high-pressure fluctuations at 323 Hz. The intermittency between the five states (including the two stable states) is investigated.

This paper presents a joint experimental and numerical study on premixed laminar ammonia/methane/air flames, aiming to characterize the flame structures and NO formation and determine the laminar flame speed under different pressure, equivalence ratio, and ammonia fraction in the fuel. The experiments were carried out in a lab-scale pressurized vessel with a Bunsen burner installed with a concentric co-flow of air. Measurements of NH and NO distributions in the flames were made using planar laser-induced fluorescence. A novel method was presented for determination of the laminar flame speed from Bunsen-burner flame measurements, which takes into account the non-uniform flow in the unburned mixture and local flame stretch. NH profiles were chosen as flame front markers. Direct numerical simulation of the flames and one-dimensional chemical kinetic modeling were performed to enhance the understanding of flame structures and evaluate three chemical kinetic mechanisms recently reported in the literature. The stoichiometric and fuel-rich flames exhibit a dual-flame structure, with an inner premixed flame and an outer diffusion flame. The two flames interact, which affects the NO emissions. The impact of the diffusion flame on the laminar flame speed of the inner premixed flame is however minor. At elevated pressures or higher ammonia/methane ratios, the emission of NO is suppressed as a result of the reduced radical mass fraction and promoted NO reduction reactions. It is found that the laminar flame speed measured in the present experiments can be captured by the investigated mechanisms, but quantitative predictions of the NO distribution require further model development.

No abstract available

Structures of laminar non-premixed ethanol/air spray flames in the axisymmetric counterflow configuration are studied under fuel-rich conditions by means of numerical simulations. The monodisperse ethanol spray is carried by air and directed against an air stream. Both streams enter at 300 K, and the system is at atmospheric pressure. Up to three different structures of these flames for identical boundary and initial conditions are identified, and regime diagrams are presented that show their conditions of existence in terms of the gas strain rate on the spray side of the configuration, a − ∞ , starting from 55/s at an initial spray velocity of 0.44 m/s. The equivalence ratio on the spray side, E − ∞ , is varied between 1.1 and 1.6, and initial droplet radii, R 0 , from 10 to 50  μ m are considered. The most stable spray flame structure is characterized by two chemical reaction zones. For some conditions, single chemical reaction zones on either side of the counterflow configuration are found. Conditions under which these different flame structures exist are analyzed. Previous studies identified only two different structures for non-identical boundary conditions, and in this study, three different structures are presented for the first time. Moreover, the transition mechanisms of one structure to another are analyzed. The competition between the energy-consuming spray evaporation and the exothermic chemical reaction rates as well as the location of the spray determines the existence of the different flame structures. This transition of the different flame structures may explain spray flame characteristics such as flame pulsation or flame instabilities.

Polyhedral Bunsen flames, induced by hydrodynamic and thermo-diffusive instabilities, are characterized by periodic trough and cusp cellular structures along the conical flame front. In this study, the effects of flow velocity, hydrogen content, and equivalence ratio on the internal cellular structure of premixed fuel-lean hydrogen/methane/air polyhedral flames are experimentally investigated. A high-spatial-resolution one-dimensional Raman/Rayleigh scattering system is employed to measure the internal scalar structures of polyhedral flames in troughs and cusps. Planar laser-induced fluorescence of hydroxyl radicals and chemiluminescence imaging measurements are used to quantify the flame front morphology. In the experiments, stationary polyhedral flames with varying flow velocities from 1.65 to 2.50 m/s, hydrogen contents from 50 to 83%, and equivalence ratios from 0.53 to 0.64 are selected and measured. The results indicate that the positively curved troughs exhibit significantly higher hydrogen mole fractions and local equivalence ratios compared to the negatively curved cusps, due to the respective focusing/defocusing effect of trough/cusp structure on highly diffusive hydrogen. The hydrogen mole fraction and local equivalence ratio differences between troughs and cusps are first increased and then decreased with increasing measurement height from 5 to 13 mm, due to the three-dimensional effect of the flame front. With increasing flow velocity from 1.65 to 2.50 m/s, the hydrogen mole fraction and local equivalence ratio differences between troughs and cusps decrease, which is attributed to the overall decreasing curvatures in troughs and cusps due to the decreased residence time and increased velocity-induced strain. With increasing hydrogen content from 50 to 83%, the hydrogen mole fraction and local equivalence ratio differences between troughs and cusps are amplified, due to the enhanced effects of the flame front curvature and the differential diffusion of hydrogen. With increasing equivalence ratio from 0.53 to 0.64, a clear increasing trend in hydrogen mole fraction and equivalence ratio differences between troughs and cusps is observed at constant flow velocity condition, which is a trade-off result between increasing effective Lewis number and increasing curvatures in troughs and cusps.

Self-excited thermoacoustic instability (SETAI) is an undesirable and dangerous phenomenon in combustion systems. However, its control is difficult, thus greatly limiting the development of combustion technology. Our previous works clarified how the premixed chamber length (LP) and equivalence ratio (φ) influence SETAI behavior in a symmetrical hedge premixed combustion system. On real-world sites, however, the supply structure or combustion condition in a multi-flame system could be asymmetric due to space limitations or combustion adjustment needs. This paper aims to clarify the SETAI behavior of a combustion system with an asymmetric supply structure or an asymmetric combustion condition. The results indicate that the sound pressure amplitude under strong oscillation can reach 160 dB, which is about 5% of the total pressure. The SETAI state under the asymmetric condition is determined by the coupling between the heat release oscillation and sound pressure oscillation on each side and their cooperation. The asymmetric supply structure leads to asynchronous heat release oscillations between the two sides; it may be that one promotes oscillation and that the other suppresses it, or that both have a promotion effect but with asynchronous action, thus partly canceling each other out to lower the system’s oscillation intensity. This brings an advantage for controlling SETAI, which can be achieved by only changing one side of the structure. The oscillation amplitude can be reduced by 80–90% by appropriately changing one LP only by ~20%. Under an asymmetric combustion condition with φ differing between the two sides, the heat release oscillation on each side is dependent on the local φ but not the global φ. Consequently, SETAI can also be controlled by changing the distribution but maintaining a constant fuel feeding rate and φ. The concepts identified in this paper demonstrate that SETAI can be effectively controlled by adopting an asymmetric φ distribution or an asymmetric structure of the supply system. This provides a convenient SETAI control approach without affecting the equipment’s thermal performance.

In recent years, the need for low-carbon power has seen hydrogen emerge as a potential fuel to replace conventional hydrocarbons in combustion to limit CO2 emissions in several sectors, including aeronautics. The challenges posed by hydrogen combustion are similar to the issues of kerosene flames but more challenging, like nitrogen oxide (NOx) emissions and flame flashback. One potential solution to address these problems is to burn a rich mixture of hydrogen and air in globally lean conditions on a coaxial injector to obtain a stable and staged combustion and attempt to reduce emissions. In this article, the evolution of NOx production as more air is mixed into the fuel is studied, as well as the changes in flame size and structure. In particular, the appearance of a secondary flame front is observed and increasing the proportion of air in the fuel mixture both shortens the flame and reduces the NOx emission index. Additionally, the effect of the global equivalence ratio and flame thermal power are studied. Finally, existing models for NOx emission of hydrogen flames on a coaxial injector based on average flame residence time and strain rate are tested and shown to have promising results.

The utilization of liquid ammonia in gas turbines can reduce energy loss and start-up time. However, the flash boiling phenomenon and the high latent heat of liquid ammonia make the spray flame difficult to stabilize. Increasing the preheated air temperature or adding a small amount of hydrogen as a piloted fuel are considered as effective methods to enhance the stability. To understand the flame topological structure, simultaneous Mie scattering and planar laser-induced fluorescence of OH (OH-PLIF) techniques were used to visualize the liquid ammonia spray structure and flame region information. Results show that the liquid ammonia swirl spray flame exhibits the flame topological structure of distinct zoning characteristics, including the droplet zone, the mixing zone, and the flame zone. Increasing the preheated air temperature accelerates the evaporation of liquid ammonia, leading to an increase in the local equivalence ratio and radial flame splitting. At lower air temperature conditions, increasing the hydrogen blending ratio has minimal impact on the flame topological structure. However, at higher temperature conditions, hydrogen blending significantly promotes reaction intensity upstream and reduces the flame lift-off height, which makes the mixing zone smaller. In general, to achieve a better flame stability effect, the two factors need to be reasonably matched, which has important reference value for the development of liquid ammonia fueled gas turbine combustors.

This study investigates the control of methane/air inverse diffusion combustion using surface dielectric barrier discharge (SDBD) plasma technology to enhance methane fuel combustion performance in rocket engines. Under lean combustion conditions (Φ=0.76), forward SDBD dissociates methane C-H bonds via high-energy electrons, generating CH₃ radicals and forming a stable conical flame at 16 kV, while reverse SDBD suppresses turbulence to reduce flame height by 41.7%. At an optimal equivalence ratio (Φ=1), the reverse structure achieves flame height reduction from 130 mm to 94 mm, whereas the forward structure exacerbates flame nonuniformity due to aerodynamic effects. In rich combustion (Φ≥1.5), both plasma configurations inhibit methane inverse diffusion combustion, with the forward structure prone to causing flame instability. Analysis confirms that oxygen content is critical to the divergent control effects: forward SDBD excels in high-oxygen environments for combustion enhancement, while reverse SDBD is more effective for flow control in low-oxygen conditions. This research provides experimental insights and technical references for optimizing plasma-assisted combustion in rocket engines.

Coke oven gas (COG) is the main by-product of the coking industry, with a hydrogen content of more than 50%. Upon purification, COG can be used as a hydrogen-rich, efficient, high-quality alternative fuel, to improve the energy structure, alleviate global warming, and reduce polluting emissions. In the present paper, a constant-volume combustion experiment bench was established to carry out the premixed laminar combustion experiments of the COG-air mixture. A high-speed camera was used to record the flame propagation process and the effects of the influencing factors such as fuel-air equivalence ratio, initial pressure and H2 concentration on the flame speed. The results indicated that when Φ= 1.1, i.e., for a relatively concentrated fuel, the flame speeds of the stretched and unstretched flame both reached a maximum. With the increase in initial pressure, the flame speeds of both the stretched and unstretched flame decreased. As the H2 concentration increased, the propagation speeds of the stretched and unstretched flame both increased and the enhancements were more obvious; mainly since hydrogen combusts faster than methane.

The high emission of nitrogen oxides (NOx) is one of the major obstacles to the practical application of ammonia as a carbon-free fuel. Improving the flow distribution and structure has been demonstrated to achieve low NOx emissions. However, in comparison to hydrocarbon flames, the influence of swirl intensities on ammonia NOx emissions is still not well understood. This study builds upon previous research by further exploring the effects of swirl intensity on NO production in ammonia-methane-air premixed swirling flames. A new adjustable axial swirler was designed to achieve a wide range of swirl numbers. We measured flame morphology, NO emissions, and NO and OH planar laser-induced fluorescence (PLIF) images over extensive ranges of equivalence ratio and ammonia fraction. The study found that increasing the swirl number from 0.6 to 1.0 resulted in a more compact flame, with enhanced reactions in the corner recirculation zone. Varying the swirl number significantly alters the NO concentration in the exhaust gas. The concentration of NO was significantly reduced at an equivalence ratio of 0.90 and an ammonia fraction greater than 80%. NO/OH-PLIF indicated that NO was primarily formed in the main reaction zone, with NO-PLIF intensity in the post-flame zone almost remaining constant at different heights. The integrated intensities of NO and OH-PLIF were obtained at different heights above the nozzle. A positive linear correlation was observed between NO-PLIF plateau intensity and NO mole fraction. The increased heat loss to the wall at larger swirl intensities reduces the flame temperature in the main reaction zone, which inhibit the formation of OH radicals, ultimately resulting in low NO emissions. DOI: https://doi.org/10.18573/jae.14

Ammonia-air flames are known for low reactivity and have been posing as a huge hindrance in employing the chemical as a sustainable fuel of tomorrow. Curvature is a parameter that could influence the flame structure and so the position of the maximum heat release rate. Flame-acoustic interactions on a Bunsen burner are performed to study the local flame response to highly perturbed flows. NH2* chemiluminescence is used to study the reactivity of these flames. Non-perturbed flames are used as a reference to understand the inherent behaviour of Bunsen ammonia flames. A case study has been chosen for an equivalence ratio ranging between 1.0 and 1.4 at atmospheric conditions to study perturbed flames. The objective is to study the effect of curvature induced by the perturbations on the reactivity of the flame. It was seen that this given case study was quite complex as the flame response was to multiple factors like the effect of Lewis number, convective-diffusion velocities, decomposition of ammonia into hydrogen, thereby, promoting preferential diffusion of hydrogen in both large-scale and locally for certain cases apart from the generated acoustic perturbation which itself dictates the flow regime of the fresh gases, etc. Since the Damköhler number was around 1, the perturbation time scales and the reactivity time scales were comparable and so none of the effects could be ignored. It was concluded that for richer flames where Le>1, the negative curvature promoted the production of hydrogen leading to local enhancement in reactivity. A change in the local thickness due to the induced curvature was seen for all conditions. DOI: https://doi.org/10.18573/jae.16

Achieving full premixing and complete evaporation in lean prevaporized premixed combustors is challenging as it depends on the spray injector characteristics, prevaporisation strategy, and flow conditions. This article experimentally explores the stability and structure of a turbulent swirling n-heptane spray flame under various degrees of prevaporization. The results show that preheating the air to 343 K and 393 K has little effect on the lean blow-off velocity, while recessing the fuel injection significantly decreases the lean stability limit. To correlate these limits, various attempts to define a Damköhler number were made, but unlike previous studies with no prevaporisation, the difficulty in defining laminar flame speed in the present case does not allow a single correlation to work for all degrees of prevaporization. Four stable cases that differ in equivalence ratio, air preheat temperature, and fuel injection recess are investigated using one-dimensional PDA, OH* chemiluminescence and CH 2 O-planar laser-induced fluorescence (PLIF). Cases without fuel injection recess or air preheat exhibit a conical-shaped heat release zone near the shear layers. Preheating the air to 393 K reduced the Sauter mean diameter, increased prevaporization, and enabled a second heat release zone downstream of the fuel injection. Recessing the fuel injection by 25 mm reduced droplet velocities and led to a semi-spherical instead of a conical heat release zone. The CH 2 O-PLIF signal without injection recess was high along the central axis and its distribution resembled that observed for spray jet flames. In contrast, with recessed spray injection, CH 2 O was mainly found outside the central recirculation zone and only appeared inside during lean blow-off; similar to previous work with premixed flames. These findings show that different methods of prevaporization, which only differ by subtle changes in droplet characteristics, strongly impact flame stability. The present data can be used for turbulent flame modelling focusing on sprays and finite-rate kinetics.

Abstract To optimize the integrated flameholder, PIV was used to study flow fields of V-gutter and integrated flameholder under both non-reacting and reacting conditions. PLIF, high-speed cameras, and TDLAS were adopted to capture OH distribution, flame structure, and temperature distribution. Comparative analysis of flow fields, combustion characteristics and flame stabilization mechanisms were analyzed. Results show that heat release increases adverse pressure gradient, which can enlarge the recirculation zone size and recirculation rate compared to non-reacting flow field. The flames of both flameholders exhibit symmetrical structures distributed near the shear layers. The blockage ratio dominates the non-reacting flow field, while the expansion angle dominates the reacting flow field, which can further increase the adverse pressure gradient under reacting condition. The V-gutter flameholder demonstrates better fuel/air mixing and larger recirculation than the integrated flameholder. The combustion performance of the integrated flameholder is inferior to the V-gutter flameholder, albeit with better flow resistance properties.

Low emission combustion technology for gas turbine and aero-engine industrial has attracted more and more attention by the aeronautic community. Hydrogen fuel is believed being a promising alternative fuel for gas turbines and aero-engines in the next generation of propulsion system. In this work, a novel type of annular combustor with multi-point direct injection (MDI) burners for burning pure hydrogen is proposed for gas turbines and aero-engines. The MDI burner is featured with several multi-point injectors arranged in inclined direction, which forms a swirling flow to enhance the mixing process of hydrogen and air. Meanwhile, the MDI burners are beneficial for flame stability and preventing flashback. The flow structure, combustion process and NOx emission of a single MDI burner are numerically studied with hydrogen fuel under different air flow rate Qair and the equivalence ratio $\phi$. Then, the cold flow and combustion characteristics of the annular combustor with 12 MDI burners are investigated by the steady RANS method to understand the flow structure, temperature field and NOx emission under different operation conditions. Furthermore, the thermoacoustic property of the annular combustor is investigated to obtain the acoustic modes, including the longitudinal modes and azimuthal modes.

In this study, the optical method of Mach-Zehnder interferometry (MZI) is utilized in order to explore the flame structure and temperature field of syngas/air and hydrogen/air flames. Two axisymmetric burners with inner diameters of 4 mm and 6 mm are used for temperature field measurement of hydrogen and syngas, respectively. The effects of fuel composition, equivalence ratio, and Reynolds number (Re) are investigated at ambient condition (P=0.87 bar, T=300  K). Three different H2/CO fuel compositions with hydrogen fractions of 30%, 50%, and 100% are studied. Temperature profiles are reported at four different sections above the burner tip. Measured temperatures using the interferometry method are compared with thermocouple data and good agreement between them is observed. The results obtained in this investigation indicated that the MZI can be applied for accurate determination of flame front and temperature field, especially for high-temperature flames where other methods cannot be properly utilized. Analyses of the data reduction method revealed that the exact determination of the refractive index distribution and reference temperature is critical for accurate determination of the temperature field. The results indicated that by increasing the Re, the maximum flame temperature is enhanced. Increasing the equivalence ratio leads to expansion of the flame radial distribution (at the same distance from the burner tip). At higher distances from the burner tip, temperature increases uniformly from the flame boundary toward the flame axis, while at lower heights it shows reduction at the burner axis. By increasing the CO content of fuel, the maximum flame temperature increases at all equivalence ratios except at the stoichiometric condition, where SH100 illustrates the highest maximum flame temperature.

The flame structure characteristics of the RP-3 fueled dual-swirl direct-mixing combustor are studied experimentally. The flame shape is marked by the OH* radical, which is captured by a CMOS camera with an image intensifier. The flow fields and spray distributions are obtained by particle image velocimetry. The variation of pilot/main-stage flame structure with global fuel-air ratio (FAR), fuel ratio (FR), pilot/main-stage swirl numbers (Sp and Sm), and Δp/p (total pressure loss coefficient) is further investigated. The typical flame structure, consisting of two main-stage combustion zones (pilot-stage and main-stage combustion zones), is first analyzed. Then, according to the relationship between integral OH* intensity and global heat release rate, increasing swirl number will weaken the effect of strain rate on OH* chemiluminescence. The global FAR has little impact on the flame structure, while modifying the FR will alter the flame mode. The influence of the Sp on the flame structure is more significant than that of the Sm. Within the range of experimental conditions, the greater the swirl number, the smaller the Δp/p is required to obtain the maximum OH* radical intensity. The addition of non-swirl flow in the main-stage swirler can improve the stability of the pilot-stage flame. The equivalent swirl number is further evaluated by the neural network.

