流场火焰对比度增强

The relationship between the flow field and flame propagation is essential in determining the dynamics and effects of turbulent flow in an optical SI engine. In this study, the high-speed PIV technique was applied to evaluate the incylinder velocity and turbulence distribution under firing condition. The simultaneous measurement technique of incylinder flow and flame propagation characterized the relationship between flow field and flame propagation in the same cycle. A 500 cc single-cylinder optical engine was used for this experiment. The bore diameter and stroke length were 86 mm and 86 mm respectively, and compression ratio was 10.4. The flame front configuration was extracted from the PIV image based on planar laser tomography method. The strong contrast between the intensity of burned and unburned region in the captured images due to low gas and seed density of the former was utilized to derive this flame front. In order to evaluate the interaction between flame and flow with high resolution, the interrogation area was set to 16 × 16 pixels. This corresponds to 0.75 × 0.75 mm. The spatial filter of 6 mm was used to separate the instantaneous flow velocity into a low frequency component and a high frequency component. The engine speed was 1200 rpm and absolute intake pressure was 60 kPa. The equivalence ratio was stoichiometry condition. The ignition timing were set to 19 deg.BTDC. The characteristic cycles in the same test condition were extracted and discussed. The bulk flow with the large scale as large as the size of the combustion chamber influences the overall shape of the flame propagation, and the state of the flame propagation greatly fluctuates from cycle to cycle. The flame propagates with enhancing the large scale existing tumble flow that exists in earlier crank angle locations. Focusing on the local flame structure, the high-frequency velocity component was strong near the local flame peak and weak near the local flame valley. The flame at the local peak region propagates while pulling the valley region.

Diffusion flame streets, observed in non-premixed micro-combustion devices, align parallel to a shear flow. They are observed to occur in mixtures with high Lewis number ($Le$) fuels, provided that the flow Reynolds number, or the Peclet number $Pe$, exceeds a critical value. The underlying mechanisms behind these observations have not yet been fully understood. In the present paper, we identify the coupling between diffusive-thermal instabilities and Taylor dispersion as a mechanism which is able to explain the experimental observations above. The explanation is largely based on the fact that Taylor dispersion enhances all diffusion processes in the flow direction, leading effectively to anisotropic diffusion with an effective (flow-dependent) Lewis number in the flow direction which is proportional to $1/Le$ for $Pe\gg 1$. Validation of the identified mechanism is demonstrated within a simple model by investigating the stability of a planar diffusion flame established parallel to a plane Poiseuille flow in a narrow channel. A linear stability analysis, leading to an eigenvalue problem solved numerically, shows that cellular (or finite wavelength) instabilities emerge for high Lewis number fuels when the Peclet number exceeds a critical value. Furthermore, for Peclet numbers below this critical value, longwave instabilities with or without time oscillations are obtained. Stability regime diagrams are presented for illustrative cases in a $Le$-$Pe$ plane where various instability domains are identified. Finally, the linear analysis is supported and complemented by time dependent numerical simulations, describing the evolution of unstable diffusion flames. The simulations demonstrate the existence of stable cellular structures and show that the longwave instabilities are conducive to flame extinction.

The stability of a thick planar premixed flame, propagating steadily in a direction transverse to that of unidirectional shear flow, is studied. A linear stability analysis is carried out in the asymptotic limit of infinitely large activation energy, yielding a dispersion relation. The relation characterises the coupling between Taylor dispersion (or shear-enhanced diffusion) and the flame thermo-diffusive instabilities, in terms of two main parameters, namely, the reactant Lewis number $ {{Le}} $ Le and the flow Peclet number $ {{Pe}} $ Pe. The implications of the dispersion relation are discussed and various flame instabilities are identified and classified in the $ {{Le}} $ Le- $ {{Pe}} $ Pe plane. An important original finding is the demonstration that for values of the Peclet number exceeding a critical value, the classical cellular instability, commonly found for $ {{Le}} \lt 1 $ Le<1, exists now for $ {{Le}} \gt 1 $ Le>1 but is absent when $ {{Le}} \lt 1 $ Le<1. In fact, the cellular instability identified for $ {{Le}} \gt 1 $ Le>1 is shown to occur either through a finite-wavelength stationary bifurcation (also known as type-I $ _s $ s) or through a longwave stationary bifurcation (also known as type-II $ _s $ s). The latter type-II $ _s $ s bifurcation leads in the weakly nonlinear regime to a Kuramoto-Sivashinsky equation, which is determined. As for the oscillatory instability, usually encountered in the absence of Taylor dispersion in $ {{Le}} \gt 1 $ Le>1 mixtures, it is found to be absent if the Peclet number is large enough. The stability findings, which follow from the dispersion relation derived analytically, are complemented and examined numerically for a finite value of the Zeldovich number. The numerical study involves both computations of the eigenvalues of a linear stability boundary-value problem and numerical simulations of the time-dependent governing partial differential equations. The computations are found to be in good qualitative agreement with the analytical predictions.