No abstract available

This study applies experimental methods to investigate the partially premixed ignition characteristics of a bluff-body flameholder with a pilot stage. The ignition fuel–air ratio (FAR) under different igniter and inlet conditions is obtained, while the ignition process is recorded with a high-speed photography device. Numerical simulations are carried out to investigate the relationship between the flow field, fuel distribution, and ignition process. The results show that a higher total capacitance energy storage of the igniter, inlet Mach number, or total pressure inside the combustion chamber will help increase the ignition performance, accelerate the development of the flame, and shorten the ignition delay. The flame propagation routine of the flameholder is controlled by several pairs of symmetrical recirculation zones behind the flameholder structure and the specific uneven fuel distribution caused by the flow field. This study provides a detailed understanding of the ignition process for the bluff-body flameholder.

No abstract available

This paper presents experimentally studied environmental and radiation parameters of hollow cylindrical burners during operation with LPG-air fuel mixture. Two combustion modes have been examined – an external combustion mode where the flame anchored near the outer surface of the burner, and an internal combustion mode when the combustion takes place in the inner cavity of the burner. The dependences of CO/NOx emissions and radiation efficiency on a firing rate in the range of 160-420 kW/m2, air-fuel equivalence ratio in the range of 1.0 - 1.4, as well as the porous structure of a burner have been analyzed. An influence of a flow deflector installed in front of the burner inlet in order to distribute the flow over the inner cavity of the cylindrical burner on environmental characteristics of the burner is discussed. The necessary condition that ensures CO emission below 50 ppm, NOx emission below 20 ppm and radiation efficiency in the range of 45-55 % is described in the paper.

This study proposes a deep neural network (DNN)–based regression model for predicting the excess air ratio, which is a critical indicator for optimizing combustion efficiency and minimizing harmful emissions in industrial combustion systems. The chemiluminescence signals of the OH∗ radicals and fuel pressure were used as the input features for the prediction model. To evaluate the effect of the multidimensional input, Case 1, with only the OH∗ radical signal as a single input, was compared with Case 2, with the OH∗ radical signal and fuel pressure as the inputs. The results showed that the Case 2 model reduced the mean absolute error (MAE), mean relative error (MRE), and root mean squared error (RMSE) by approximately 40.71%, 41.85%, and 19.69%, respectively, compared to Case 1, and the average relative prediction error rate was also 2.25% lower. These results demonstrate the potential for improving the accuracy and generalization ability of the model by incorporating multidimensional input features. Therefore, DNN models using multidimensional inputs can contribute to the design and implementation of combustion control systems to optimize the combustion efficiency and reduce harmful emissions in industrial combustion systems by predicting the excess air ratio.

This numerical simulation studied the effect of H2-CH4 flame equivalence ratio on turbulent premixed combustion at 50%-50% concentration (by volume). The equivalence ratio was varied from 0.45 to 1.0 in 0.5 increments for 126 kW operating power, matching a 3.3 bar inlet reactant pressure. Tests utilized a gas turbine combustor. The thermal field, flow field, and pollutant emissions (NOx and CO) underwent rigorous analysis. The modelling framework applied steady Reynolds-Averaged Navier-Stokes (RANS) equations coupled to a probability density function (PDF) approach for turbulence-chemistry interactions and a NOx formation model. Results showed increasing equivalence ratio from 0.45 to 1.0 elevated temperature approximately 900 K, significantly promoting NOx up to 1600 ppm and CO beyond 1900 ppm. However, equivalence ratio changes minimally impacted the overall flow field, maintaining stabilized flames. These findings provide new insight on thermochemical effects and flame stability in gas turbine (G.T) combustors across a range of equivalence ratios relevant for clean, high-pressure H2-CH4 combustion. The combined RANS-PDF methodology enables predictive simulation of turbulence, kinetics, emissions, and flame stability to guide optimal fuel-air ratio selection and low-emission combustor design.

The pilot stage plays a crucial role in central-staged combustion technology. This study aimed to investigate the impact of the jet-type pilot stage on the flame structure and combustion instability in a novel strong coupled centrally staged swirl gas turbine combustor, using both experiments and large eddy simulations (LES). Nonlinear dynamic analyses of dynamic pressure, including phase and recurrence plots, were performed alongside a proper orthogonal decomposition of the dynamic flame structures. It is indicated that a richer pilot stage worsens the instability of the centrally staged combustion system. An increase in the equivalence ratio of the pilot stage leads to enhanced non-premixed combustion and a downstream shift in the heat release region. The transition results in the shift of flame shape from an attached V-shaped flame to an intermittent lifting U-shaped flame. The flame surface statistics from LES results including the strain rate and progress variable gradient of lean and rich pilot conditions were compared. Under richer pilot conditions, the lifting U-shaped flame demonstrates increased sensitivity to flow field fluctuations, intensifying vortex–flame interactions. This interaction causes the large-scale flame surface stretching and even extinction of the pilot stage flame, exacerbating combustion instability observed in this study. These insights offer a deeper understanding of the impact of the jet-type pilot stage on the novel multi-staged central combustion systems.

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This study investigated premixed NH3 combustion in a closed circular duct using two-dimensional numerical simulations. By varying the equivalence ratio and the oxygen volume fraction from 21% to 30%, the evolution of flame morphology, flame propagation velocity, flame surface area, as well as the temporal variations in duct-averaged temperature and pressure were systematically examined. In addition, sensitivity analysis and reaction-pathway analysis based on a detailed chemical kinetic mechanism were performed to clarify the coupling between local chemical reactions and global flow dynamics. The results showed that the flame generally evolves through a sequence of hemispherical, finger-shaped, wall-attached skirt, and planar finger- and tulip-shaped structures. Well-developed tulip flames are mainly observed under conditions close to stoichiometric composition with moderate to elevated oxygen enrichment, corresponding to an intermediate overall reactivity. As the oxygen volume fraction increases from 21% to 30%, flame propagation becomes markedly faster. The tube-averaged temperature and the peak overpressure show an overall increasing trend. This increase in overpressure is most pronounced at equivalence ratios of 1.0–1.2. This study identifies hazardous parameter ranges in oxygen-enriched NH3 combustion that are prone to producing strong tulip flames and high overpressure, providing useful guidance for explosion risk assessment and safety-oriented design of NH3-fueled combustion systems.

Flame synthesis using liquefied petroleum gas (LPG) as the precursor gas to produce carbon nanofibers (CNFs) is an economical alternative to conventional chemical vapor deposition methods using single-component fuels such as methane and ethylene. Though LPG is a commercially viable source for carbon-based nanomaterials, the understanding of the effects of a LPG flame on CNF growth is very limited. Therefore, the present study is to analyze the feasibility of CNF growth in a premixed LPG flame using a one-dimensional flame at varying equivalence ratios. The effects of flame equivalence ratio on the CNF morphology and crystallinity are then analyzed systematically. In the present study, a diffusion flame was used to check the stability of the flame at different flow rates, followed by establishing a premixed flat flame of LPG. An optimum height above burner of 10 mm at which the temperature is around 650 °C was used in the synthesis process. Zirconia beads impregnated with nickel nitrate catalyst have been employed. Dense CNF growth with an average diameter of 77.9 nm was observed at an equivalence ratio of 1.8; as the equivalence ratio was reduced to 1.6, the average diameter of CNF increased by 46% to 114 nm, with amorphous carbon observed. The said observation is due to the effects of the increased flame temperature as the equivalence ratio approaches stoichiometry conditions from the rich side. This increases the nucleation rate, which in turn increases the catalyst particle size and the amount of free carbon atoms, producing CNFs with larger diameters and amorphous carbon. According to Raman analysis, the grown CNFs have a high number of defects, which may be good for applications where defective nanomaterials are desirable to improve the component performance. The work has proven that flame synthesis of CNFs using commercial LPG is feasible, paving the way for further exploration into cost-efficient CNF production with potential industrial applications.

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Flame stability is crucial for many industrial and aviation applications. Flow parameters, notably Reynolds number and velocity, significantly affect combustor power and flame characteristics. Thus, the influence of the Reynolds number ratio (RR = Reout/Rein) from 1 to 2.5 was investigated for CH4 and CH4/H2 fuel in a dual annular jet burner. Twenty percent of the fuel volume for CH4/H2 combustion was hydrogen. For each RR, the global equivalence ratio varied from stoichiometric to lean blowoff (φg = 0.48), while power ranged from 2.9 to 17.8 kW. The static stability analysis indicates that increasing the RR decreases the lean blowoff limit by approximately 2%. Reynolds number ratios also change the flame macrostructure transitioning from M-shaped flame to V-shaped flame for methane combustion at stoichiometric conditions. At RR = 1, three shear layers are anchored at the central bluff body and inner annulus in the M-shaped flame. However, at higher RR, only the center shear layer is anchored on the bluff body forming V-shaped flame. For CH4/H2 combustion at stoichiometric conditions, the flame shape forms the M shape consistently with the increased flame front for RR of 1–2.5. Additionally, emission analysis showed that higher RR produced more thermal NOx at higher combustor power, while CO emission was found to be highest at RR = 1.5 for methane combustion. For hydrogen-blended methane combustion, the emission of CO and NOx were found to be decreased compared to methane combustion.

Spark ignition (SI) and subsequent flame front development exert a significant influence on cyclic variability of internal combustion engines (ICEs). The increasing exploitation of lean air-fuel mixtures in SI engines to lower fuel consumption and CO2 emissions is driving the scientific community towards the search for innovative combustion strategies. Moreover, although lean combustion has been widely investigated and an important number of studies is already present in literature, the high cyclic variability typical of this combustion process still represents a major hinder to its exploitation. This study aims to investigate the effects of increasing ignition energy on combustion characteristics of lean mixtures. Tests were performed on an optically accessible gasoline direct injection (GDI) engine that allowed to investigate the correlation between the thermodynamic results and spark arc-flame morphology. Engine speed was fixed at 2000 rpm, a relative air fuel ratio (AFRrel) of about 1.3 was selected and ignition timing was set at 12 crank angle degrees (CAD) bTDC. Coil charge duration was swept from 10 to 40 CAD. Two intake pressure levels were investigated, the first corresponding to wide open throttle under naturally aspirated operating mode, the second with an intake pressure of 1.2 bar, thus corresponding to a boosted operating condition. Two dedicated scripts built using NI Vision were employed for image processing, allowing the evaluation of temporal and spatial evolution of the early stages of combustion. Arc elongation and flame front contour were used as correlation parameters that characterize flame kernel inception and development. The results confirm that, as expected, the increase of the coil charge duration tends to reduce cyclic variability in terms of engine output. The optical investigations revealed that for both examined cases the standard deviation related to the wrinkling effect on flame edge at CA5 decreased as the coil charge duration increased.

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Pure hydrogen combustion is a critical pathway to achieving zero-carbon emissions for the gas turbine industry. Micro-mixing combustion is one of the most widely attractive hydrogen combustion methods in gas turbines. This study investigates pure hydrogen flame in a 3 × 3 matrix micro-mix combustor. The setup includes nine micro-mix injectors, each equipped with a bluff body and a hydrogen injection tube. The OH* chemiluminescence imaging and PIV (Particle Image Velocimetry) techniques were employed to visualize the single- and triple-flame morphology and flow field under various operating conditions. The results show that equivalence ratio, flow rate, and air injector exit angle can influence the flame structure and combustion characteristics, providing an insightful understanding of micro-mix pure hydrogen combustion.

This study investigates the effects of hydrogen addition on the laminar flame speed of natural gas using computational simulations performed with Cantera. The calculated laminar flame speed of hydrogen-enriched natural gas mixtures was simulated across a range of equivalence ratios (0.6-2) and hydrogen fractions (0-25% by volume). The modified GRI-Mech 3.0 mechanism was employed. Results demonstrate a significant increase in laminar flame speed with increasing hydrogen content, with the effect more pronounced at higher equivalence ratios. The peak flame speed was observed in stoichiometric conditions with increasing hydrogen fraction. The highest speed is achieved with a 25% hydrogen concentration in a natural gas mixture, reaching 45.219 cm/s at an equivalence ratio of 0.9. This study provides insights into the combustion characteristics of hydrogen-enriched natural gas and demonstrates the utility of Cantera for such simulations.

ABSTRACT The combustion dynamics of methane-oxygen inverse diffusion flames stabilized over a swirl coaxial injector are investigated in detail. Mapping of different regimes of flame dynamics was carried out by classifying the flame into stable flames, oscillating flames, and unstable flames. The dominant frequency of the acoustics associated with different flame characteristics was determined from a free-field microphone measurement of combustion noise, and the respective flame morphology was studied by high-speed imaging of OH* chemiluminescence. Experiments were conducted for different oxidizer jet Reynolds numbers corresponding to each of the three different fuel jet Reynolds numbers. For all three power levels, flame transition from stable to unstable was observed as the oxidizer Reynolds number and equivalence ratio increased from fuel-rich to oxidizer-rich conditions. Strong acoustic-combustion heat release rate coupling was observed in the unstable flames. Modal decomposition techniques were employed to study the spatio-temporal evolution of the flame. The energetic coherent modes corresponding to the frequency of oscillation reveal two different modes of flame oscillation: longitudinal and transverse. All three fuel Reynolds numbers under investigation showed a predominant longitudinal mode of oscillation, while only the highest fuel flow conditions, corresponding to a fuel jet Reynolds number of 765 showed a transverse mode flame oscillation, as the oxidizer Reynolds number increased above 8000.

Laminar flame speeds of methanol/air mixtures at 338–398 K are measured by the heat flux method, extending the range of equivalence ratio up to 2.1. And a new optimized methanol mechanism with 94 reactions is proposed by using the particle swarm algorithm, adjusting 20 Arrhenius pre‐exponential factors in their uncertainty domains. The optimized model is compared with eight methanol combustion mechanisms and experimental data published in recent years, covering a wide range of initial temperatures (298–1537 K), pressures (0.04–50 atm) and equivalence ratios (0.5–2.1). The results show that the optimized mechanism not only improves the accuracy of ignition delay time with rapid compression machine at low temperature but also moderately improve the description of laminar flame speed in lean and stoichiometric conditions. Meanwhile, the optimized model significantly enhances the prediction accuracy of CH3 and CH2O radical, and perfectly captures the evolution trend of HCO radical in laminar flat flame. Overall, the optimized mechanism provides the best overall description of the currently available measurements, leading to more accurate and comprehensive prediction of ignition delay time, laminar flame speed and species concentration.

This article presents simulations of a turbulent lifted flame using the large eddy simulation-transport probability density function-discretized population balance equation approach. This approach takes into account the interaction between turbulent reacting flow and soot particle formation. A reduced chemical kinetics mechanism including a series of polycyclic aromatic hydrocarbons (PAHs) species linked to soot formation is generated employing the approach of the directed relation graph error propagation and is tested on a perfectly stirred reactor under varying equivalent ratio conditions and premixed flames. The soot kinetics model includes the PAH-based nucleation and surface condensation, the hydrogen abstraction acetylene addition surface growth and oxidation mechanism, and the size-dependent aggregation. The soot morphology considers the surface area and other geometrical properties for both spherical primary particles and fractal aggregates. The simulation results show, in general, reasonably good agreement with experimental measurements in terms of lifted height, flame shape, flow-field velocity, the hydroxyl radical, and soot volume fraction. A discussion of micromixing and its modeling in the context of the Interaction by Exchange with the Mean model is also presented. To investigate the effect of the soot micromixing frequency factor on soot particles, an additional simulation is conducted where this factor is reduced by a factor of 10 for the soot particles. The maximum soot volume fraction is observed to increase slightly. However, compared with the impact of kinetics on soot modeling, this effect is a minor one.

In combustion systems, determining the flammability limits of the fuel–air mixture to be used, or establishing the conditions that enable combustion within the desired limits, is of critical importance. By doing so, instabilities that may occur in the system can be minimized, and operational safety can be ensured. In the present study, the flammability limits and instantaneous flame images of premixed burners were experimentally investigated as a function of the amount of H₂ added to pure LPG. The H₂ content was increased up to 80%, while the swirl number and thermal power were kept constant at 1 and 4 kW, respectively, under all experimental conditions. Instantaneous flame images were taken at different equivalence ratios at 0.1 intervals below stoichiometric conditions and at 0.2 intervals above stoichiometric conditions. Flame brightness and RGB (red-green-blue) values were evaluated quantitatively in detail by image processing with HSV and grayscale detection methods using MATLAB. For 100% LPG, the stable flammability limits are ϕ=0.8–2.8, while with hydrogen enrichment the lower flammability limit expands to ϕ=0.4, and the upper flammability limit decreases to ϕ=2.0 (for 20% LPG–80% H₂). The results revealed that hydrogen addition significantly affects the LPG flame behavior, alters the stable flammability limits, and increases flame brightness up to ϕ=1.6.

We present a combined experimental and modeling study of premixed atmospheric-pressure tetralin flames. Chemical speciation in near-stoichiometric (φ = 0.8–1.0) tetralin/O2/Ar flames was characterized by probe-sampling molecular-beam mass spectrometry (MBMS) with soft ionization (12.3–18 eV). Total ionization cross-sections (TICSs) for heavy intermediates were computed ab initio to enable quantitative MBMS processing. Laminar burning velocities (LBVs) of tetralin/air flames were measured in a range of equivalence ratios (φ = 0.75–1.5) on a nozzle burner via the stretch-corrected total area method. This is the first reported LBV data for tetralin/air flames (maximum LBV was 47.3 ± 2 cm/s at φ = 1.1). The experimental mole fraction profiles and LBVs were interpreted using three detailed mechanisms. None of the mechanisms were able to correctly describe the LBV profile, and a number of discrepancies were observed in the mole fraction profiles. Reaction network and sensitivity analyses were performed to identify specific sub-mechanisms requiring refinement. In particular, the subchemistry of naphthalene and indene strongly affects the accuracy of model predictions, whereas the flame speciation data indicate large uncertainties in the simulated concentrations of these species.

A reliable combustion monitoring system is essential to satisfy global carbon neutrality trends. As the concentrations of emissions and flame stability are associated with the air–fuel ratio, the equivalence ratio should be continuously evaluated. In this study, a deep neural network- (DNN-) based regression model is proposed to predict the equivalence ratio of turbulent diffusion flames. Chemiluminescence signals from the OH ∗ , CH ∗ , and C2 ∗ radicals were acquired as input features. In addition, three different optical sensing views were applied to consider the future general measurement conditions. Furthermore, a loss function comparison for model training and hyperparameter tuning techniques, such as random search and Bayesian optimization, were used to improve the prediction performance. Consequently, the enhanced DNN model showed reductions in the mean absolute error and root mean square error of ~17.84% and ~12.06%, respectively, compared with the initial model. In addition, a mean absolute percentage error and R -squared value of ~3.61% and ~0.9311, respectively, were obtained. Thus, a novel sensing method has been proposed for flame monitoring systems to realize future digital transformations in the combustion industry.

ABSTRACT The C2*/CH* model is often used to measure the equivalence ratio of combustion. But only the band of C2*(0,0) at 516.5 nm of the C2* swan band system was employed as the input of the C2*/CH* model for equivalence ratio measurement. The relations of other components of the C2 swan band system with flame equivalence ratios and their impact on this equivalence ratio measurement are unreported. In this study, the visible emission spectrum of premixed methane flame, ranging from 425 to 700 nm, was measured/obtained by a hyperspectral imaging system under equivalence ratios from 0.72 to 1.52. The influence of the C2* swan bands on the C2*/CH* model was analyzed, and the intrinsic characteristics of the C2* swan bands were found. After that, a more sensitive soft measurement model for equivalence ratio measurement based on the ratio of C2*(0,0)/C2*(2,0) was proposed. The monotonicity and stability of the proposed model were discussed. At last, the proposed model was further validated by the random forest (RF) algorithm.