No abstract available

This study measures in-flame flow fields in a single-cylinder small-bore optical diesel engine using Flame Image Velocimetry (FIV) applied to high-speed soot luminosity movies. Three injection pressures were tested for a two-hole nozzle injector to cause jet-wall interaction and a significant jet-jet interaction within 45° inter-jet spacing. The high-pressure fuel jets were also under the strong influence of a swirl flow. For each test condition, soot luminosity signals were recorded at a high framing rate of 45 kHz with which the time-resolved, two-dimensional FIV post-processing was performed based on the image contrast variations associated with flame structure evolution and internal pattern change. A total of 100 combustion events for each injection pressure were recorded and processed to address the inherent cyclic variations. The ensemble-averaged flow fields were used for detailed flow structure discussion, and Reynolds decomposition using a spatial filtering method was applied to obtain high-frequency fluctuations that were found to be primarily turbulence. The detailed analysis of flow fields suggested that increased injection pressure leads to enhanced jet flow travelling along the bowl wall and higher flow vectors penetrating back towards the nozzle upon the impingement on the wall. Within the jet-jet interaction region, the flow vectors tend to follow the swirl direction, which increases with increasing injection pressure. The FIV also captured a turbulent ring vortex formed in the wall-jet head, which becomes larger and clearer at higher injection pressure. A vortex generated in the centre of combustion chamber was due to the swirl flow with its position being shifted at higher injection pressure. The bulk flow magnitude indicated significant cyclic variations, which increases with injection pressure. The turbulence intensity is also enhanced due to higher injection pressure, which primarily occurs in the wall-jet head region and the jet-jet interaction region.

The flame stabilization mechanism was investigated under the conditions of a Mach number of 2.52, a stagnation temperature of 1650 K, a stagnation pressure of 1.30 kPa, and a mass flow rate of 0.69 kg/s. Specifically, the study was conducted with nitrogen throttling (accounting for 3.4% of the inflow mass flow rate) and utilizing a rearwall-step cavity as the flameholder. The control experiments without throttling were performed under the same equivalence ratios of 0.20, 0.24, 0.28, 0.32, 0.36, and 0.40. The static pressure distribution, schlieren images, and CH* chemiluminescence images were employed to elucidate the combustion stabilization mechanism. As the equivalence ratio increases, the “Scram-Dual” and “Dual-Ram” processes exhibit intermittency under the no-throttling condition. Herein, “Scram-Dual” denotes the transitional phase between scram mode and dual mode, while “Dual-Ram” refers to the transitional stage between dual mode and ram mode. Under nitrogen throttling, by contrast, the processes tend to stabilize, with combustion oscillations effectively suppressed. The combustion zone can be subdivided into two distinct regions: the shear layer zone, characterized by stable flame intensity, and the jet wake zone, which exhibits strong oscillations that vary with the combustion mode. Throttling enhances flame stability, particularly within the shear layer zone. In contrast, flame instability in the jet wake zone is primarily influenced by flame geometry, as an increase in the windward area intensifies the blowoff effect induced by the inflow. In the dual mode, flame instability tends to intensify as the equivalence ratio increases. In the ram mode, by contrast, the effect of throttling is significantly diminished, and flame oscillations in the jet wake zone reach their peak, with negligible differences observed between the no-throttling and nitrogen-throttling conditions.