This study investigates the combustion characteristics of hydrogen-enriched low-calorific landfill gas (LFG) in a double-layer porous media burner by using numerical simulations. The research addresses challenges related to flame instability and pollutant emissions during low-calorific LFG combustion. A two-dimensional axisymmetric numerical model was developed in ANSYS Fluent, incorporating a skeletal chemical reaction mechanism and the standard k-ε turbulence model. Simulations were performed with LFG composed of 30% methane and 70% carbon dioxide (LFG30) under varying hydrogen blending ratios (0 to 20%), an equivalence ratio of 1.5, and an inlet gas velocity of 0.15 m/s. The results demonstrate that increasing hydrogen concentrations shifts the flame upstream, lowers both combustion and exhaust gas temperatures, and significantly decreases CO and NO x emissions. When the hydrogen blending ratio reaches 20%, the mole fractions of CO and NO x at the outlet are reduced by 22.14 and 72.65%, respectively, compared with the pure LFG30. The findings indicate that hydrogen enrichment significantly enhances the combustion stability and emission performance of low-calorific LFG in porous media burners, providing an effective approach for efficiently utilizing low-calorific-value fuels even at extreme operating conditions. This study offers novel insights toward the development of effective burners aimed at increasing the utilization of this underutilized renewable energy resource and addressing environmental concerns.

The stability, combustion, and emission features of stratified oxy-methane (CH4/O2/CO2) flames stabilized over a dual annular counter-rotating swirl (DACRS) burner, developed for gas turbine combustion applications, were investigated experimentally. The experiments were performed at fixed velocity ratio (Vr=Vp/VS=3.0) in both the primary and secondary streams at a constant primary stream velocity, Vp of 5 m/s and at fixed primary stream equivalence ratio, φP=0.9, and over ranges of oxygen fractions (OFP for the primary stream, OFS for the secondary stream) and secondary stream equivalence ratios. Measurements of flame macrostructure, temperature profiles, and exhaust emissions were recorded to characterize the flames and validate future numerical models. The testing findings revealed no flame flashback within the operational ranges of OFP and OFS and up to φs = 1.0. However, the near stoichiometric operation of the primary stream (φp = 0.9) at OFp =0.38, permitted the main secondary flame to tolerate exceptionally lean conditions (φs = 0.397 at OFs =0.34 and φs = 0.223 at OFs =0.39), raising the thresholds for flame blowout. Increasing OFP from 0.21 to 0.38 significantly reduced φS at blowout from 0.537 to 0.223, corresponding to a decrease in the combustor's global equivalence ratio (φg) at blowout from 0.554 to 0.254 at global oxygen fraction (OFg) from 0.38 to 0.39. Lower OFp values caused earlier flame lift-off, indicating the greater influence of OFp on flame macrostructures.

Combustion oscillation is a very common phenomena in many unsteady or transient HVAC applications (e.g., start-up operation). Combustion driven oscillations can occur once the sound (thermo-acoustically driven) is reflected from the combustion chamber back into the mixture supply region (and propagate even further upstream). The reflected sound causes a fluctuation in inlet mixture composition or mixture flow. In either cases, the equivalence ratio is changed and consequently the amount of heat release changes. This phenomenon could easily trap the combustion process in a loop that can easily result in combustion oscillations with higher amplitude. It can also produce unwanted transient noise tones and can raise NOx emission levels. The current work is aimed at providing a simulation model for diagnosing and preventing combustion driven oscillation using a modified positive feedback loop. This feedback loop is developed using upstream/downstream impedances (obtained from low-order acoustic simulations), and a flame transfer function. The value-add of this study (model-based approach) is that it can help design engineers to reduce the number of test iterations and optimize the design.

Ammonia is a promising energy carrier for energy system decarbonization, although several drawbacks affect its combustion process. Coupling moderate or intense low-oxygen dilution (MILD) combustion with the use of high reactivity fuels allows to improve NH3 combustion. In particular, H2 addition may be a feasible strategy, considering the high proportion of H2 achievable by NH3 partial cracking. The present study focuses on MILD combustion effectiveness in ensuring high stability and low-NO x emissions for NH3/H2 blends. Influence of both equivalence ratio and H2 addition was experimentally investigated in a cyclonic reactor. Furthermore, the results were directly compared with those obtained with cracked NH3 mixtures (NH3/H2/N2). Results for NH3/H2 blends strengthen the fuel flexibility of the cyclonic reactor, which allows total conversion of the fuel mixtures by ensuring operating temperatures always lower than 1400 K, independently of the equivalence ratio and the fuel blend composition. In particular, H2 addition increases NH3 reactivity, whereas increasing NO x emissions with respect to pure ammonia. Instead, for pure H2 and pure NH3, they always stay lower than 40 and 100 ppm, respectively. For cracked NH3 mixtures, the fuel dilution content by N2 does not affect the NH3/H2 combustion behavior under MILD conditions. Instead, for 100% NH3 cracking (75%H2-25%N2 mixture), H2 dilution by N2 entails a more uniform reaction zone than not diluted H2 case, further limiting NO x formation by avoiding the occurrence of hot-spot regions within the reactor.

This study investigated the effects of equivalence ratio and central bluff body on the instability of premixed swirl propane/air flame. First, a low-order thermoacoustic network model based on the n−τ model was employed for a premixed swirl combustor, and several potential longitudinal modes were predicted within the system. Subsequently, self-excited oscillation experiments revealed mode switching phenomena between two longitudinal modes (f1 and f2). Moreover, further analysis demonstrated that this mode transition was related to both the equivalence ratio of premixed gas and the characteristics of the swirler. The central bluff body in the swirler was found to modify the gain and time delay of flame transfer function, thereby influencing the stability of thermoacoustic modes of the combustor. Finally, thermoacoustic mode analysis incorporating experimentally measured flame transfer functions showed a good agreement between the model predictions as well as the thermoacoustic oscillation experimental results.

This study demonstrates that hydrogen enrichment in lean-burn spark-ignition engines can simultaneously improve three key performance metrics, thermal efficiency, combustion stability, and nitrogen oxide emissions, without requiring modifications to the engine hardware or ignition timing. This finding offers a novel control approach to a well-documented trade-off in existing research, where typically only two of these factors are improved at the expense of the third. Unlike previous studies, the present work achieves simultaneous improvement of all three metrics without hardware modification or ignition timing adjustment, relying solely on the optimization of the air–fuel equivalence ratio λ. Experiments were conducted on a six-cylinder engine for combined heat and power application, fueled with hydrogen–natural gas blends containing up to 30% hydrogen by volume. By optimizing only the air–fuel equivalence ratio, it was possible to extend the lean-burn limit from λ≈1.6 to λ>1.9, reduce nitrogen oxide emissions by up to 70%, enhance thermal efficiency by up to 2.2 percentage points, and significantly improve combustion stability, reducing cycle-by-cycle variationsfrom 2.1% to 0.7%. A defined λ window was identified in which all three key performance indicators simultaneously meet or exceed the natural gas baseline. Within this window, balanced improvements in nitrogen oxide emissions, efficiency, and stability are achievable, although the individual maxima occur at different operating points. Cylinder pressure analysis confirmed that combustion dynamics can be realigned with original equipment manufacturer characteristics via mixture leaning alone, mitigating hydrogen-induced pressure increases to just 11% above the natural gas baseline. These results position hydrogen as a performance booster for natural gas engines in stationary applications, enabling cleaner, more efficient, and smoother operation without added system complexity. The key result is the identification of a λ window that enables simultaneous optimization of nitrogen oxide emissions, efficiency, and combustion stability using only mixture control.

Cycle-to-cycle variations in spark ignition (SI) internal combustion engines (ICE) fueled with hydrogen (H 2 ) were investigated in the context of enhancing combustion stability. In the first part, the effects of boost pressure, exhaust gas recirculation (EGR), spark advance (SA), and lambda on cycle-to-cycle variations based on Matekunas plots were investigated. In the second part, the integral of the coefficient of variation of pressure (COV P ) curve (ICOV) by varying SA, equivalence ratio (ER), and volumetric efficiency (VE) were examined and their impacts were evaluated. The experimental data were acquired through a test campaign performed on a single-cylinder PFI-SI ICE for 100 consecutive cycles. The Matekunas plots showed that as combustion stability increases, cycle-to-cycle variations decrease, and vice versa. Additionally, a decrease in cycle-to-cycle variations is accompanied by a decrease in maximum pressure ( P max ) values. The increase in the maximum value of COV P results in the increase of the ICOV with a relatively strong linear relationship demonstrated by correlation coefficient (CC) of 0.62 between ICOV and COV P . Therefore, it can be concluded that if the COV P is higher, it results in high cycle-to-cycle variations. The higher values of P max lead to high COV P with a strong linear correlation coefficient (CC) value of 0.78 between P max and COV P . As SA and VE increase, and ER decreases, cycle-to-cycle variations during combustion duration increase, concluding less repetitive combustion.

Micro-mixing combustion is promising because of its non-premix like combustion stability while having the potential to achieve NOx emissions near premix levels. However, based on our previous work, the heavier the hydrocarbons, the higher the thermal NOx because of the increase of the mixing timescale. Consequently, there is a need to achieve faster mixing with heavy hydrocarbons to allow their pairing with micro-mixing combustion. This paper presents an experimental proof-of-concept of a combustor architecture composed of a micro-mixing injector coupled with a pre-reactor operating under ultra-rich conditions. Air is introduced in the fuel at high equivalence ratio, between 15 and 25, and the reaction is activated with the heat of the main combustion. The thermally coupled reactor forms lighter species from heavy hydrocarbons through partial oxidation of the fuel, balancing the effects of heavier fuels and decreasing mixing timescale. Experimental tests with propane at atmospheric pressure, and at adiabatic flame temperatures between ~1500 K and ~2000 K, were performed to demonstrate the concept. NOx emissions were reduced by a factor up to 3 with this combustion architecture compared to baseline tests, achieving near premix NOx emissions. Increasing the air injected in the reactor decreased further the NOx emissions. Additionally, tuning of the combustion stability was possible by adjusting the reactor parameters.

This study systematically investigates the combustion characteristics of co-firing Coke Oven Gas (COG) and ammonia (NH 3 ), a promising low-carbon fuel blend for decarbonising the steel industry. Experiments are conducted using a 10 kW tangential swirl burner, varying the ammonia fraction (X NH3 ) and equivalence ratio ( Φ ). Results demonstrate a significant synergistic effect, where blending expands the flame stability range; ammonia addition suppresses flashback from the high-hydrogen COG, while COG enhances the reactivity of ammonia. The widest stability range is achieved at X NH3 = 0.2. An analysis of exhaust gas emissions reveals that increasing X NH3 not only suppresses the peak NO concentration but also shifts the equivalence ratio at which NO concentration is negligible on the fuel-rich side closer to the stoichiometric condition. Furthermore, a Chemical Reactor Network (CRN) analysis identifies that the HNO + OH ↔ NO + H 2 O reaction, promoted by OH radicals from COG, is a crucial NO formation pathway, a novel finding for this fuel blend. These fundamental data contribute to advancing the practical application of COG/NH 3 co-firing.

An investigation into the non-premixed combustion characteristics of methane in a planar micro-combustor with a splitter was performed. The impact of blending methane with hydrogen on these characteristics was also analyzed. Additionally, the effects of inlet velocity and global equivalence ratio on flame location, flame temperature, combustion efficiency and outer wall temperature were studied for three different fuel compositions: pure methane (MH0), 60% methane with 40% hydrogen (MH40), and 40% methane with 60% hydrogen (MH60)). A heat recirculation analysis of the combustor wall was conducted to determine the amount of heat recirculated into the unburnt gas at various inlet velocities for all three fuel compositions. The results demonstrated that the stability limit of methane in terms of inlet velocity (1–2 m/s) and global equivalence ratio (1.0–1.2) was significantly enhanced to 1–3 m/s and 0.8–1.2, respectively, with the addition of hydrogen. At an inlet velocity of 2 m/s, the flame location of 3.6 mm for MH0 was significantly improved to 2.2 mm for MH60. Additionally, outer wall temperature exhibited a rise of 100 K for MH60 compared to MH0. Furthermore, from heat recirculation analysis, when the ratio of heat recirculated to heat loss exceeded unity, the flame started exhibiting the lift-off phenomenon for all the fuel compositions.

Combustion modes of kerosene spray in a scramjet combustor condition with different injection schemes are experimentally investigated at Mach 2.52. The study is based on two single injectors with nozzle diameters of 0.79 and 1.14 mm and two dual injectors with nozzle diameters of 0.56 and 0.72 mm, respectively. The results show that the weak combustion mode has little effect on the flow field, while the intensive combustion mode has the opposite effect. The dual injector can promote evaporation and mixing of the kerosene spray. Compared with the dual injector, intensive combustion cannot occur when a single injector is used, and the flame stability range is also narrower. As the nozzle diameter of the injector increases, the distribution and oscillation of kerosene spray change significantly, transition from the weak to intensive combustion mode occurs at a higher equivalence ratio, and the flame stability range increases. However, change in the nozzle diameter does not affect the overall process of combustion mode transition. For the single injector, intensive combustion still cannot occur when the nozzle diameter changes. In addition, change in the nozzle diameter has little effect on combustion heat release when the combustion mode remains unchanged.

Microchannel burners suffer from low combustion efficiency and poor stability in applications. In order to explore the effect of wall reaction on methane/air premixed combustion performances in the microchannel, the effects of wall activity, inlet velocity, pressure, and equivalence ratio on the temperature and radical distribution characteristics were studied by CFD computational simulations. It is found that as the reaction pressure increases, there are more free-radical collisions, causing the reaction temperature to rise. The OH radicals participate in the reaction at the active near wall so that the mass fraction of the OH radical on the active wall is lower than that on the inert wall. As the equivalence ratio increases from 0.6 to 1.2, the high-temperature regions increase but the maximum temperature decreases. The mass fraction of OH radical increases with the increase of the equivalence ratio, and the increase of OH radical near the inert wall is larger than that of the active wall. As the flow rate increases, the disturbance increases, and the combustion reaction becomes more intense, resulting in an increase in the temperature and the mass fraction of OH radicals. The mass fraction of H, O, OH, and CH3 radicals in the inert wall was slightly higher than that in the active wall, in which the peak mass fraction of CH3 radical appeared at the axial position closest to the entrance, while the other three radicals reached the peak at about the same axial position. This study provides a reference for combustion stability in microcombustors.

ABSTRACT Porous media combustion (PMC) has made a comeback as a practical technology for the utilization of low-concentration methane (LCM) from coal mining. However, the conventional direct-fired PM burner still suffers from the rampart of flame stability and combustion efficiency not addressing industrial demands. Herein, a four-layer gradually-varied porous burner was innovatively developed for LCM combustion to investigate combustion performance under lean combustion conditions (CH4 volume fraction below 5%). The methane conversion, pollutant emissions, and flue gas temperature were also evaluated in detail. The results indicated that the burner offered a favorable combustion resistance due to the gradually-varied PM arrangement to heighten combustion stability with subtle temperature fluctuation and flame migration. The flammability limit of LCM was extended to the lowest equivalence ratio of 0.43 with stationary combustion at 240°C for 120 min. The energy efficiencies of the LCM combustion under lean combustion conditions were greatly boosted with the highest combustion and thermal efficiencies attained at 99.52% and 70.05%, respectively. The maximum 99.93% CH4 conversion was acquired at an equivalence ratio of 0.45 and a flow rate of 80 L/min. LCM combustion in the burner achieved extremely low pollutant emission levels and the overall CO and NOx emissions were 58.04 ppm and below 23 ppm respectively under the experimental conditions. In addition, the high-quality flue gas with an average temperature of more than 516°C was detected in the operating process, which allowed the available heat utilization at the coal mine scenes or in other industries.

Porous media burners offer significant advantages due to their high efficiency and low emissions. Nevertheless, a substantial challenge in the field of porous media burner research pertains to the precise control of crucial operating parameters, with the objective of further reducing pollutant emissions while maintaining combustion stability. The establishment of a numerical model of a cylindrical porous media burner enabled the simulation of the premixed combustion process, with a focus on the analysis of the effects of equivalence ratio, inlet velocity, and temperature on combustion characteristics and pollutant emissions. The findings of the study demonstrate that augmenting the equivalence ratio and inlet velocity results in elevated flame temperature and heightened NO and CO emissions. Conversely, an increase in inlet temperature has been shown to enhance both flame and outlet temperatures while concomitantly reducing pollutant emissions. The system reveals the variation patterns of combustion stability and emission characteristics across various operating conditions, with the findings offering theoretical support for optimizing burner design and operational control. This finding is of considerable significance for the achievement of highly efficient, low-pollution combustion.

A vortex flows in micro/meso scale combustors for small-scale power generation play a crucial role in enhancing combustion efficiency and stability. They enhance mixing between fuel and air, promoting better combustion and help stabilize flames by maintaining consistent fuel-air ratios. The temperature are significantly impacts the reactant temperature due to heat conduction wall in Cylindrical Vortex Combustor (CVC). This phenomenon, known as preheating, occurs as the wall transfers heat to the reactants. ANSYS Fluent software is used for conducted a numerical investigation on a CVC. The combustor was characterized by a prescribed mass flow rate of 40 mg/s and an equivalence ratio (j) ranging from 0.5 to 1.5. Our analysis aimed to understand the combustion behavior within this confined geometry, considering factors such as heat loss and temperature behavior. The numerical findings indicate that elevated equivalence ratios correlate with the highest flame temperature in micro-combustion. Specifically, at an equivalence ratio of j=0.5, the flame temperature remains consistently low compared to the higher value of j=1.5. However, when accounting for wall temperature effects, the maximum flame temperature occurs at an equivalence ratio of j=1.3. The heat dissipation region is quite limited, especially at low equivalence ratio. In summary, heat transfer in cylindrical vortex combustors (CVC) contribute to reliable and efficient power generation, making them essential for portable energy systems.

The combustion characteristics in two geometrically similar kerosene-fueled scramjet combustors with mass flow rates of 0.69 and 1.41 kg/s are experimentally investigated to explore the scale effects of flame stabilization at Mach 2.52 condition. As the equivalence ratio increases, the combustion usually changes from weak to intensive to blow-out mode. The weak combustion has little effect on the flow field, whereas the intensive combustion has the opposite effect. The transition combustion tends to occur between different modes. When the single injector is used, compared with the small-scale combustor, intensive combustion cannot occur in the large-scale combustor, and the flame stability range is also narrower. One probable reason is that as the combustor scale increases, the boundary layer becomes relatively thinner, resulting in a smaller low-velocity zone and a faster mainstream velocity at the downstream wall of the cavity, which is not conducive to the flame propagation upstream to form the intensive combustion. After shortening the isolator, all cases with intensive combustion in the small-scale combustor are transformed into weak combustion, further confirming the speculation. Compared to the single injector, the dual injector is required in the large-scale combustor to achieve intensive combustion and a wider flame stability range.