Recent progress in hydrogen combustion indicates that hydrogen could partially or fully replace traditional fuels in power plants, but maintaining stable flames remains a major challenge for many combustion systems. This study presents the effect of hydrogen enrichment of Liquid Petroleum Gas (LPG) on the low-swirl flame structure and flame temperature at different hydrogen mass fractions and equivalence ratios (φ = 0.501 and 1.04). The experimental observations for low-swirl flames under various conditions, including the effect of increasing hydrogen enrichment from 0% to ~20%, were discussed. Experiments were performed using a swirl burner, flame photography, and temperature measurements to evaluate the dynamic swirl flame, stability, and flame temperature distribution. The results show that moderate hydrogen enrichment (5–15%) improves flame stability and delays blow-off. In contrast, very high hydrogen concentrations may destabilize the flame due to higher reactivity and enhanced sensitivity to flow perturbations. Also, hydrogen enrichment up to ~20% enhances flame compactness, intensifies heat release, and reduces oscillatory instability without triggering blow-off or flashback, making hydrogen blending a promising strategy for stabilizing swirl flames at rich operating conditions. Finally, hydrogen enrichment consistently increases swirl flame temperature at both equivalence ratios.

No abstract available

Fluidic obstacles, employed as a form of turbulence generator, are often utilized to facilitate flame acceleration and deflagration-to-detonation transition (DDT). This paper conducts a detailed numerical study focusing on the position (L1) and the delayed injection time. The results indicate that reducing L1 leads to an earlier interaction between the flame and vortex structures, thus enhancing the initial flame acceleration effect. However, this also results in a reduction in the movement of vortex structures, which prevents the enhancement of turbulence intensity within the channel. Conversely, notwithstanding increasing L1 can improve the turbulence intensity within the channel, vortex structures fail to interact with the flame, which is unfavorable for the DDT process. Therefore, an optimal L1 exists which not only improves initial flame acceleration but also accommodates DDT. Furthermore, from a comprehensive perspective, the effectiveness of the delayed injection strategy is constrained by the range of L1. When L1 is small, the delayed injection strategy can enhance the time window for the flame–jet interaction, thereby improving turbulence and finally enhancing the DDT performance. However, as L1 increases, this improvement gradually diminishes and ultimately disappears. Regarding the DDT process, this study reveals that the distribution and strength of the wave structure in the channel, the size of the recirculation zone, the motion effect of the vortex structure, and various flow instability are the internal causes of DDT. The intensity of the pressure and velocity fields in the unburned zone ahead of the flame plays a crucial role in the DDT process.

This study investigates the effects and mechanisms of wall obstacle arrangements on flame acceleration and deflagration-to-detonation transition in spiral curved channels using quasi-direct numerical simulations. The results reveal that obstacle performance is strongly influenced by the curvature-induced asymmetric flow field, which fundamentally differs from that of straight tubes. Inner-wall obstacles couple effectively with the primary acceleration region, significantly enhancing flame wrinkling, vortex–flame interactions, and shock focusing, thus achieving the shortest initiation distance and earliest detonation onset among the uniform arrangements. In contrast, outer-wall obstacles interact weakly with the flame during the early stages and mainly contribute through delayed shock reflection, while symmetric double-wall obstacles provide intermediate performance due to dispersed disturbances. Mechanistic analysis shows that inner-wall obstacles trigger multi-point localized explosions through shock convergence, whereas outer-wall obstacles initiate detonation via reflection-induced hotspots. These findings highlight the necessity of adapting obstacle strategies to asymmetric flow fields. Furthermore, a novel segmented obstacle strategy is proposed and verified, demonstrating that placing inner-wall obstacles upstream and outer-wall obstacles downstream significantly enhances initiation performance. This work provides new insights into detonation control in curved micro-scale channels and offers guidance for the design of compact high-performance pulsed detonation devices.

Abstract This paper presents a hybrid and unsupervised approach to flame front detection for low signal-to-noise planar laser-induced fluorescence (PLIF) images. The algorithm combines segmentation and edge detection techniques to achieve low-cost and accurate flame front detection in the presence of noise and variability in the flame structure. The method first uses an adaptive contrast enhancement scheme to improve the quality of the image prior to segmentation. The general shape of the flame front is then highlighted using segmentation, while the edge detection method is used to refine the results and highlight the flame front more accurately. The performance of the algorithm is tested on a dataset of high-speed PLIF images and is shown to achieve high accuracy in finely wrinkled turbulent hydrogen-enriched flames with order of magnitude improvements in computation speed. This new algorithm has potential applications in the experimental study of turbulent flames subject to intense wrinkling and low signal-to-noise ratios. Graphic abstract