ABSTRACT Combustion in a porous medium burner is an effective technology that deals with low heat capacity fuels, producing low emissions of pollutant gases. Experimental investigations of filtration combustion were carried out on a laboratory-scale prototype developed by our research group. This prototype is outfitted with a novel spark ignition system attached in the middle of the reactor, which does not change the porous medium properties because it works as a flamethrower. The ignition system produces a flame front into the porous matrix that quickly reaches the operational combustion temperature of 1200 K. By applying pure methane and biogas with different CO2 contents (15% − 40%) for combustion processes, the influence of equivalence ratio, gas flow velocity, energy extraction efficiency and NOx emissions was evaluated. Experimental results such as temperature profiles, reaction stability and flammability limits also have been evaluated. According to the operational parameters tested in this study, this prototype setup presented a low time-consuming process to reach the temperature of combustion, almost ten times faster than those with electrical resistance. Furthermore, the prototype was capable of operating at an equivalence ratio of 0.4 and gas flow velocity of 0.2 m/s, maintaining high CO2 content in the fuel, to reach NOx emission values lower than 1.0 ppm.

No abstract available

Self‐excited thermoacoustic instability (SETAI) is a dangerous phenomenon in combustion equipment. While it is widely acknowledged that SETAI behavior is determined by the couple between pressure and heat release oscillation, their phase difference is difficult to predict, which impedes the development of SETAI control technology. With the aim of passive control technology development, this paper conducted experiment on a premixed hedge combustor to explore the mechanism whereby premixed chamber length (LP) and equivalence ratio (φ) collaboratively influence SETAI behavior. Results showed LP mainly affects the pressure mode shape within premixed chamber and consequently alters the phase difference between pressure and flowrate oscillation at combustion chamber inlet. Changing φ gives rise to different reaction time‐lag (τ), thus altering the phase difference between flowrate and reaction heat release oscillation. By introducing this flowrate oscillation, how LP and φ collaboratively determine phase difference between pressure oscillation and heat release oscillation was clarified. The mechanisms identified in this study are consistent with the emerging rationalization of the factors contributing to SETAI, and also provides better understanding on Rayleigh criterion and guidance for SETAI control. With further work on heat release and flow rate measurement, as well as the development on τ description, SETAI can be better predicted and controlled.

Gas turbine operation increasingly relies on lean premixed (LPM) combustion to reduce harmful emissions, which is susceptible to thermoacoustic instabilities. Most combustion systems are technically premixed and exhibit a degree of equivalence ratio inhomogeneity. Thermoacoustic pressure oscillations can couple with the heat release oscillations through the generation of equivalence ratio fluctuations at fuel injection sites, which are then convected to the flame front. Previous experimental studies have shown that porous inert media (PIM) can passively mitigate these instabilities by adding acoustic damping and by reducing the thermoacoustic feedback mechanism. To understand the role of PIM on these equivalence ratio oscillations, spatially resolved, phased averaged equivalence ratio fluctuations are measured using the ratio of OH*/CH* chemiluminescence. Spatial imaging of OH* or CH* radicals produce integrated line of sight intensity values and an Abel transformation is used to obtain spatially resolved values. Phase averaged images are synced with dynamic pressure measurements, and an axisymmetric atmospheric burner is used to study the effects of ring-shaped PIM on the spatially resolved equivalence ratio field with self-excited thermoacoustic instabilities. The results show that PIM significantly reduces these fluctuations, and the effects on the stability of the system are discussed.

The main focus of this paper is to discover the link between flame macrostructure and thermoacoustic instability in a centrally staged swirl burner. In practical combustors, the flow rate in the pilot stage is much smaller than that in the main stage. However, the modification in the pilot stage could alter the flame macrostructure while maintaining a similar total flow rate. Therefore, the thermoacoustic instability was examined at different flame macrostructures by varying the pilot stage equivalence ratio under identical main stage inlet conditions. High-frequency planar laser measurements and chemiluminescence measurement were conducted to enhance spatial and temporal accuracy, providing a more comprehensive understanding of thermoacoustic instability. Two different flame macrostructures, S-type and I-type flames, were identified based on the preheating zone distribution. They exhibit distinct thermoacoustic instabilities, with the I-type flames demonstrating more intense instability than S-type flames. The results indicate that the variation of flame macrostructure influences the coupling of flame heat release and flow field. Specifically, the preheating zone and heat release of I-type flames exhibit greater sensitivity to flow field fluctuations, resulting in a more intense and complex fluctuation of the flame. This discrepancy leads to variations in thermoacoustic instability intensity, as well as the changes in the phase coupling between heat release and acoustic pressure, which in turn impact the total Rayleigh index. Meanwhile, significant differences exist in the distribution pattern and range of flow field fluctuations between I-type and S-type flames.

With the growing emphasis on developing clean fuels to reduce emissions, hydrogen has become a critical focus due to its potential for lowering environmental impact. However, the operational challenges in lean fuel systems, particularly combustion instability, highlight the necessity of parametric studies to ensure stable performance with alternative fuels. This study numerically examines the thermoacoustic instability of pure hydrogen and methane in a Rijke tube combustor using the Unsteady Reynolds-Averaged Navier-Stokes method. The investigation focuses on two distinct instability behaviors: beating and limit cycle oscillations. Key findings reveal that for both fuels, increasing the equivalence ratio toward richer mixtures tends to damp thermoacoustic instabilities. It was observed that a 20% increase in fuel flow rate caused a transition from beating to limit cycle oscillations. A comparative analysis shows that lean methane flames produce stronger fluctuations, whereas hydrogen has a wider instability range under fuel-rich conditions. Advanced modal analysis using Dynamic Mode Decomposition identifies the dominant longitudinal acoustic modes and localized oscillations near the fuel nozzle that influence flame structure. These results provide critical insights into the unique thermoacoustic characteristics of hydrogen and methane, which can inform the design of stable, next-generation combustion systems.

This paper presents an experimental study of thermoacoustic oscillations of synthetic gas (syngas) micromixed combustion, in which the excitation and evolution characteristics of thermoacoustic oscillations were obtained as a function of flame equivalence ratio. The results show that the flame initiates thermoacoustic oscillations when the equivalence ratio drops below 0.80, with the oscillations undergoing mode transferring as the flame equivalence ratio decreases. When the equivalence ratio is 0.70, the second-order mode of thermoacoustic oscillation begins to dominate. At an equivalence ratio of 0.50, the second-order oscillation mode (640 Hz) begins to dominate the first-order oscillation mode (320 Hz). Flame flashback coexists with thermoacoustic oscillation starting at an equivalence ratio of 0.5, where the chemiluminescence distribution of the flame indicates that the flame heat release rate pulsation is maximal at the burner outlet, and other radicals vary with the equivalence ratio. Low equivalence ratio promotes the generation of amino group radicals, thereby interfering with the production of nitrogen oxides. The flame shape also depends on the flame equivalence ratio. The flame is initially conical and flattens as the equivalence ratio decreases. The innovation of this study lies in its first exploration of the thermoacoustic oscillations and flashback characteristics of syngas micromixed combustion, which contributes to the formulation of design criteria for micromixed burners. These results clarify the evolution of syngas micromixed combustion and facilitates the prediction and control of flashback or thermoacoustic oscillations.

This study investigates the characteristics of thermoacoustic instability in an annular combustor fueled with ammonia-methane mixtures, with a particular focus on the impact of ammonia enrichment on longitudinal instability. The experiments revealed distinct transitions in combustion states as the equivalence ratio increased, following the sequence: lean blowout – stable combustion – intermittent oscillation – limit cycle oscillation. Ammonia enrichment was found to reduce oscillation amplitudes while increasing the lean blowout limit. Using integrated experimental diagnostics of acoustic pressure, OH* emissions, flame structure, and phase-averaged flame dynamics, the mechanisms underlying the effect of ammonia addition on thermoacoustic coupling were analysed. The instability maps identified at various equivalence ratio and ammonia content suggested that ammonia addition remarkably weakened the thermoacoustic coupling, which was attributed to the significant increase in convection delay time and ignition delay time by ammonia addition. This work highlights the role of ammonia in mitigating thermoacoustic instability and provides valuable insights for optimizing ammonia-methane combustion systems.

This paper presents the first numerical evidence of an intermittency route to period-2 thermoacoustic instability in a subcritical single-element liquid rocket engine burning hydrogen peroxide/kerosene as we decrease the equivalence ratio (ϕ) from fuel-rich to fuel-lean. To achieve this, three-dimensional compressible large eddy simulation algorithms combined with the Euler–Lagrangian framework are used. A one-equation eddy sub-grid turbulence model with a partially stirred reactor sub-grid combustion model is employed to simulate the spray turbulent combustion process in a high-pressure liquid-fueled combustor based on open-source platform OpenFOAM. This paper focuses on examining the transition process of the dynamical states in the thermoacoustic system and the synchronization between multiple subsystems. The results indicate that, as the equivalence ratio reduces continuously (1.5 ≤ ϕ ≤ 0.5), the system dynamics shift from period-1 oscillations (ϕ = 1.5) to period-2 oscillations (ϕ = 0.5) via intermittency (1.3 ≤ ϕ ≤ 0.9). Under the equivalence ratio of 0.7 (ϕ = 0.7), a transient mode switching between period-1 and period-2 was also observed. The synchronization processes between the pressure and combustion subsystems in terms of phase-locking and frequency-locking are responsible for the emergence of complex dynamical states. The cycle snapshots analysis also provides more details on the synchronization processes between the pressure and the multiple subsystems, such as vortex dynamics, mixture fraction, and combustion heat release. In summary, this paper sheds light on the complex non-linear thermoacoustic oscillations and the underlying physical mechanisms related to the two-phase flow of spray combustion in liquid rocket engines using three-dimensional large eddy simulations, paving the way for developing passive or active control methods.

This experimental study investigates the dynamical transition from stable operation to thermoacoustic instability in a turbulent bluff-body stabilised dump combustor. We conduct experiments to acquire acoustic pressure and local heat release rate fluctuations and use them to characterise this transition as we decrease the equivalence ratio towards a fuel-lean setting. More importantly, we observe a significant increase in local heat release rate fluctuations at critical locations well before thermoacoustic instability occurs. One of these critical locations is the stagnation zone in front of the bluff-body. By strategically positioning slots (perforations) on the bluff-body, we ensure the reduction of the growth of local heat release rate fluctuations at the stagnation zone near the bluff-body well before the onset of thermoacoustic instability. We show that this reduction in local heat release rate fluctuations inhibits the transition to thermoacoustic instability. We find that modified configurations of the bluff-body that do not quench the local heat release rate fluctuations at the stagnation zone result in the transition to thermoacoustic instability. We also reveal that an effective suppression strategy based on the growth of local heat release rate fluctuations requires an optimisation of the slots' area-ratio for a given bluff-body position. Further, the suppression strategy also depends on the spatial distribution of perforations on the bluff-body. Notably, an inappropriate distribution of the slots, which does not quench the local heat release rate fluctuations at the stagnation zone but creates new critical regions, may even result in a dramatic increase in the amplitudes of pressure oscillations.

An experimental study on a turbulent, swirl-stabilized backward facing step combustor is conducted to understand the spatiotemporal dynamics during the transition from combustion noise to thermoacoustic instability. By using a turbulence generator, we investigate the change in the spatiotemporal dynamics during this transition for added turbulence intensities. High-speed CH* images of the flame (representative of the field of local heat release rate fluctuations ( q · ' (x,y,t))) and simultaneous unsteady pressure fluctuations ( p ' (t)) are acquired for different equivalence ratios. In the study, without the turbulence generator, as the equivalence ratio is reduced from near stoichiometric values, we observe an emergence of coherence in the spatial dynamics during the occurrence of intermittency, enroute to thermoacoustic instability. As the turbulence intensity is increased using the turbulence generator, we find that there is an advanced onset of thermoacoustic instability. Spatial statistics and the instantaneous fields of p ' ( t ) q · ' ( x , y , t ) show that during the transition from combustion noise to thermoacoustic instability, the emergence of coherent spatial structures in the instantaneous fields of p ' ( t ) q · ' ( x , y , t ) for the experiments with higher turbulence intensities is advanced. However, as the equivalence ratio is reduced further, we notice that higher turbulence intensities result in the reduction of the strength of the pressure oscillations during the state of thermoacoustic instability. We find that, at these low equivalence ratios, there is a decrease in the coherence due to the dispersal of p ' ( t ) q · ' ( x , y , t ) , which explains the reduction in the strength of the pressure oscillations.

This paper investigates the thermoacoustic stability of non-premixed flames in a tangential swirler combustion chamber. Thermoacoustic instabilities, characterized by intense pressure oscillations, pose significant challenges in the design and operation of combustion devices such as gas turbines and rocket engines. This study focuses on understanding the dynamics of flame heat release rate (HRR) and its interaction with acoustic waves under varying operating conditions. The experimental setup involves a single-injector tangential swirler non-premixed flame with detailed measurements of pressure fluctuations, HRR, and flame dynamics using high-speed imaging and chemiluminescence techniques. The results reveal distinct thermoacoustic instability modes, influenced by factors such as inlet velocity and equivalence ratio. This paper highlights the complex interplay between flame structure, HRR fluctuations, and acoustic coupling, providing insight into the mechanisms that govern thermoacoustic stability in non-premixed combustion systems. This research contributes to the development of strategies for mitigating thermoacoustic instabilities in practical combustion applications.

To investigate the thermoacoustic instability characteristics of a jet multi-nozzle array combustor, this study experimentally measured the Flame Describing Function (FDF) of the model combustor using methane fuel experiments, to investigate the effects of equivalence ratio and bulk velocity of nozzle jet on the flame response. The measured FDFs were then incorporated into the low-order thermoacoustic network model, OSCILOS (Open Source Combustion Instability Low Order Simulator), to predict thermoacoustic oscillations of the model combustor. Results showed that within the bulk velocity range of 30 m/s to 60 m/s and equivalence ratio range of 0.64 to 0.76, the FDF gain decreased with increasing jet velocity and equivalence ratio when u^/u¯ = 0.1, while the characteristic peak frequency increased. The delay times extracted from the phase curves indicated that equivalence ratio fluctuation mechanism could play a dominant role in the generation and development of thermoacoustic oscillations. The FDF gain exhibited nonlinear characteristics when u^/u¯ = 0.1 to 0.5, and thermoacoustic oscillations predictions of frequency using OSCILOS achieved an error margin within 8.0%.

Replacing methane by hydrogen in premixed burners is a typical decarbonation scenario. This is usually done by ensuring a constant flame speed, i.e. by using ultra-lean H 2 mixtures. This change can modify the thermoacoustic properties of burners. This study focuses on one class of thermoacoustic instabilities, Intrinsic ThermoAcoustic (ITA) modes, for flames stabilized on a diaphragm. An acoustic network approach is built to construct an ITA stability criterion (called ITAS criteria) for a three-duct configuration and a fully analytical expression is obtained in the limit of thin diaphragms. It shows that instability will grow when the Flame Transfer Function (FTF) gain 𝑛 (cid:57) 𝜋 , located at the frequency at which 𝑢 ′ - 𝑞 ′ phase ϕ = − 𝜋 , is greater than a critical gain 𝑛 𝑐 expressed by the ITAS criteria. Two-dimensional DNS, with anechoic terminations, of methane-air and hydrogen-air flames stabilized on a diaphragm are simulated with similar laminar flame speed 𝑠 0 𝐿 and theoretical flame length, with an equivalence ratio of 0.73 and 0.4 respectively. The theory developed allows to analyse the differences with these two strong unstable ITA modes. For ultra-lean hydrogen flame, preferential diffusion near the diaphragm lips and tip opening reduces the flame time delay and shifts the ITA frequency towards higher values compared to methane. The assessment of two new control strategies is possible with the help of the analytical network model by increasing 𝑛 𝑐 . A first strategy involves the use of preheated leaner mixtures: increasing 𝑇 𝑢 and reducing 𝜙 while keeping the flame speed 𝑠 0 𝐿 constant. A second strategy consists of modifying the combustor geometry by changing the section ratio 𝑆 2 / 𝑆 0 , between plenum and combustion chamber. Both strategies successfully mitigate the hydrogen-air ITA modes and seems promising strategies for methane cases despite the fact that instabilities are not fully mitigated for this fuel at the conditions of interest in this study

We experimentally study the transition from a state of combustion noise to azimuthal thermoacoustic instability in a laboratory-scale turbulent annular combustor. This combustor has sixteen swirl-stabilized burners to facilitate continuous and spatially distributed combustion along the annular region. Our approach involves simultaneous measurement of CH* chemiluminescence emission of the flame using two high-speed cameras and the acoustic pressure fluctuations using eight piezoelectric pressure transducers mounted on the backplane of combustor. We observe that the transition from combustion noise to azimuthal instability occurs through mode shifting, where the system switches from a longitudinal mode to an azimuthal mode as the equivalence ratio is decreased. Throughout this progression, the combustor exhibits various dynamical behaviors, including intermittency, dual-mode instability, standing azimuthal instability, and beating azimuthal instability. These dynamical states are determined from the acquired pressure signals by decomposing the acoustic pressure fluctuations into clockwise (CW) and counterclockwise (CCW) waves, enabling a reconstruction of the amplitude of acoustic pressure fluctuations, nature angle, (anti-)nodal line location, and spin ratio. The global heat release response is then examined during various dynamical states, contrasting their behavior at different non-dimensional time steps by phase-averaging the fluctuations of the heat release rate over the acoustic pressure cycle. Distinctive flame behaviors were observed based on the direction of pressure wave propagation, showcasing characteristic CCW spinning, standing, and CW spinning heat release patterns. Moreover, our examination of relative phase distributions during various dynamical states, computed by analyzing the phase of heat release rate fluctuations across all burners with respect to one burner, reveals the emergence of diverse patterns in the interaction of neighboring flames influenced by acoustic field.

No abstract available

Lean premixed (LPM) combustion is very effective at mitigating emissions but is vulnerable to strong thermoacoustic instabilities. A porous insert in the shape of an annular ring placed at the dump plane of the combustor has been proven to be an effective passive technique for mitigating these instabilities across a wide range of operating conditions. However, it is unclear if the change results from the insert geometry or porosity of the insert. In this study, swirl-stabilized LPM combustion is investigated for three configurations — without any insert, with a porous insert, and with a geometrically similar solid insert. Acoustics, flow, and heat release rate behavior of the three test geometries are investigated using diagnostics including dynamic pressure and acoustic probes, particle image velocimetry (PIV), and OH* chemiluminescence (OH*CL) imaging. Synchronized measurements at a fixed equivalence ratio were acquired at 40 kHz using sound probes and at 3.5 kHz using PIV and OH*CL. Results include time-series and spectral measurements of pressure, velocity, and OH*CL, and mode analysis by proper orthogonal decomposition (POD). In addition, the dynamics of the instability are investigated by high-resolution phase reconstructions of velocity and OH*CL data using a novel implementation of POD introduced in this work. Results show two different instability modes: a longitudinal instability for the solid insert case and a helical, precessing vortex driven instability for the no insert case. In both cases, the flow field and heat release rate oscillations are coupled to produce the instability. No such coupling or oscillations are observed for the porous insert case. These results ascertain the unique capabilities of the porous insert in protecting against instability from different, simultaneous driving mechanisms and demonstrate that the insert porosity and flow dynamics associated with it are the primary mitigating factors.