A computational fluid dynamics (CFD) numerical simulation methodology was implemented to model transient heating processes in steel industry reheating furnaces, targeting combustion efficiency optimization and carbon emission reduction. The effects of oxygen concentration (O2%) and different fuel types on the flow and heat transfer characteristics were investigated under both oxygen-enriched combustion and MILD oxy-fuel combustion. The results indicate that MILD oxy-fuel combustion promotes flue gas entrainment via high-velocity oxygen jets, leading to a substantial improvement in the uniformity of the furnace temperature field. The effect is most obvious at O2% = 31%. MILD oxy-fuel combustion significantly reduces NOx emissions, achieving levels that are one to two orders of magnitude lower than those under oxygen-enriched combustion. Under MILD conditions, the oxygen mass fraction in flue gas remains below 0.001 when O2% ≤ 81%, indicating effective dilution. In contrast, oxygen-enriched combustion leads to a sharp rise in flame temperature with an increasing oxygen concentration, resulting in a significant increase in NOx emissions. Elevating the oxygen concentration enhances both thermal efficiency and the energy-saving rate for both combustion modes; however, the rate of improvement diminishes when O2% exceeds 51%. Based on these findings, MILD oxy-fuel combustion using mixed gas or natural gas is recommended for reheating furnaces operating at O2% = 51–71%, while coke oven gas is not.

Numerical and experimental studies were conducted to uncover the physical aspects of a liquid jet injected into a supersonic crossflow with gas throttling systematically. The results were obtained with the inflow conditions of a Mach number of 2.0, a total temperature of 300 K, and a total pressure of 0.55 MPa. The results show that fuel–air mixing is considerably enhanced due to shock-induced flow distortion by adding gas throttling. The strength of downstream backpressure determines the distance of forward movement of the throttling shock wave train and the flowfield structure in the channel. When the mass flux of gas throttling is high, the influence of throttling gas spreads across the expansion section, resulting in significant flow separation in front of the liquid jet. It is found that the spray flashback phenomenon is similar to the flame flashback phenomenon that occurs in the supersonic combustion process under the action of a precombustion shock train. The wall counterrotating vortex pair and induced cavity streamwise vortices are enhanced with the increase of the flux of gas throttling. The relatively high-pressure environment generated by gas throttling promotes the atomization of droplets. As a result, the mixing enhancement mechanism of a liquid jet in a supersonic crossflow with gas throttling is mainly due to the combined effects of 1) the shock waves separating the side wall boundary layer and modifying the local flow state of air in the combustor, which lead to a dramatic increase in fuel–air mixing, and 2) the streamwise vorticity values as well as the residence time resulting from channel blockage elevating.

Hydrogen, with its carbon-free composition and the availability of abundant renewable energy sources for its production, holds significant promise as a fuel for internal combustion engines (ICEs). Its wide flammability limits and high flame speeds enable ultra-lean combustion, which is a promising strategy for reducing NOx emissions and improving thermal efficiency. However, lean hydrogen-air flames, characterized by low Lewis numbers, experience thermo-diffusive instabilities that can significantly influence flame propagation and emissions. To address this challenge, it is crucial to gain a deep understanding of the fundamental flame dynamics of hydrogen-fueled engines. This study uses high-speed planar SO2-LIF to investigate the evolutions of the early flame kernels in hydrogen and methane flames, and analyze the intricate interplay between flame characteristics, such as flame curvature, the gradients of SO2-LIF intensity, tortuosity of flame boundary, the equivalent flame speed, and the turbulent flow field. Differential diffusion effects are particularly pronounced in H2 flames, resulting in more significant flame wrinkling. In contrast, CH4 flames, while exhibiting smoother flame boundaries, are more sensitive to turbulence, resulting in increased wrinkling, especially under stronger turbulence conditions. The higher correlation between curvature and gradient of H2 flames indicates enhanced reactivity at the flame troughs, leading to faster flame propagation. However, increased turbulence can mitigate these effects. Hydrogen flames consistently exhibit higher equivalent flame speeds due to their higher thermo-diffusivity, and both hydrogen and methane flames accelerate under high turbulence conditions. These findings provide valuable insights into the distinct flame behaviors of hydrogen and methane, highlighting the importance of understanding the interactions between thermo-diffusive effects and turbulence in hydrogen-fueled engine combustion.