In 2020, energy-related CO2 emissions reached 31.5 Gt, leading to an unprecedented atmospheric CO2 level of 412.5 ppm. Hydrogen blending in natural gas (NG) is a solution for maximizing clean energy utilization and enabling long-distance H2 transport through pipelines. However, insufficient comprehension concerning the combustion characteristics of NG, specifically when blended with a high proportion of hydrogen up to 80%, particularly with minority species, persists. Utilizing the heat flux method at room temperature and 1 atm, this experiment investigated the laminar burning velocities of CH4/NG/H2/air/He flames incorporating minority species, specifically C2H6 and C3H8, within NG. The results point out the regularity of SL enhancement, reaching its maximum at an equivalence ratio of 1.4. Furthermore, the propensity for the enhancement of laminar burning velocity aligned with the observed thermoacoustic oscillation instability during fuel-rich regimes. The experimental findings were contrasted with kinetic simulations, utilizing the GRI 3.0 and San Diego mechanisms to facilitate analysis. The inclusion of H2 augments the chemical reactions within the preheating zone, while the thermal effect from temperature is negligible. Both experimental and simulated results revealed that CH4 and NG with a large proportion of H2 had no difference, no matter whether from a laminar burning velocity or a kinetic analysis aspect.

We present a detailed study on the characterization of the degeneration process in combustion instability based on dynamical systems theory. We deal with combustion instability in a lean premixed-type gas-turbine model combustor, one of the fundamentally and practically important combustion systems. The dynamic behavior of combustion instability in close proximity to lean blowout is dominated by a stochastic process and transits to periodic oscillations created by thermoacoustic combustion oscillations via chaos with increasing equivalence ratio [Chaos 21, 013124 (2011); Chaos 22, 043128 (2012)]. Thermoacoustic combustion oscillations degenerate with a further increase in the equivalence ratio, and the dynamic behavior leads to chaotic fluctuations via quasiperiodic oscillations. The concept of dynamical systems theory presented here allows us to clarify the nonlinear characteristics hidden in complex combustion dynamics.

We propose an online method of detecting combustion instability based on the concept of dynamical system theory, including the characterization of the dynamic behavior of combustion instability. As an important case study relevant to combustion instability encountered in fundamental and practical combustion systems, we deal with the combustion dynamics close to lean blowout (LBO) in a premixed gas-turbine model combustor. The relatively regular pressure fluctuations generated by thermoacoustic oscillations transit to low-dimensional intermittent chaos owing to the intermittent appearance of burst with decreasing equivalence ratio. The translation error, which is characterized by quantifying the degree of parallelism of trajectories in the phase space, can be used as a control variable to prevent LBO.

This work investigates a high-frequency thermoacoustic instability in an atmospheric laboratory-scale combustion test rig. The flame is stabilized by a single-jet burner operated with pure hydrogen at thermal powers between 60 and 120 kW. A parametric study is performed by varying the equivalence ratio and bulk flow velocity. The measurements show a clear correlation between the equivalence ratio and the occurrence of high-frequency thermoacoustic instabilities, and with increasing thermal power the amplitude of pressure fluctuations rises. A stable and an unstable operating condition with identical bulk velocity but different equivalence ratios are examined in detail using optical and acoustic methods. The power spectra reveal two closely spaced peaks in the 4 kHz range, indicating that multiple modes in close proximity may be simultaneously unstable. This is supported by a beating pattern in the response of azimuthally distributed microphones, which also suggests the presence of transverse acoustic modes. High-speed OH* chemiluminescence imaging from lateral and aft-end views is used to describe the mode shapes. The phase-averaged images show an axisymmetric OH* distribution propagating at approximately the bulk jet velocity, indicating a longitudinal or radial mode. A spectral proper orthogonal decomposition of the chemiluminescence signals confirms that the leading mode is rotationally symmetric, while a secondary mode of transverse shape carries less energy.

This paper is about the characteristics of and a method to recognize the onset of limit cycle thermoacoustic oscillations in a gas turbine-like combustor with a premixed turbulent methane/air flame. Information on the measured time series data of the pressure and the OH* chemiluminescence is acquired and postprocessed. This is performed for a combustor with variation in two parameters: fuel/air equivalence ratio and combustor length. It is of prime importance to acknowledge the nonlinear dynamic nature of these instabilities. A method is studied to interpret thermoacoustic instability phenomena and assess quantitatively the transition of the combustor from a stable to an unstable regime. In this method, three-phase portraits are created on the basis of data retrieved from the measured acoustics and flame intensity in the laboratory-scale test combustor. In the path to limit cycle oscillation, the random distribution in the three-phase portrait contracts to an attractor. The phase portraits obtained when changing operating conditions, moving from the stable to the unstable regime and back, are analyzed. Subsequently, the attractor dimension is determined for quantitative analysis. On the basis of the trajectories from the stable to unstable and back in one run, a study is performed of the hysteresis dynamics in bifurcation diagrams. Finally, the onset of the instability is demonstrated to be recognized by the 0-1 criterion for chaos. The method was developed and demonstrated on a low-power atmospheric methane combustor with the aim to apply it subsequently on a high-power pressurized diesel combustor.

The propagation mechanism of flow disturbance under acoustic excitations plays a crucial role in thermoacoustic instability, especially when considering the effect of non-premixed combustion on heat release due to reactant mixing and diffusion. This relationship leads to a complex coupling between the spatial distribution of the equivalence ratio and the propagation mechanism of flow disturbance. In the present study, the response of a methane-air non-premixed swirling flame to low-frequency acoustic excitations was investigated experimentally. By applying Proper Orthogonal Decomposition (POD) analysis to CH* chemiluminescence images, the harmonic flame response was revealed. Large Eddy Simulation (LES) was utilized to analyze the correlation between the vortex motion within the shear layers and the harmonic response under non-reacting conditions at excitation frequencies of 20 Hz, 50 Hz, and 150 Hz. The results showed that the harmonic flame response was mainly due to the harmonic velocity pulsations within the shear layers. The acoustically induced vortices within the shear layer exhibited motion patterns susceptible to harmonic interference, with spatial distribution characteristics closely related to the oscillation modes of the non-premixed combustion.

We investigate the route to self-excited thermoacoustic instability in a laminar flow multiple flame matrix burner. With an increase in the equivalence ratio, the thermoacoustic system that is initially quiet (stable operation) transitions to limit cycle oscillations through two distinct dynamical states, namely, bursting oscillations and mixed mode oscillations. The acoustic pressure oscillations transition from quiescence to large amplitudes during bursting oscillations. Such high amplitude bursting oscillations that occur well ahead of the onset of limit cycle oscillations can potentially cause structural damage. The thermoacoustic system exhibits hysteresis. The transition to limit cycle oscillations is replicated in a phenomenological model containing slow-fast time scales.

No abstract available

Combustion instabilities in a high-pressure, multi-element combustor are studied in order to understand the relationship between the chamber and injector dynamics. A linear array of seven injectors supplies premixed natural gas and air into a rectangular combustion chamber designed to promote high-frequency, transverse thermoacoustic instabilities. The effect of equivalence ratio on the combustion dynamics was investigated for two injector lengths, 62.5 and 125 mm. For all operating conditions, the 125 mm injectors promote high-amplitude instabilities of the fundamental transverse (1T) mode, which has a frequency of 1750–1850 Hz. Reducing the injector length significantly lowers the instability amplitudes for all operating conditions and, for lower equivalence ratio cases, excites an additional mode near 1550 Hz. The delineating feature controlling the growth of the instabilities in each injector configuration is the coupling with axial pressure fluctuations in the injectors that occur in response to the transverse modes in the chamber.

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The effect of temporal increase in the equivalence ratio on the combustion instability of a lean-premixed low-swirl hydrogen jet flame in a low-swirl combustor (LSC) is investigated in detail using a high-fidelity Large-Eddy Simulation (LES). The equivalence ratio is linearly increased from 0.3 to 0.5 over a duration of 0.4 s. The results show that the pressure oscillation amplitude in the combustor increases significantly when the equivalence ratio at the combustor inlet (ERCI) exceeds 0.42, and the maximum pressure amplitude and the combustion instability mode exhibit trends consistent with those in a previous experiment and numerical simulation conducted with the same LSC setup at a fixed equivalence ratio of 0.39. Temporal variations in the equivalence ratio and consequently the temperature inside the combustor cause the drastic amplification of pressure oscillation (when the ERCI exceeds 0.42) whose amplitude is larger than that at the fixed equivalence ratio (= 0.39). Prior to the onset of this exceptionally strong combustion instability, a transient irregular oscillation phenomenon comprising instantaneous changes in the pressure oscillation frequency is observed. While the pressure oscillations in the combustor and in the injector channel are in phase after the onset of strong combustion instability, they are in opposite phases during the occurrence of the irregular oscillation phenomenon prior to the onset of strong combustion instability. This irregular oscillation phenomenon predicted by the LES may play a crucial role in the mechanism of transition from stable combustion to combustion instability.

This study explored an estimation method for equivalence ratio in a thermoacoustic combustor by combining emission spectroscopy and a random forest (RF) model. Spectra signals under different equivalence ratios were experimentally acquired using a spectrograph. The measured signals were primarily processed and five obvious spectral components were observed, including OH<inline-formula> <tex-math notation="LaTeX">$\ast $ </tex-math></inline-formula> (309.348 nm), CH<inline-formula> <tex-math notation="LaTeX">$\ast $ </tex-math></inline-formula> (430.482 nm), <inline-formula> <tex-math notation="LaTeX">$\text{C}_{{2}}\ast $ </tex-math></inline-formula> (516.192 nm), K (766.188 nm), and <inline-formula> <tex-math notation="LaTeX">$\text{H}_{{2}}\text{O}$ </tex-math></inline-formula> (927.119 nm). Characteristic peaks of the spectral components at various equivalence ratios were extracted to establish the raw features. Before regression modeling, the feature importance of the spectral components was analyzed and taken as a reference to further optimize the raw features. A nonlinear regression model was then established based on the optimized features and RF algorithm. Results demonstrate that the equivalence ratio can be predicted by the proposed model with an average determination coefficient higher than 98%. The spectral information proved an effective method for the prediction of the equivalence ratio in the thermoacoustic pulse combustor.

The stability of biogas–hydrogen combustion is critical for advancing hydrogen-enriched fuel applications in sustainable energy systems. This study investigates the downward propagation dynamics of biogas–hydrogen premixed flames in a vertical tube, focusing on thermoacoustic instabilities under varying hydrogen fractions (0–40 vol. %) and equivalence ratios (0.8–1.2). Four distinct flame propagation regimes were identified, transitioning from continuous oscillation to Helmholtz-driven pulsations as hydrogen content increased. Spectral analysis revealed secondary frequency components (≈2 × primary frequency), attributed to acoustic resonance modes. A novel criterion for flat-flame instability was established by correlating laminar burning velocities (SL) with equivalence ratios, proposing a critical SL threshold of 24 cm/s for regime transition. Theoretical analysis further quantified the role of the Lewis number (Le) in combustion instability: higher Le amplified oscillatory intensity, while elevated SL increased dimensionless frequencies across modes. Growth rate calculations demonstrated that acoustic losses (1/τloss ≈ 24 s−1) and the combined Zel'dovich–Mach parameter (βM ≈ 0.009) are pivotal in suppressing flat-flame instabilities.

Combustion instability is one of the prominent and unavoidable problems in the design of high-performance propulsion systems. This study investigates the heat release rate (HRR) responses in a triple-nozzle swirling nonpremixed combustor under various thermoacoustic self-excited instability modes. Dynamic pressure sensors and high-speed imaging were employed to capture the pressure oscillations within the combustion chamber and the characteristics of flame dynamics, respectively. The results reveal nonlinear bifurcations in the self-excited thermoacoustic instabilities at different equivalence ratios. Significant differences in flame dynamics were observed across the instability modes. In lower frequency modes, the fluctuations in flame length contribute to the driving force of thermoacoustic instability. In relatively high-frequency modes, HRR fluctuations are dominated by the rolling up and convective processes of wrinkles on the flame surface. Alternating regions of gain and damping are observed on the flame surface. At even higher frequencies, both aforementioned HRR fluctuation patterns are simultaneously observed. These findings provide a deeper understanding of the complex interactions between flame dynamics and thermoacoustic instabilities, offering new insights into the design and optimization of nonpremixed combustion systems. The study underscores the importance of considering the spatial and temporal variations in flame behavior to effectively predict and control thermoacoustic instabilities.

No abstract available

Fuel-lean hydrogen combustion systems hold significant potential for low pollutant emissions, but are also susceptible to intrinsic combustion instabilities. While most research on these instabilities has focused on flames without wall confinement, practical combustors are typically enclosed by walls that strongly influence the combustion dynamics. In part I of this work, the flame-wall interaction of intrinsically unstable hydrogen/air flames has been studied for a single operating condition through detailed numerical simulations in a two-dimensional head-on quenching configuration. This study extends the previous investigation to a wide range of gas turbine and engine-relevant operating conditions, including variations in equivalence ratio (0.4 - 1.0), unburnt gas temperature (298 K - 700 K), and pressure (1.01325 bar - 20 bar). These parametric variations allow for a detailed analysis and establish a baseline for modeling the effects of varying instability intensities on the quenching process, as the relative influence of thermodiffusive and hydrodynamic instabilities depends on the operating conditions. While the quenching characteristics remain largely unaffected by hydrodynamic instabilities, the presence of thermodiffusive instabilities significantly increases the mean wall-heat flux and reduces the mean quenching distance. Furthermore, the impact of thermodiffusive instabilities on the quenching process intensifies as their intensity increases, driven by an increase in pressures and a decrease in equivalence ratio and unburnt gas temperature.

ABSTRACT Given unsteady operating conditions, the combustion process in gas turbine combustor often suffers from equivalence ratio fluctuations (ERFs), which induce combustion instability. In this study, the effects of ERFs on a swirl-stabilized premixed flame were studied by large eddy simulation (LES). A new combustion model with turbulence modification and a two-step methane oxidation mechanism were employed to simulate the interaction between turbulence and chemical reactions. In this study, LES was first validated by comparing the simulation results with the corresponding experimental data of a baseline case. Furthermore, two types of ERFs with different frequencies (40 and 160 Hz) were set on the inlet surface of the combustor, and flame responses were predicted by LES. With fluctuation frequencies of 40 and 160 Hz, the inner shear layer was strengthened, and evident corresponding vortices were generated. Furthermore, a more conspicuous variation was observed at 40 Hz, and it meant that a stronger combustion instability was induced by ERFs with a lower frequency.

The effects of dimethyl ether (DME) addition to methane and ethylene fuels on the combustion characteristics of heat release, soot emissions, and flame temperature were investigated experimentally and numerically in a non-premixed laminar flame configuration. The flame-heat release soot-volume fraction was measured experimentally using CH*, OH*, and C2* chemiluminescence and planar two-color soot pyrometry, respectively. The CH*, OH*, and C2* were used to locate flame-heat release regions as well as to investigate the soot signal’s effect on their measurements. The ratios of the chemiluminescence pairs (OH*/CH* and OH*/C2*) were studied for the feasibility of map local equivalence ratios. Numerical calculations across a full range of DME mixing ratios were performed through 1D laminar flame simulations implemented with a detailed mechanism to provide an indication of the flame structures and profiles of key species including OH*, OH, CH*, CH, CH3, C3H3, C2H2, heat release rate (HRR), and flame temperature. An existing developed soot model was used in a 2D computational study to investigate its validity for modeling soot for DME (oxygenated fuel)/C2H4/N2 flames. Parametric studies have been carried out on some key parameters in the soot model to find optimum values that can be used in future studies. Although soot radiation intensities increased at a small amount (25%vol) of DME addition in the DME/methane flames, the soot pyrometry results showed a reduced soot volume fraction with an increased DME mixture ratio in both DME/methane and DME/ethylene flames studied, agreeing with the key conclusion of 1D numerical results. The flame HRR decreases with the increasing addition of DME to methane and ethylene flames and correlates with the trend of OH* and CH* profiles. The 1D simulation showed a non-monotonic correlation between OH*/CH* ratios and equivalence ratios, implying a limited use of OH*/CH* for the equivalence ratio measurement in non-premixed flames with DME additions.

This paper presents an analysis of the unsteady heat release rate response of premixed flames to equivalence ratio perturbations for an industrial premixed swirl-based burner. During this investigation, perfectly and technically premixed flames were acoustically forced via fuel/air mixture flow and air flow modulations respectively, at the same operating conditions. From the resulting flame transfer functions (FTFs), measured using the multi-microphone method, the equivalence ratio driven FTF was isolated and extracted by removing the velocity driven component, i.e. the measured FTF from the perfectly premixed flame, from the technically premixed FTF with two novel extraction techniques. The results are compared with FTFs obtained directly in a previous experimental campaign [1] where the fuel flow was acoustically forced, the resulting equivalence ratio fluctuations measured via an IR absorption technique, and the heat release rate response to the forcing was quantified using chemiluminescence measurements. The results from both measurement approaches agreed well highlighting the validity of the techniques. Further, to understand the governing features of the equivalence ratio driven FTF, a physics-based analytical model following the G-equation approach was developed. The contributions from flame surface area, flame speed, and heat of reaction oscillations were modeled to describe the heat release rate dynamics. A limited number of physical parameters in the analytical model were anchored on one test condition, optimized and restricted to values which were all physically reasonable, and were subsequently used for model predictions at other operating conditions. The FTF model predictions compared well with experimental data across a range of different operating conditions. Finally, the relative contributions from flame surface area, flame speed, and heat of reaction oscillations on the features of the FTFs were identified and explored.

Radiant porous burners are favored in household and industrial applications for their high flexibility, minimal emissions, and excellent fuel adaptability. Enhancing heating temperature uniformity improves the heat transfer efficiency and heating quality. This work presents a two-dimensional numerical study on the non-uniformity of the heating temperature (i.e., solid temperature at the burner exit) in a cylindrical porous burner. The root mean square (RMS) of the heating temperature (Ts,rms) is used to quantify the non-uniformity. From the numerical results, the cylindrical porous burner exhibits a non-uniform heating temperature and a non-uniform mass flow rate of flue gas at the sub-outlets. The surface emissivity and equivalence ratio have minor effects on the Ts,rms. However, increasing the volumetric flow rate and solid temperature leads to a more uniform distribution of heating temperature. The Ts,rms decreases from 40 K to 4.4 K as the volumetric flow rate increases from 26.75 slm (standard liters per minute) to 87 slm at an equivalence ratio of 0.96 and emissivity of 0.85. Meanwhile, the results demonstrate that increasing the thermal conductivity of the porous shell improves the heating temperature uniformity. This study presents guidelines for enhancing the uniformity of heating temperature for the cylindrical porous burner.

The broad-band direct combustion noise is an important problem for industrial and domestic burners. The power spectral density (PSD) of this noise is related to the local spectral density of fluctuating heat release rate (HRR) ( $\psi _{\dot {q}}$ ), which is challenging to measure but is readily available from large eddy simulations (LES) results. The behaviour of $\psi _{\dot {q}}$ for a wide range of thermochemical and turbulence conditions is investigated. Three burners are studied, namely a dual-swirl burner, a bluff-body burner and a jet in cross-flow burner, operating at atmospheric conditions with $\textrm {CH}_4$ –air and $\textrm {H}_2$ –air mixtures. In contrast to the classical $f^{-5/2}$ scaling, the far-field sound pressure level and volume-integrated HRR ( $\psi _{\dot {Q}}$ ) spectra reveal a universal $f^{-5}$ scaling for high frequencies. This differing spectral decay rate for $\psi _{\dot {Q}}$ compared to the classical scaling is due to multi-regime combustion, related to either partial premixing or the local turbulence intensity. The dependence of $\psi _{\dot {q}}$ on the chosen spatial locations, flame configuration and its relation to velocity spectra are studied. A simple model for $\psi _{\dot {q}}$ involving the velocity spectra is found that compares well with LES results. The characteristic frequency involved in this model is related to the time scale of the coherent structures of the flow.

Addressing climate change and reducing greenhouse gas emissions are critical priorities. Utilizing hydrogen-rich methane or pure hydrogen as fuels within gas turbines, facilitated by array micro-tube premixed combustion technology, is anticipated to markedly accelerate the decarbonization process of the energy sector. In this study, the flame structure of the array micro-tube premixed burner under various fuel compositions was examined using OH-Planar Laser-Induced Fluorescence and Particle Image Velocimetry measurement techniques. The effects of the equivalence ratio (φ) and the hydrogen power ratio (HPR) on the characteristics of the flame front, including its curvature, density, volume, and the associated flow field properties, were discussed. As φ and HPR increase, the wrinkled structure of the flame front is significantly enhanced, with a more pronounced effect on smaller scales. This enhancement leads to the separation of the unburned pockets from the main flame. Concurrently, both the flame length and the flame area decrease with the augmentation of φ and HPR, indicating a more concentrated combustion process and increased combustion intensity under hydrogen-enriched and pure hydrogen conditions. The study also observed a slight increase in both the negative and positive curvatures of the flame front, with a more notable increase in the negative curvature. The increased negative curvature results in an elevated degree of wrinkling and a higher value of Σ (flame surface density), reaching a maximum of 0.876 mm−1 under the conditions where φ is 0.8 and ⟨c⟩ (mean progress variable) is 0.5, resulting in the smallest observed flame volume of 100.6 mm3. Upon coupling the flame with the flow field, it was discovered that the exit flow field of the array micro-tube exhibits symmetry and a characteristic conical flame shape. The burning velocity of the side flame brushes increases, and the velocity peak shifts upstream. The aforementioned findings confirm that the addition of hydrogen increases the laminar flame velocity, enabling the flame to stably anchor to the microtube outlet and thereby enhance the flame's robustness and stability.

A new optical diagnostic method that predicts the global fuel–air equivalence ratio of a swirl combustor using absorption spectra from only three optical paths is proposed here. Under normal operation, the global equivalence ratio and total flow rate determine the temperature and concentration fields of the combustor, which subsequently determine the absorption spectra of any combustion species. Therefore, spectra, as the fingerprint for a produced combustion field, were employed to predict the global equivalence ratio, one of the key operational parameters, in this study. Specifically, absorption spectra of water vapor at wavenumbers around 7444.36, 7185.6, and 6805.6 cm–1 measured at three different downstream locations of the combustor were used to predict the global equivalence ratio. As it is difficult to find analytical relationships between the spectra and produced combustion fields, a predictive model was a data-driven acquisition. The absorption spectra as an input were first feature-extracted through stacked convolutional autoencoders and then a dense neural network was used for regression prediction between the feature scores and the global equivalence ratio. The model could predict the equivalence ratio with an absolute error of ±0.025 with a probability of 96%, and a gradient-weighted regression activation mapping analysis revealed that the model leverages not only the peak intensities but also the variations in the shape of absorption lines for its predictions. Graphical abstract This is a visual representation of the abstract.

Free‐piston engine generator is a new type of hybrid power device and is regarded as the next‐generation energy conversion device which can replace the traditional internal combustion engine. This paper focused on the combustion stability and combines experimental results to study the key factors affecting the stability of the free‐piston engine, such as ignition time, intake pressure, equivalence ratio, and operating frequency. The simulation results showed that as the ignition advance angle increased, the indicated mean effective pressure increased significantly and the coefficient of variation of the indicated mean effective pressure was effectively reduced from 15.55% to 1.02% as the advance in ignition time from −15 to −30°ECA. When the intake pressure was increased to 1.2 bar, the average value of indicated mean effective pressure reached about 5.15 bar. When the equivalence ratio was in the range of 1.0–1.4, the coefficient of variation of the indicated mean effective pressure can be kept below 10%. The indicated mean effective pressure decreased monotonically from 4.79 to 3.62 bar, and the coefficient of variation increased by five times as the engine speed increased as the engine speed increased from 1,000 to 2,500 RPM.

An experimental method based on chemiluminescent measurements is developed to determine the heat release rate produced by a two-phase flow kerosene/air flame. This quantity is known to be proportional to the air mass flow rate and the equivalence ratio. Experimental studies are carried out downstream of a liquid fuel injector used in aeronautical combustion chambers. The chemiluminescent spectra of the flame are analyzed for different air mass flow rates and equivalence ratios ranging from 0.4 to 0.71 in the steady-state flame configuration. The broadband background emission due to [Formula: see text] emission (where [Formula: see text] indicates an electronically excited specie) and soot radiation is first evaluated. Then, the analysis of the chemiluminescent emission from [Formula: see text], [Formula: see text], and [Formula: see text] indicates that the [Formula: see text] may be used to determine both the instantaneous equivalence ratio and the air mass flow rate. An example of the application of this method to measure fluctuations in the heat release rate induced by acoustic excitation of the flame is shown.

This work numerically examines the premixed combustion of partially cracked ammonia/air in a cavity-stabilized micro-combustor. Effects of the equivalence ratio (Φ) and inlet temperature (Tin) on the combustion features, flame–wall heat transfer and nitrogen-containing emissions are investigated quantitatively at a cracking ratio of 0.6. Results show that increasing Φ from 0.8 to 1.2 shifts the high-temperature region downstream and causes it to elongate axially. This spatial expansion decreases peak temperatures and distributes heat release over a longer distance. Mean wall temperature and overall heat loss are thus decreased due to weakened near-wall thermal interaction. NO formation closely follows the high-temperature and OH-rich zones. However, at Φ = 1.2, oxygen limitation suppresses NO production and redirects fuel-bound nitrogen towards N2O, enhancing its outlet emissions. As Tin increases from 300 K to 500 K, the improved reactivity of the mixture promotes an upstream shift of the main reaction zone. The reaction zone becomes more concentrated within the cavity. Such structural changes intensify NO formation but simultaneously compress the high-temperature zone, which reduces the wall-averaged temperature and overall heat loss. In the extended downstream post-flame region, lower temperatures and limited radical activity suppress NO2 formation and N2O decomposition. As a result, NO2 emissions decrease monotonically, while N2O emissions exhibit a gradual increase. These findings provide useful insights into the effects of operating parameters on combustion stability, heat transfer and nitrogenous pollutant evolution in microscale partially cracked ammonia flames.

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The double‐layer porous media burner is considered as an effective way to realize stable lean‐burn. In order to quickly achieve a stable combustion state in a double‐layer porous media burner, this work investigated the dynamic characteristics of methane/air premixed combustion in a bench‐scale double‐layer porous media burnspanning from ignition to stable combustion and ultimately flameout. The experimental results indicate that regulating the equivalence ratio and the inlet velocity enables the establishment of a stable flame front and the φ = 0.75 and the Vin = 0.20 m/s are the appropriate start‐up conditions. The average propagation velocity of the combustion wave variation along the axial direction and ranged approximately from −0.022 to −0.078 mm/s. Moreover, the transition time to a stable combustion state is reduced by nearly 47.14% as the equivalence ratio increases from 0.60 to 0.70. During start‐up stage, there are significant fluctuations in CO and NOx concentrations, but both emissions remain low during steady combustion state, with the maximum concentrations of 37.5 and 40.2 mg/m3, respectively. Furthermore, the porous media combustion exhibits a pronounced re‐ignition capacity. At higher equivalence ratios, longer interruptions of premixed gas are allowed.

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This study presents a comprehensive a priori analysis of tabulated-chemistry models for both laminar and turbulent lean premixed hydrogen flames in strained counterflow configuration. Particular focus is drawn on differential and preferential diffusion effects and the synergistic interaction of thermodiffusive instabilities and turbulence that existing models struggle to capture. Through detailed assessment of various modelling approaches at unfiltered and filtered grids, we identify significant limitations in traditional unstretched flamelet manifolds, particularly their strong filter dependence and systematic reaction rate mispredictions. To address these challenges, we introduce and evaluate novel strained flamelet approaches, including: (1) a one-dimensional manifold constructed from a single strained flamelet that provides computationally efficient and reliable consumption speed predictions at coarser grids, and (2) a two-dimensional manifold combining fixed strain with varying equivalence ratio that demonstrates improved performance in predicting the local reaction rates across multiple grid resolutions. Additionally, we develop a correction methodology derived from laminar simulations that significantly improves consumption speed predictions of unstretched flamelet manifolds in turbulent settings. Unlike previous works, our solutions maintain computational efficiency without increasing manifold dimensionality, keeping memory costs unchanged. These advancements provide guidance for developing reliable LES models that properly account for differential and preferential diffusion and strain effects in practical hydrogen combustion systems.

Premixed flames of partially cracked ammonia (NH3) hold significant promise for the decarbonization of internal combustion engines and gas turbines, since they can burn at a similar laminar flame speed to methane but have notably high blow-out resistance. Understanding turbulent premixed flames with partially cracked NH3 is highly relevant from both academic and application perspectives. This study aims to enhance our understanding of such premixed NH3/H2/N2-air flames subjected to increasing turbulence. For this purpose, a specific fuel mixture, consisting of 40vol% NH3, 45vol% H2, and 15vol% N2, is selected to match the laminar flame characteristics of methane at the same equivalence ratio. Turbulent jet flames are stabilized in a piloted burner with increasing bulk velocities from 30 to 180 m/s and Karlovitz numbers from approximately 75 to 2,140. One-dimensional (1D) simulations of freely propagating flames and strained counter-flow flames are performed, emphasizing temperature and species axial profiles and flame response to strain rate. Further, turbulent flow and flame structures are characterized using simultaneous particle image velocimetry (PIV) and laser-induced fluorescence of OH radicals (OH-LIF) measurements. Flame surface density  and curvature  distributions are evaluated, revealing the dominant role of turbulence over differential diffusion in shaping the flame surface topology. It is also found that the OH intensity gradient  serves as a marker for local reactivity and thermo-diffusive instabilities, being higher at positive curvatures than at negative ones. Flat flames dominate the surface topology but show significant discrepancies in as they appear both upstream and downstream of leading edges. The thickness of the OH layer is not broadened by turbulence, even at Ka = 2,140, suggesting that eddies cannot penetrate into the main reaction zone marked by OH radicals, which are formed at higher temperatures than the preheat layer.

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The understanding of processes that govern soot production in aero-engines is fundamental for the design of new combustion systems with low environmental impact. For this, the development of numerical models that allow for soot accurate prediction for a limited computational cost is required. Many combustors, more specifically those used in aero-engines, feature rich flame regions typically exploited in the so-called Rich-Quench-Lean (RQL) technology. Thus, it is important to complete the data on soot formation in turbulent flames that exist in the literature, by considering rich turbulent flames operating in the premixed mode. To this purpose, a model scale swirled combustor, called EM2Soot, was designed at the EM2C laboratory to analyze soot production under perfectly premixed rich conditions. In this work, the effect of the equivalence ratio on soot production in this burner is experimentally characterized and numerically simulated. Measurements of Planar Laser Induced Fluorescence of Polycyclic Aromatic Hydrocarbons (PLIF-PAH) were performed to examine soot precursors presence, whereas soot volume fraction is measured with Planar Laser Induced Incandescence (LII). Large Eddy Simulations (LES) are carried out using an analytically-reduced chemistry for the gas phase simulation in combination with a three-equation model for the solid phase description. By considering a range of equivalence ratios, the soot volume fraction from the experiments was found to reach a maximum near ϕ = 1.8, whereas a lower level of soot volume fraction was measured for lower and for higher equivalence ratios. The large eddy simulations performed in parallel are found to be in qualitative agreement with experimental data in terms of PAHs location and soot presence. The soot volume fractions fv are notably overestimated with respect to the LII measurements, suggesting the need for improved modeling schemes. However, the numerical results correctly retrieve a reduction of soot production for the highest considered equivalence ratio value and can be used to explain the experimental behaviour. Results indicate that LES gives access to a qualitative understanding of the interactions between flame, turbulent flow, and precursors governing soot formation in this system.

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In this study, the flame dynamics of lean premixed hydrogen jet flames are experimentally investigated. Acoustic and optical measurements are used to capture the response of a bundle of jet flames to acoustic forcing. Using helium as a fuel surrogate, we simulate the change in acoustic properties in the burner during the determination of cold burner transfer matrix measurements. We investigate the influence of the equivalence ratio and the addition of methane as well as the interaction of the individual flames to evaluate the scalability of the results to systems with more flames. It is shown that the changes in the dynamic flame response can primarily be explained by the flame length, which changes both with the methane share and with the equivalence ratio. It can be observed that with small changes in the equivalence ratio, the flame length and the flame transfer function (FTF) change in the same way as with a small change in gas composition. To assess the scalability of these results, we deactivate some of the jet flames and analyze how the overall response to acoustic forcing changes. We find that the FTF phase is not affected by the number of active flames. Analyzing the respective gain values, significantly stronger responses are measured for a few flames, but only small difference can be measured above a certain number of neighboring flames so that the lab scale results can also be expected to be valid for industrial configurations with a high number of flames.

The introduction of the pilot flame can stabilize the lean premixed flame and promote its industrial application. However, the interaction mechanism between the pilot and main flames is complicated. To reveal the effect of the pilot flame on the main flame, a laminar lean premixed flame adjacent to a rich premixed pilot flame on one side and a similar lean premixed flame on the other side was considered. A two-dimensional numerical model was adopted with detailed chemistry and species transport, also with no artificial flame anchoring boundary conditions. The results show that the pilot flame could promote the main flame stabilized in different locations with various shapes, by adjusting the stretch, heat transfer, and preferential diffusion in a complicated manner. The pilot flame improves the local equivalence ratio and transfer more heat to the main flame. The growth of the pilot flame equivalence ratio and inlet velocity enhances the combustion on the rich side of the main flame and helps it anchor closer to the flame wall. Both the curvature and strain rate show a significant effect on the flame root, which contributes to the main flame bending towards the pilot flame.

The interrelation between Reynolds stresses and their dissipation rate tensors for different Karlovitz number values was analysed using a direct numerical simulation (DNS) database of turbulent statistically planar premixed H2-air flames with an equivalence ratio of 0.7. It was found that a significant enhancement of Reynolds stresses and dissipation rates takes place as a result of turbulence generation due to thermal expansion for small and moderate Karlovitz number values. However, both Reynolds stresses and dissipation rates decrease monotonically within the flame brush for large Karlovitz number values, as the flame-generated turbulence becomes overridden by the strong isotropic turbulence. Although there are similarities between the anisotropies of Reynolds stress and its dissipation rate tensors within the flame brush, the anisotropy tensors of these quantities are found to be non-linearly related. The predictions of three different models for the dissipation rate tensor were compared to the results computed from DNS data. It was found that the model relying upon isotropy and a linear dependence between the Reynolds stress and its dissipation rates does not correctly capture the turbulence characteristics within the flame brush for small and moderate Karlovitz number values. In contrast, the models that incorporate the dependence of the invariants of the anisotropy tensor of Reynolds stresses were found to capture the components of dissipation rate tensor for all Karlovitz number conditions.

Porous media combustion greatly improves the combustion of low calorific value gas (LCG), and appropriate heat control contributes to optimizing the flame evolution. To obtain the dynamic characteristics of lean‐rich combustion, an enhanced heat‐recirculating burner is built by embedding the cylindrical rod with high thermal conductivity. The temperature distribution and gas products are investigated under different rod parameters and operating conditions. The results indicate that the reduction of the equivalence ratio and inlet velocity are both beneficial for the upstream propagation of rich‐methane flame, which has an opposite trend to lean combustion. Regardless of the direction in which the rich‐methane flame propagates, the flame propagates the fastest as the diameter of the cylindrical rod is 8 mm. When the 8–120 cylindrical rod is embedded in the burner, the downstream propagation time is shortened by 73.40%. The appropriate embedment of the cylindrical rods in porous media not only accelerates the rich‐methane flame propagation but also promotes the conversion of methane to syngas. Moreover, the decrease in pellet diameter is also conducive to increasing syngas production. The above conclusions provide theoretical support for the efficient and clean utilization of LCG in the porous media.

This study quantifies the behavior of a lean premixed prevaporized combustor operating at elevated temperatures and pressures, using both conventional and sustainable aviation fuel. Three different fuel compositions, 100% Jet-A, 100% hydrogenated esters and fatty acids (HEFA), and a 50/50 volumetric blend, are tested at various equivalence ratios and air preheat temperatures. Mie scattering is used to examine the liquid fuel spray pattern in the combustor; phase Doppler particle analysis is used for droplet sizing; 10-kHz OH planar laser induced fluorescence is used to determine the flame surface distribution and to examine the lean blowoff process. Experimental data show that the pure HEFA fuel had smaller droplets and lower fuel penetration depths because of favorable physical properties, but did not otherwise affect the combustor behavior. Despite the differences in fuel spray, the flame surface density fields are similar across fuels. Furthermore, the lean blowoff Damköhler vs Reynolds-number behavior is the same for all fuels within experimental uncertainty. These results demonstrate promising fuel flexibility for low-emission lean premixed prevaporized combustor technologies.

This study explores the impact of blockage ratio on the stability of swirl (axial swirl generator with S1.5) stabilized turbulent premixed n-butane/air flames at 1 bar, 300 K, and ϕ = 1.4 and ϕ = 0.8. Particle image velocimetry experiments and delayed detached eddy simulation simulations are employed to reveal the underlying mechanisms. Increasing the blockage ratio leads to (1) a single broader central recirculation zone (CRZ) to an elongated CRZ with a recirculation zone behind the bluff body and (2) higher turbulence and strain levels generated an intense and narrow flame (jet spread rate = 22°–15°). An adverse effect of enhanced strain rate with an increasing blockage ratio narrowed the measured lean blowoff limits (ϕ = 0.78–0.86). For a higher blockage ratio, the local equivalence ratio (ϕlocal) to the reaction side decreased due to (1) air entrainment and (2) diffusion of deficient species O2 toward the reaction zone. The entrainment of ambient air into the flame was quantified by estimating root mean square local equivalence ratio (ϕrms) from predictions, which showed a 12.1% increase at the outer shear layer of the burner having the highest blockage ratio. Furthermore, the Lewis number effect on a low blockage ratio burner revealed preferential diffusion of product species H2O ahead of CO2 toward the preheat zone for Le < 1 condition (ϕ = 1.4, Le = 0.93). However, based on the local equivalence ratio analysis, no preferential diffusion of the deficient reactant O2 was observed within this regime. The present study with premixed swirl n-butane-air unconfined flames indicated that a higher blockage ratio is beneficial to anchor a stable turbulent flame at ϕ = 1.4, which entrained a large amount of ambient air. In contrast, at lean mixture conditions, the air entrainment decreased the lean blowoff limits at a higher blockage ratio, and hence, a lower blockage ratio is preferable.