The ignition of a dual-combustion-zone supersonic combustor featuring a cavity and a backward-facing step (BFS) are investigated experimentally and numerically. The inflow conditions are Mach 2.52, stagnation pressure 1.6 MPa, and stagnation temperature 1486 K, with ethylene as the fuel at global equivalence ratios ([Formula: see text]) of 0.30, 0.35, and 0.40. The results show that cavity ignition is achieved by spark discharge for all tested conditions, where the spark generates an initial flame kernel confined in the recirculating flow. After a short growth phase limited by radical dissipation, the flame develops into a shear layer flame. In contrast, BFS ignition depends on the equivalence ratio. At [Formula: see text], ignition remains spark-driven, while at [Formula: see text] and 0.40, the BFS undergoes self-ignition followed by flame backpropagation. The self-ignition originates from combustion-induced flow restructuring. The lifted cavity flame compresses the mainstream, forming a high-temperature, high-pressure separation zone that traps fuel–air mixture and enables flame kernel formation. As the energy accumulates, coupled flow and thermal choking stabilize the flame kernel, which transitions to a self-sustained flame. It then propagates upstream through a fuel-rich channel and stabilizes as a BFS flame. These findings provide guidance for optimization and design of novel supersonic combustors.

Abstract Optical tomography, a critical component of process tomography, is an important tool for determining the absorption coefficient of cross-sectional media, with significant applications in multi-phase flow analysis, chemical processing, and combustion studies. However, the precise reconstruction of these media distributions is a major challenge. In this work, a sophisticated optical tomography (OT) system coupled with an innovative reconstruction algorithm is presented. The system architecture includes 25 light sources and 25 strategically placed fan-beam receivers. We present a convolutional neural network (CNN) with an encode-decode configuration, augmented by residual connections and a squeeze-and-excitation (SE) attention mechanism. Initial evaluations performed using MATLAB simulations showed the algorithm's superior performance compared to existing methods, with notable improvements in relative error (RE) and correlation coefficient metrics. Subsequent practical experiments validated these findings and emphasized the efficiency of the residual and SE components in improving reconstruction accuracy. While this study focuses on high-contrast binary scenarios, the proposed RESE-CNN framework provides a basic architecture for future extensions to weakly absorbing and scattering media where nonlinear reconstruction problems dominate.

The ignition of spray flames is a process of stochastic nature, especially relevant to the high-altitude relight of aeroengines. The present work aims to quantify the role of spray inhomogeneities in a realistic droplet-laden ignition set-up through Direct Numerical Simulations (DNS). Computations of the ignition process of weakly turbulent ethanol spray flows are performed using complex chemistry, a realistic energy deposition model and a polydisperse description of the spray. A good qualitative agreement of the flame radius evolution is found between the DNS and experiments, and both long-time failure and ignition modes are recovered with comparable radii, hence reproducing the experimentally-observed range of behaviour of ignition kernels. In contrast to experiments where the presence of large droplets at the spark impact both the actual energy deposited in the flow and the local fuel-air equivalence ratio, the effect of both parameters on the ignition outcome are assessed independently in this work by carefully choosing ignition locations in the flow representative of time-wise fluctuations. The minimum ignition energy (MIE) representative of the global flow condition is then evaluated using results from the DNS simulations and flow and spark characteristics. In order to understand the origin of the ignition stochasticity observed experimentally, local values of the fuel-air equivalence ratio $ \Phi _t $ Φt and turbulence level $ u' $ u′ at the spark plug are measured in the simulations using an averaging in volumes of size $ \Delta _b $ Δb. It is found that $ \Phi _t $ Φt best correlates to the MIE at three different locations in the domain for a size around 4 mm, which corresponds approximately to the kernel size at 0.1 ms, that is, the typical time scale of plasma processes and right when the flame becomes toroidal. The results give support to the experimental conjecture that stochasticity is largely due to the spatial fluctuations of $ \Phi _t $ Φt induced by polydispersity and in a lesser extent to those of $ u' $ u′.