Abstract This work investigated the effect of high-pressure (10 MPa) methane gas jet impinging on the methane lean-burn premixed flame (equivalent ratio of 0.7) based on a three-dimensional numerical simulation by CONVERGE software. The results show that laminar premixed flame is accelerated to develop into a stable turbulent flame under the action of the methane jet, the whole process of flame front development is divided into three stages: laminar (Reynolds number maintains stable, 1–1.3 ms), transition (Reynolds number shows an increasing trend, 1.4–8 ms), and turbulent (Reynolds number tends to stabilize at a high value, 1.9–3 ms). The effect of high-pressure jet on flame development along jet direction (Z axis) is greater than that on vertical direction (Y axis). During turbulence stage, the momentum and kinetic energy of Z axis are 2.7 and 6.3 times greater than that of Y axis, respectively. The high-pressure methane jet causes a change in heat distribution, resulting in local flameout. The rate of change in the local flameout area is greater than that in the flame area, causing a temperature drop in Z axis. This temperature drop increases with the increase in equivalence ratio and with the decrease in distance between cross-section position and ignition center.

Abstract The reactive Navier–Stokes equations with adaptive mesh refinement and a detailed chemical reactive mechanism (11 species, 27 steps) were adopted to investigate a detonation engine considering the injection and supersonic mixing processes. Flame acceleration and deflagration-to-detonation transition (DDT) in a premixed/inhomogeneous supersonic hydrogen–air mixture with and without transverse jet obstacles were addressed. Results demonstrate the difficulty in undergoing DDT in the premixed/inhomogeneous supersonic mixture within a smooth chamber. By contrast, multiple transverse jets injected into the chamber aid detonation transition by introducing perturbed vortices, shock waves and a suitable blockage ratio. Increasing distance between the leading shock and the flame tip impedes detonation transition due to an insufficient blockage ratio. The extremely perturbed distributions of fuel-lean and fuel-rich mixtures lead to more complicated flame structures. Also, a larger flame thickness appears in the inhomogeneous mixture compared with the premixed mixture, resulting in a lower combustion temperature. The key findings are that the DDT, detonation quenching and reinitiation are generated in the inhomogeneous supersonic mixture, but both DDT mechanisms are ascribed to a strong Mach stem with the Zel'dovich gradient mechanism. Additionally, the obtained results demonstrate that an intensely fuel-lean mixture (equivalence ratio = 0.15) results in a partially decoupled flame front. However, detonation reinitiation and subsequent self-sustained detonation occur when a fierce shock wave propagates through a highly sensitive mixture, even within a smaller and elongated area. Moreover, the inhomogeneous mixture also augments the propagation speed and detonation cell structure instabilities and delays the sonic point resulting from the extending non-equilibrium reaction.

The stability of lean premixed turbulent swirl flames has been investigated using a laboratory-scale swirl-stabilized gas turbine combustor by varying the position of the bluff body inside the premixing tube. The relative location of the bluff body with respect to the dump plane is characterized in terms of a recess length, which is varied during the experiment. The resultant flame structure and dynamics have been studied for different bluff body recess lengths and bluff body shapes based on highspeed OH*-chemiluminescent images. Herein, we employ two different tapered bluff body shapes of an area ratio of 40% (B40) and 60% (B60). For the bluff body B40, a stable flame is mainly attached to the burner at a higher equivalence ratio. For a lower equivalence ratio, a lifted and elongated flame is formed. The increase in the recess length brings the flame closer to the burner surface, and a stable and intense flame is observed. For the bluff body B60, a less stable elongated and columnar type of flame is formed, and the flame stability slightly improves with the increase in recess length. The change in recess length significantly improves combustor performance when B40 is used instead of B60. The increase in recess length is supposed to bring the recirculation created in the wake of the bluff body close to the exit of the premixing tube, resulting in better mixing of the inlet reactants and hot combustion products. This effect is less prominent for the B60 case, and high bulk velocity squeezing from the edges of B60 may not result in the effective mixing of inlet reactants and hot combustion products. The high bulk velocity for the B60 case may result in the delayed expansion of the inflow reactants, and all these combined effects may render ineffective for the B60 case.

The lean blow-off (LBO) behavior of turbulent premixed bluff-body stabilized hydrocarbon flames and ammonia/hydrogen/nitrogen flame is investigated and compared both experimentally and numerically. Simultaneous high-speed PIV and OH-PLIF are employed to resolve temporal flame and flow field information, allowing the curvature and hydrodynamic strain rates along the flame surfaces to be calculated. OH* and NH2* chemiluminescence images are also used to examine flame structures at the same bulk flow velocity but at four equivalence ratios from far away from to near LBO. A NH3/H2/N2 (70%/22.5%/7.5%) flame is slightly more resilient to LBO compared with methane and propane flames at 20 m/s. The hydrocarbon flame structures change from 'V-shape' to 'M-shape' when approaching lean blow-off, resulting in incomplete reactions and finally trigger the LBO. However, the strong OH* intensity in the shear layer near flame root for the ammonia blend flames indicate a robust reaction which can increase flame stability. Widely-distributed positive curvature along the flame surface of the NH3/H2/N2 flames (Le<1) may also enhance combustion. The less strain rates change along NH3/H2/N2 flames fronts due to less dramatic changes to the flame shape and position can extend the stability limits. Furthermore, the faster consumption rates of hydrogen near the flame root for the ammonia blend flames, and the lower temperature loss compared with the adiabatic temperature also contribute to the stabilization of ammonia blends near lean blow-off.

NO$_{\rm x}$ formation in lean premixed and highly-strained pure hydrogen-air flamelets is investigated numerically. Lean conditions are established at an equivalence ratio of 0.7. Detailed-chemistry, one-dimensional simulations are performed on a reactants-to-products counter-flow configuration with an applied strain rate ranging from $a=100 \, {\rm s}^{-1}$ to $a=10000 \, {\rm s}^{-1}$ and the \texttt{GRI3.0} mechanism. Following a similar setup, two-dimensional direct numerical simulations are also conducted for representative strain rates of $2000 \, {\rm s}^{-1}$ and $5000 \, {\rm s}^{-1}$. Both solutions show a decreasing NO$_{\rm x}$ trend as the applied strain rate is increased. This decreasing emission outcome is highlighted for the first time in this study for lean pure-hydrogen flamelets. A deep analysis of the 2D solution underlines that there is no production of NO$_{\rm x}$ in the second dimension, thus proving that the emission trend is not a result of a setup preconditioning, but is instead a direct physical effect of stretch on the flame. Furthermore, a detailed analysis of the NO$_{\rm x}$ formation pathways at $a=2000 \, {\rm s}^{-1}$ and $a=5000 \, {\rm s}^{-1}$ is performed. Thermal NO$_{\rm x}$ and NNH pathways are shown to both contribute significantly to the total NO$_{\rm x}$ production. While the NNH route contribution is roughly constant at different strain rates, a significant decrease is observed along the thermal NO$_{\rm x}$ route. Overall, results show that lean and highly-strained hydrogen flames experience a significant decrease of NO$_{\rm x}$. This property is discussed and analysed in the paper.

No abstract available

This paper describes the use of experimentally validated computational fluid dynamics methods to study the similarity performance of various models scaled by the DaI criterion. First, the numerical method is validated by particle image velocimetry and CH* chemiluminescence data under the reaction state. Combustor prototypes and models are then simulated under different equivalence ratios (ERs) and swirl numbers (SWs) with the geometric scaling factor (Q) ranging from 0.1 to 1. When Q < 0.3, the reaction zone is obviously stretched. Changes in Q produce large deviations in the velocity distribution. Increasing either ER or SW increases the deviation in the velocity distribution in the outer shear region in front of the combustor but reduces that in the recirculation zone and jet zone at the back of the combustor. The scaling law changes with ER and SW. To distinguish whether the reaction flow field of a model maintains similarity with respect to the prototype, a novel concept called “degree of similarity” is proposed. The “non-similarity range” for geometric scaling factors under different conditions is further clarified. When ER = 0.55, the range of non-similarity of the combustion flow field is Q ≤ 0.3. As ER increases, the range of non-similar intervals decreases, and when ER reaches 0.95, the non-similarity range is Q ≤ 0.1. When SW = 0.42, the non-similarity range is Q ≤ 0.4, and when SW ≥ 0.42, the non-similarity range is Q ≤ 0.3.

The present study investigates the flame shape topology in a lean premixed swirl stabilized combustor for various hydrogen thermal powers (Pth.H2) and equivalence ratios (ϕ) using CNG/H2-air mixtures at atmospheric conditions. High-speed OH*-chemiluminescence (OH*-CL) imaging is employed to study flame dynamics. It is found that at a fixed value of hydrogen mole fraction (XH2), an intense and short flame is found for a higher value of ϕ. Also, at a fixed value of ϕ, flame length shortens, and an enhanced heat release is observed near the burner exit with an increase in XH2. A V-shaped flame with the flame tip pointing toward the exhaust port is witnessed at low values of ϕ and XH2. Moreover, the V-shaped flame transitioned into an M-shaped flame with a flickering flame tip near the wall with an increase in ϕ and XH2. Further, the heat release zone gets extended and distributed over the entire length of the combustion chamber for small values of ϕ and XH2. Finally, the stable flame topology is correlated to the non-dimensional parameters Damköhler number (Da), and the wall temperatures (Tw).

The Head-on Quenching (HoQ) of laminar premixed ammonia–hydrogen-air flames under lean to stoichiometric condition is numerical investigated. Detailed chemistry including 34 reactive species and detailed multi-component transport model including thermal diffusion (Soret effect) are applied. The quenching distance is considered as a representative quantity for the HoQ process, and the influence of different system parameters on it has been investigated. These parameters involve fuel/air equivalence ratios, hydrogen content in gas mixture and pressure. It was found that an increase of quenching distance can be caused by a lower hydrogen addition and a leaner mixture condition. Furthermore, it was found that, regardless of the gas mixture, the quenching distance decreases monotonically with increasing pressure, obeying a power function with the exponent -\documentclass[12pt]{minimal} \usepackage{amsmath} \usepackage{wasysym} \usepackage{amsfonts} \usepackage{amssymb} \usepackage{amsbsy} \usepackage{mathrsfs} \usepackage{upgreek} \setlength{\oddsidemargin}{-69pt} \begin{document}$$-$$\end{document} 0.7. Moreover, numerical results show a relation between the quenching Peclet number and the dimensionless wall heat flux normalized by the flame power. Additionally, sensitivities of quenching distances with respect to the transport model, considering the heat loss in the wall and the chemical kinetics are studied.

The flame holder configuration can significantly affect the performance of a combustion system. In this study, the influence of fuel composition and bluff body geometry on the lean premixed combustion system performance indicators, including temperature, stability, NOx production, and unburned hydrocarbons, are numerically investigated. The premixed combustion of five pure hydrocarbon fuels, namely H2, CH4, C2H2, C2H4, and C2H6, is simulated using a three-dimensional coupled stoichiometric-CFD model. The holder bluff body cross-section shape is varied to assess the impact of the flow patterns on the combustion parameters. The numerical model provided satisfactory agreement with experimental data for propane-air combustion. Results indicate that lighter fuels sustain higher combustion completion and higher flame stability. The bluff body shape significantly impacted temperature distribution and flame shape compared to the plate flame holder design, which produced a broad flame along the burner. The findings establish a relationship between bluff body geometry, enhanced flame stability, and the conditions that influence pollutant formation, providing a basis for designs that mitigate NOx emissions during stable operation at leaner equivalence ratios.

This study is dedicated to understanding the combustion characteristics of turbulent premixed C3H8‐Air‐CO2 and C3H8‐Air‐NH3 swirl flames in a rich‐lean combustor at atmospheric pressure. In this study, the emission characteristics of both flames were obtained through two‐dimensional numerical simulations based on the RANS approach with Realizable k‐ϵ turbulence model for turbulence closure, and the P1 radiation model for the flame radiation inside the combustor. The turbulence‐chemistry interaction was modeled using the Finite‐Rate Eddy Dissipation Model (FR/EDM) model with a reduced reaction mechanism (Jones‐Lindstedt). The study was conducted for five volumetric fractions of CO2 or NH3, XCO2/NH3 = 0,4%, 8%, 12%, 16%, two swirl numbers (Sn = 0.6 and 1.05), and four equivalence ratios, ϕ = 0.4 (with dilution), 0.5, 0.8, and 1. The results show that the addition of NH3 to C3H8‐Air flames promotes the production of CO, whereas the minimum NOx emission (0.14 ppm) was obtained for a dilution rate of 16% at ϕ = 0.8 and Sn = 0.6 corresponding to an outlet temperature of Tout = 1652 K.

No abstract available

Lean premixed flames are useful for low nitrogen oxide (NOx) emissions but more prone to induce combustion instability in gas turbines. Combustion instability of a lean premixed swirling flame (LPSF) with hydrogen–methane was investigated experimentally. The effects of hydrogen addition on combustion instability with equivalence ratios 0.75–1 were investigated with acoustic frequencies (90–240 Hz) and acoustic amplitudes (the ratio of velocity fluctuation to an average velocity of 0–0.5), respectively, which are characterized by the gain and phase of the flame describing function (FDF). The evolution of vortex and the flame morphologies were observed by the particle image velocimetry (PIV), intensified charge-coupled device (ICCD), photomultiplier tube (PMT), and Cassegrain optical systems. The global and local heat release fluctuations of the LPSF were shown by CH*/OH* chemiluminescence and temperature measurements. Results show that the FDF features maximum and minimum gain values in the acoustic frequency range of 90–240 Hz and reaches local maximum peaks at 110 and 180 Hz and local minimum peaks at 160 Hz. It can also be observed that varying velocity amplitudes (0–0.5) have greater effects on the gain and phase of FDF than changing equivalence ratios (0.75–1) for lean swirling flames. Higher velocity amplitudes more effectively intensified the compression of the flame length, which enhanced the mixing of the high-burning gas and the unburned gas, and then heat release fluctuations increased. However, it is more interesting that the effects of hydrogen addition on the combustion instability of the LPSF show a completely opposite phenomenon due to acoustic frequency under all experimental conditions. The FDFs were compared at typical frequencies of 140 and 180 Hz, and it was found that combustion instability enhanced with increasing hydrogen content at 140 Hz while weakened at 180 Hz. The flow field of PIV images shows that it is related to the location and development of vortices in the flame with varying acoustic frequencies. The intensity of OH*/CH* chemiluminescence, local temperature, and heat release rate show the same changing trend with the flame morphology for two acoustic parameters with the increasing hydrogen content in the LPSF. This directly affects the compression and curvature of the LPSF and thereby changes the mixture and temperature of the combustible gas, which influence the heat release fluctuation of the LPSF.

In the present study, a laboratory-scale swirl-stabilized dump combustor was used to investigate the dynamic characteristics. The dynamics of the flame was studied for progressively decreasing equivalence ratios till blowout occurred. The equivalence ratio was varied by decreasing the fuel flow rate at a given air flow rate. The combustion dynamics was represented in terms of time series data of pressure fluctuations, measured with a microphone, and fluctuations of intensity of flame emissions (CH* chemiluminescence), measured with a photomultiplier tube. The dynamics was characterized by means of power spectrum analysis and short time Fourier transform. The statistical moments are calculated based on fluctuation which gives the clear picture flame dynamics.

In the present study, a laboratory-scale swirl-stabilized dump combustor was used to investigate the dynamic characteristics. The dynamics of the flame was studied for progressively decreasing equivalence ratios till blowout occurred. The equivalence ratio was varied by decreasing the fuel flow rate at a given air flow rate. The combustion dynamics was represented in terms of time series data of pressure fluctuations, measured with a microphone, and fluctuations of intensity of flame emissions (CH* chemiluminescence), measured with a photomultiplier tube. The dynamics was characterized by means of power spectrum analysis and short time Fourier transform.

This experimental study investigates the effects of hydrogen and oxygen fractions on stability and combustion characteristics of oxy-methane flames in a dual annular counter-rotating swirl combustor for wider operability of emission-free gas turbines. The pilot stream, comprising a mixture of H2 and CO2, serves as the pilot flame, while the mainstream, consisting of a mixture of CH4, CO2, and O2, is the dominant flame. The experiments were conducted at a fixed velocity ratio of 2.27 across a range of mainstream equivalence ratio. The results demonstrated that increasing hydrogen and oxygen fractions significantly enhanced the combustor's lean blowout stability. At hydrogen fraction of 15% and oxygen fraction of 34%, the blowout equivalence ratio was reduced to 0.35, compared to 0.50 at zero hydrogen enrichment, representing a ∼ 43% decrease. The highest temperature of 1763°C was recorded at equivalence ratio of 0.73, showcasing enhanced combustion intensity at richer equivalence ratios. Comparatively, at oxygen fractions of 34% and 30 %, with hydrogen fraction of 15%, the CO emissions dropped to a range of 2 to 4 ppm at equivalence ratio of 0.63, significantly lower than 33 ppm recorded at oxygen fraction of 26% and zero hydrogen enrichment. Flame imaging revealed that the flames became brighter, more compact, and stable with increasing hydrogen and oxygen fractions. These findings underscore the synergistic role of hydrogen and oxygen enrichment in extending lean blowout limits, improving thermal performance, and reducing emissions in dual annular counter-rotating swirl combustors.

Flashback with subsequent flame anchoring (FA) is an inherent risk of lean premixed gas turbine combustors operated with highly reactive fuel. The present study has been performed to characterize flame stabilization in the premixing zone of a lean premixed swirl stabilized burner and to identify critical combustion characteristics. An optically accessible burner was used for experimental investigations under atmospheric pressure and elevated preheat temperatures. The air mass flow rate, global equivalence ratio and preheat temperature were systematically varied to identify critical operating parameters. Hydrogen-natural gas mixtures with hydrogen mass fractions from 15 to 100 % were studied to evaluate the impact of fuel reactivity. The air-fuel mixture was ignited with a focused single laser pulse to trigger FA in the premixing zone during steady operation. High speed imaging with OH*-chemiluminescence were applied to observe flame characteristics and evaluate flame anchoring propensity. Flame anchoring limits (FAL) are reported in terms of the minimum global equivalence ratio at which the flame was blown out of the premixing zone within a critical time period. A comparison of characteristic time scales at FAL shows that the main impact during flame anchoring is given by the fuel reactivity and to some ex tent by preheat temperature. A Damköhler criterion is derived from the FAL that allows prediction of FA propensity based on operating conditions and 1-D reacting simulations.

The demand for replacing hydrocarbon fuel with hydrogen (H 2 ) in existing combustion systems has increased to achieve a carbon-neutral society, subsequently necessitating developments of advanced monitoring technologies for safer operations of combustion systems. This study proposes a method that effectively predicts key operational parameters—total combustion flow rate ( Q ), fuel-air equivalence ratio ( ϕ ), and H 2 blend ratio ( X H 2 )—for a given combustor, directly from the chemiluminescence spectrum of a model low-swirl combustor. This method is based on the fact that the combustion field is governed by these key operational parameters, and significant information about the field can be contained in the chemiluminescence spectrum. Since it is difficult to obtain an analytical relationship between the spectrum and the operational parameters, a predictive model was developed in a data-driven manner. Q , ϕ , and X H 2 were varied between 80–140 L/min, 0.7–1.0, and 0–30 mol%, respectively, resulting in 441 experimental conditions, with 500 spectra collected for each condition. Specifically, the model consisted of a convolutional autoencoder (CAE) along with separate regressors, as these predictions share the same spectral features. Once optimized, the final model was able to predict Q , ϕ , and X H 2 within ±2.994 L/min, ±0.012, and ±2.252 mol%, respectively, with 96% probability. A gradient-weighted regression activation mapping (Grad-RAM) analysis confirmed that the model employs all key features of OH*, CH*, C 2 *, CO 2 *, and H 2 O* intensities in predicting each parameter, while compensating for the effects of the others.

This study investigates the flame dynamics of lean premixed kerosene combustion for two different degrees of fuel-air premixing using a swirl stabilized burner with an axially movable twin fluid fuel injection nozzle. Thermal power, equivalence ratio and atomizing air mass flow are varied systematically for both nozzle positions investigated. Measurements of the droplet size distribution at the nozzle exit are provided for all operation points. NOx emission measurements and OH*-chemiluminescence flame images show that stationary combustion characteristics significantly change with the nozzle position. Flame Transfer Functions (FTFs) are presented and interpreted for all operation points. The FTFs for the two configurations differ most in the low frequency range where the influence of the droplet dynamics is expected to be highest. For both configurations, a change in thermal power does not affect droplet size, flame shape, NOx emissions and FTF. The observed trends in response to changes in equivalence ratio and atomizing air mass flow are opposite for both configurations. NOx emissions and flame shape are independent of the atomization air mass flow in the highly premixed configuration but not in the partially premixed configuration. In contrast to this, the FTF is affected by changes of the atomization air mass flow in both configurations, but again the trends are opposite. The observed trends for the highly premixed configuration are modeled and reproduced by a change in the phase relation between the equivalence ratio fluctuations and other instability driving mechanisms.

The generation and turbulent transport of temporal equivalence ratio fluctuations in a swirl combustor are experimentally investigated and compared to a one-dimensional transport model. These fluctuations are generated by acoustic perturbations at the fuel injector and play a crucial role in the feedback loop leading to thermoacoustic instabilities. The focus of this investigation lies on the interplay between fuel fluctuations and coherent vortical structures that are both affected by the acoustic forcing. To this end, optical diagnostics are applied inside the mixing duct and in the combustion chamber, housing a turbulent swirl flame. The flame was acoustically perturbed to obtain phase-averaged spatially resolved flow and equivalence ratio fluctuations, which allow the determination of flux-based local and global mixing transfer functions. Measurements show that the mode-conversion model that predicts the generation of equivalence ratio fluctuations at the injector holds for linear acoustic forcing amplitudes, but it fails for non-linear amplitudes. The global (radially integrated) transport of fuel fluctuations from the injector to the flame is reasonably well approximated by a one-dimensional transport model with an effective diffusivity that accounts for turbulent diffusion and dispersion. This approach however, fails to recover critical details of the mixing transfer function, which is caused by non-local interaction of flow and fuel fluctuations. This effect becomes even more pronounced for non-linear forcing amplitudes where strong coherent fluctuations induce a non-trivial frequency dependence of the mixing process. The mechanisms resolved in this study suggest that non-local interference of fuel fluctuations and coherent flow fluctuations is significant for the transport of global equivalence ratio fluctuations at linear acoustic amplitudes and crucial for non-linear amplitudes. To improve future predictions and facilitate a satisfactory modelling, a non-local, two-dimensional approach is necessary.

Alternative low carbon fuel blends are a promising way towards clean energy transition in the transportation and power generation sectors. In this work, the objective was to study the combustion characteristics of one such low carbon fuel blend (premixed Ammonia, Methane and Air) in a swirl stabilized Gas Turbine Can Combustor under varying % of pilot fuel flow (= 8 % to 10 % of the main fuel flow rate) at atmospheric pressure conditions. Pure Methane was used as the pilot flame which helped in the ignition and stabilization of the main flame and was kept on throughout the experiment. Different volume % of Ammonia and Methane blends were analyzed (starting from 10 to 50 % Ammonia in the fuel blend and the rest being Methane) at Reynolds number of the incoming air ~ 50000, and at equivalence ratio = 0.6 and 0.7. Characteristics such as Combustor liner wall heat load and flame stability were studied using the Infrared Thermography technique and High-Speed flame imaging respectively. Additionally, both carbon and NOx emission trends were estimated for selected cases using the CONVERGE CFD software under steady state conditions incorporating the RANS RNG k-ε and SAGE modeling techniques. Among all cases, wall heat load was observed to be the least for the 50 % Ammonia-50 % Methane case and for cases under reduced pilot %. Also, under reduced pilot %, flames were mostly unstable wherein the manifestation of instabilities at equivalence ratio = 0.6 and 0.7 were markedly different from one another.

Advances in high-performance computing have expanded the use of computational fluid dynamics (CFD) for reacting-flow analysis; however, simulations involving detailed flame kinetics remain computationally intensive for many practical systems. Efficient modeling approaches are therefore essential for predicting flame behavior in swirl-stabilized combustors. This study examines the influence of main-stage swirl intensity on near-lean blow-off characteristics in a multistage swirl combustor using a hybrid RANS–LES framework. The Stress Blended Eddy Simulation (SBES) model, coupled with a Flamelet Generated Manifold (FGM) combustion formulation, is employed to capture key turbulence–chemistry interactions. Results indicate that reducing swirl intensity suppresses the formation of a swirl-stabilized flame, while excessive swirl negatively affects emission performance. For the baseline (S2) and high-swirl (S3) configurations, flame lift-off height increases by 21.0% and 11.96%, respectively, for every 0.1 reduction in equivalence ratio. The S3 case also demonstrates reduced combustion efficiency, with CO emissions rising by 156.4% relative to S2. Local flame extinction is observed in regions of strong droplet–flame interaction, highlighting enhanced quenching susceptibility under near-blow-off conditions. The present study investigates the flame dynamics in a multi-stage swirl combustor using high-fidelity CFD simulations. This study has yet to be validated through experimental analysis and the results presented in this work are entirely computational. Further experimental validation is necessary to verify the results.

ABSTRACT An experimental study was performed to investigate the effects of flow swirl on flow/flame characteristics and stability of atmospheric premixed oxy-methane (CH4/O2/CO2) flames. The flames generated by two swirlers of 55° and 45° swirl angles were tested on a test stand for a dry low emission (DLE) model gas turbine combustor at constant inlet flow velocity of 5.2 m/s and over ranges of operating oxygen fraction (OF: 21% to 70% - by volume in the O2/CO2 mixture) and equivalence ratio ($\varphi $φ: 0.2 to 1.0). Combustor static stability limits (flashback and blow-out) were determined experimentally in the $\varphi $φ-OF domain to identify the operational ranges of the combustor while varying inlet flow swirl. To understand the mechanisms for flashback and blow-out, the lines representing the stability limits were displayed in the $\varphi $φ-OF domain against the contours of combustor power density (PD: MW/m3/atm), adiabatic flame temperature (AFT), and inlet flow Reynolds (Re). Comparison of flame macrostructure and measurements of local flame temperatures were performed for the two swirlers over ranges of $\varphi $φ, OF, and AFT to determine the effects of such operational parameters on flow/flame interactions and flame stability and to serve as a database for validating numerical models for such flames. The results show that, for both swirlers, the flames blow-out at a very similar AFT of ~1600 K indicating the dominant role of AFT in controlling premixed oxy-flame stability near the blow-out limit. Compared to the same combustor with a 55° swirler, the 45° swirler has a wider stable combustion zone. Comparing the flames of the same AFT, at fixed inlet flow velocity, shows almost identical flame macrostructure whatever the operating inlet flow swirl, OF and φ.

Effects of flow swirl on stability and flow/flame interactions of premixed oxy-methane flames (CH4/O2/CO2) are investigated experimentally and numerically in a premixed model gas turbine combustor. Two swirlers of 55° and 45° swirl angles were considered to perform this study over a range of combustor operating equivalence ratio (Φ=0.1-1.0) and oxygen fraction (OF=21%-70%) at constant inlet flow velocity of 5.2 m/s. Combustor stability maps (representing flashback and blowout bounds) were identified experimentally in the Φ-OF space for the two swirlers and the results were plotted over the calculated contours of adiabatic flame temperature (AFT). Specific flames were photographed using a camera to investigate the impact of flow swirl on flame macrostructure. Also, the shapes of the selected flames were calculated numerically using the contours of OH radicals and the results showed good agreement with the photographed flame shapes. Contours of temperature and flow streamlines were plotted based on numerical calculations to figure out the influence of flow swirl on flame/flow interactions. The results showed that CH4/O2/CO2 swirl flames blow out at fixed AFT of ~1600 K with no effect of swirl on flame stability near the blowout. Flow/flame interactions significantly affect flame stability near the flashback limit. Flame speed (FS) and AFT correlate with one another as log(FS) ∝ 1/AFT. The 45° swirler resulted in a wider stable combustion zone than that of the 55° swirler.

Predicting the blow-off (BO) is critical for characterising the operability limits of gas turbine engines. In this study, the applicability of a low-order extinction prediction modelling, which is based on a stochastic variant of the Imperfectly Stirred Reactor (ISR) approach, to predict the lean blow-off (LBO) curve and the extinction conditions in a hydrogen Rich-Quench-Lean (RQL)-like swirl combustor is investigated. The model predicts the blow-off scalar dissipation rate (SDR), which is then extrapolated using Reynolds-Averaged Navier–Stokes (RANS) cold-flow simulations and simple scaling laws, to determine the critical blow-off conditions. It has been found that the sISR modelling framework can predict the BO flow split ratio at different global equivalence ratios, showing a reasonable agreement with the experimental data. This further validates sISR as an efficient low-order modelling flame extinction tool, which can significantly contribute to the development of robust hydrogen RQL combustors by enabling the rapid exploration of combustor operability during the preliminary design phases.

This is the first study where a single variable sweep of swirl number (SN) is conducted to assess its impact on lean blowout limits (LBO) in a liquid fueled Lean Direct Injection (LDI) combustor. This study uses a scaled NASA SV-LDI (Swirl Venturi - Lean Direct Injection) hardware and is concerned with the impact of swirl number on the lean blow out limit of a single element LDI system at atmospheric pressure. The SN was varied from 0.31 to 0.66 using continuously variable active SN control system that was developed in-house. It is shown that the minimum operating equivalence ratio is a linearly increasing function of swirl number. While previous literature agrees with the positive slope for this correlation, past work has insisted that the LBO limit is proportional to the swirler vane angle of swirl cup flame holders which is shown to be untrue for LDI systems. By actively varying the swirl number, it is proven that LBO is proportional to SN, and it is well known that SN is not proportional to swirler vane angle. Increased SN reduces LBO margin because the better-mixed, high swirl cases dilute locally rich pockets of fuel air mixture. In addition to a baseline venturi, which was scaled from NASA's geometry, two other venturis were tested. A low pressure loss venturi with a large throat diameter showed poor blow out performance where as a parabolically profiled venturi improved LBO over the baseline for the same swirl number.

Towards the development of emission-free hydrogen gas turbines with wider operability limits, the combined effects of lean premixed combustion, dual combustion (combustion staging), hydrogen enrichment, and oxy-fuel combustion on the combustion and emission characteristics of CH4/H2/O2/CO2 flames are investigated in a dual annular counter-rotating swirl burner. The central stream is of a higher equivalence ratio (∅) of 0.9 for stable flame ignition, whereas the annular stream is of a lower ∅ of 0.65 to maintain the burner’s environmental performance. The stratified flame reactions are modelled using the partially premixed combustion model, which has been validated using the available experimental data. The inner recirculation zone becomes smaller as the hydrogen fraction (HF) becomes higher until it vanishes and there exists only an outer recirculation zone when the secondary hydrogen fraction approaches 100%. For 100% HF in both primary and secondary flow, the maximum combustion temperature within the combustor was observed to be as high as 2350 K. Increasing HF makes the flame more compact and anchored to the burner centre body with faster kinetics, less ignition delay time, and, accordingly, with increased potential for flashback. The product formation rate was found to increase in the inner shear layer at higher HF, implying a higher flame intensity and a more compact flame. The hydrogen fraction of the primary stream (maintained at higher equivalence ratio) is observed to be dominant in regulating CO mole fractions. Stable well-anchored flames are obtained over the considered fuel mixtures, indicating the role of stratified combustion in widening the operability of gas turbine combustors under stratified combustion conditions.

ABSTRACT OH* chemiluminescence and emissions of the convergent swirler module and the Venturi swirler module are studied in a single-element lean direct injection combustor. The OH* chemiluminescence images are captured by a CCD camera with an image intensifier, and the emissions are measured by a gas analysis system. The flame of the Venturi swirler module is closer to the inlet plane of the combustor compared to that of the convergent swirler module. At the equivalence ratio (ϕ) of 0.55, 0.65, and 0.75, the EINOx of the convergent swirler module is 1.02 g/kg, 1.44 g/kg, and 1.98 g/kg, and the EICO is 0.32 g/kg, 0.54 g/kg, and 3.65 g/kg separately. The EINOx of the Venturi swirler module is 0.95 g/kg, 1.37 g/kg, and 2.05 g/kg and the EICO is 0.32 g/kg, 0.93 g/kg, and 5.86 g/kg. The NOx emissions of the two swirler modules are similar. But the CO emission of the convergent swirler module is lower at the ϕ of 0.65 and 0.75. The convergent swirler module has an advantage in reducing emissions. In addition, the influence of the swirl number (S) on OH* Chemiluminescence and emissions of the convergent swirler module is studied. At the ϕ of 0.55, the EINOx is between 0.88 g/kg and 1.17 g/kg and the EICO is between 0.13 g/kg and 0.44 g/kg with different S. With the increase of the S, the NOx emission increases and the CO emission decreases. But at the ϕ of 0.65 and 0.75, with the increase of the S, the NOx emission first increases and then decreases and the CO emission first decreases and then increases.

Our study investigates the impact of ammonia- and water-based nano-particulate additives on the combustion characteristics of Jet-A1 aviation fuel, using a 300-kW liquid swirl combustor. Experiments were conducted at two global equivalence ratios (Φ = 0.24 and Φ = 0.40), focusing on laminar flame speed (LFS) and flame properties through chemiluminescence imaging and modal analysis techniques. The primary objective was to understand how these nano-additives modulate flame dynamics and internal chemical reactions, alongside evaluating the environmental implications of combustion alterations. Results showed that integrating urea and water additives into the fuel matrix affected LFS, enhancing it at the lower equivalence ratio but having detrimental effects at the higher ratio. Modal analysis revealed a notable stabilizing influence on flame behavior, especially under leaner fuel conditions. The addition of water and urea influenced combustion chemistry and spray patterns, leading to more uniform sprays and more complete combustion. Chemiluminescence imaging demonstrated higher emission intensity of NH2* radicals compared to NH* radicals, varying with the global equivalence ratio. The data indicated a significant reduction in NOx emissions, particularly at lower equivalence ratios, accompanied by a slight increase in CO2 and CO emissions. This study highlights the potential of ammonia- and water-based nano-additives to enhance the combustion performance and environmental outcomes of Jet-A1 aviation fuel, with the trade-off of increased CO2 and CO emissions requiring further consideration.

No abstract available

A variety of different flame configurations and heat release distributions exist in high swirl, annular flows, due to the existence of inner and outer shear layers as well a vortex breakdown bubble. Each of these different configurations, in turn, has different thermoacoustic sensitivities and influences on combustor emissions, nozzle durability, and liner heating. This paper presents findings on the sensitivities of the outer shear layer- stabilized flames to a range of parameters, including equivalence ratio, bulkhead temperature, flow velocity, and preheat temperature. There is significant hysteresis for flame attachment/detachment from the outer shear layer and this hysteresis is also described. Results are also correlated with extinction stretch rate calculations based on detailed kinetic simulations. In addition, we show that the bulkhead temperature near the flame attachment point has significant impact on outer shear layer detachment. This indicates that understanding the heat transfer between the edge flame stabilized in the shear layer and the nozzle hardware is needed in order to predict shear layer flame stabilization limits. Moreover, it shows that simulations cannot simply assume adiabatic boundary conditions if they are to capture these transitions. We also show that the reference temperature for correlating these transitions is quite different for attachment and local blow off. Finally, these results highlight the deficiencies in current understanding of the influence of fluid mechanic parameters (e.g. velocity, swirl number) on shear layer flame attachment. For example, they show that the seemingly simple matter of scaling flame transition points with changes in flow velocities is not understood.

No abstract available

Non-premixed swirl combustion has been widely used in pieces of industrial combustion equipment such as industrial boilers, furnaces, and certain specific gas turbine combustors. In recent years, the combustion instability of non-premixed swirl flames has begun receiving attention, yet there is still a lack of related research in academia. Therefore, in this study, we conducted experimental research on a swirl stabilized gas flame model combustor and studied the heat release response characteristics of the swirl combustor through the flame transfer function. Firstly, the flame transfer function (FTF) was measured under different inlet velocities and equivalence ratios, and the experimental results showed that the FTF gain curve of the non-premixed swirl flame exhibited a significant “bimodal” shape, with the gain peaks located around 230 Hz and 330 Hz, respectively. Secondly, two oscillation modes of the flame near the two gain peaks were identified (the acoustic induced vortex mode Mv and the thermoacoustic oscillation mode Ma), which have not been reported in previous studies on swirl non-premixed flames. In addition, we comprehensively analyzed the flame pulsation characteristics under the two oscillation modes. Finally, the coupling degrees between velocity fluctuations, fuel pressure fluctuations, and heat release fluctuations were analyzed using the Rayleigh Index (RI), and it was found that in the acoustic-induced vortex mode, a complete feedback loop was not formed between the combustor and the fuel pipeline, which was the main reason for the significant difference in the pressure fluctuation amplitude near 230 Hz and 330 Hz.

The development of lean-burn combustion systems is of paramount importance for reducing the pollutant emissions of future aero engine generations. By tilting the burners of an annular combustor in circumferential direction relative to the rotational axis of the engine, the potential of increased combustion stability is opened up due to an enhanced exhaust gas recirculation between adjacent flames. The innovative gas turbine combustor concept, called the Short Helical Combustor (SHC), allows the main reaction zone to be operated at low equivalence ratios. To exploit the higher stability of the fuel-lean combustion, a low-swirl lifted flame is implemented in the staggered SHC burner arrangement. The objective is to reach ultra-low NOx emissions by complete evaporation and extensive premixing of fuel and air upstream of the lean reaction zone. In the present work, a modeling approach is developed to investigate the characteristics of the lifted flame in an enclosed single-burner configuration, using the gaseous fuel methane. It is demonstrated that by using the Large Eddy Simulation method, the shape and lift-off height of the flame is adequately reproduced by means of the finite-rate chemistry approach. For the numerical prediction of the lean lifted flame in the SHC arrangement, the focus is on the interaction of adjacent burners. It is shown that the swirling jet flow is deflected towards the sidewall of the staggered combustor dome, which is attributed to the asymmetrical confinement. Since the stabilization mechanism of the low-swirl flame relies on outer recirculation zones, the upstream transport of hot combustion products back to the flame base is studied by the variation of the combustor confinement ratio. It turns out that increasing the combustor size amplifies the exhaust gas recirculation along the sidewall, and increases the temperature of recirculating burned gases. The present study emphasizes the capability of the proposed lean-burn combustor concept for future aero engine applications.