The homogeneous field measurement of internal flow and spray of internal combustion engine injector nozzles under high pressure has always been one of the difficulties in experimental research. In this paper, an actual-size aluminum alloy nozzle is designed, and the simultaneous measurement of internal flow and near-field spray is successfully realized with the help of synchrotron radiation X-ray phase contrast imaging technology under an injection pressure of 30~90 MPa. For a 0.25 mm aperture nozzle, different radii of the inlet corner can induce different cavitation layer thicknesses, and the measured flow section shrinkage ratio is 0.70. The flow characteristics in the nozzle are entirely connected to the jet characteristics, indicating a tight correlation between internal flow and jet morphology. Finally, the internal cavitation of the nozzle was studied by the CFD simulation, and the simulation results are in good agreement with the experiment.

No abstract available

Femtosecond laser-induced grating scattering (fs-LIGS) technique has emerged as a robust diagnostic technique for combustion flow field characterization, particularly in temperature and pressure measurements. This study investigates four deep learning architectures for the simultaneous prediction of gas-phase temperature and pressure from raw fs-LIGS signals, including fully connected neural networks (FCNN), convolutional neural networks (CNN), bidirectional long short-term memory networks (BiLSTM), and a hybrid CNN-BiLSTM model. All architectures demonstrated exceptional predictive accuracy, achieving mean percentage errors ranging from -3.94% to 0.32% for temperature estimation and -0.26% to 1.09% for pressure estimation. The detailed comparison of the four models suggests that the LSTM structures have better adaptability for the measurements of temperature and pressure. In contrast the, CNN-BiLSTM model demonstrated superior overall performance in terms of predictive accuracy, cross-validation robustness, convergence rate, memory requirements, etc. Successful integration of these deep learning approaches with an fs-LIGS system would establish a novel framework for real-time, multi-parameter combustion diagnostics.

Solid fuel ramjets are a developing technology that will enable extended range for supersonic vehicles. The simplicity of the system in terms of moving parts results in the performance being dictated by the solid fuel combustion processes. The physics near the surface, particularly in the fuel-rich recirculation zone, are challenging to resolve due to flow opacity. Here, regression rate measurements of hydroxyl-terminated polybutadiene from a miniature, portable slab burner are reported. Mass-averaged regression rates agree with correlations from the literature, suggesting that the proper physics in the burner are captured. Synchrotron-based phase-contrast imaging is used to quantify local surface features, including a multiphase froth layer, molten polymer transport, and local regression rate measurements in the recirculation zone. Regression rates near the inlet are less than the mass-averaged values, whereas near the reattachment point the local value exceeds the mass-averaged values. The observed froth layer is as large as 2 mm near the inlet and decreases to sub-millimeter thicknesses near the reattachment point. An analytical heat transfer model is used to quantify the influence of the surface froth layer on the regression rates. Results indicate that the froth layer has an insulating effect and will reduce the local regression rate.

PURPOSE To develop a regularized image reconstruction algorithm for improved scan acceleration of phase-contrast (PC) flow MRI. METHODS Based on the magnitude similarity between bipolar-encoded k-space data, magnitude-difference regularization was incorporated into the conventional compressed sensing (CS) reconstruction. The gradient of the magnitude regularization was derived so the reconstruction problem can be solved using non-linear conjugate gradient with backtracking line search. Phase contrast flow data obtained in the peripheral arteries of healthy and patient subjects were retrospectively undersampled for testing the proposed reconstruction method. Three-dimensional velocity-encoded PC flow MRI was performed with prospective 4-fold undersampling for measuring arotic flow velocity in a healthy volunteer. RESULTS In the femoral arteries of healthy volunteers, the root-mean-square (RMS) errors of mean velocities were 0.56 ± 0.09 cm/s with CS-only reconstruction and 0.46 ± 0.08 cm/s with addition of magnitude regularization for three-fold acceleration; 1.34 ± 0.17 cm/s (CS only) and 1.08 ± 0.15 cm/s (magnitude regularized) for four-fold acceleration. In the iliac arteries of the patient, the RMS errors of mean velocities were 0.72 ± 0.12 cm/s and 0.56 ± 0.10 for three-fold acceleration, and 1.75 ± 0.21 and 1.24 ± 0.19 cm/s for four-fold acceleration (in the order of CS-only and magnitude regularized reconstructions). In the popliteal arteries, the RMS errors were 0.61 ± 0.10 cm/s and 0.42 ± 0.11 for three-fold acceleration, and 1.41 ± 0.19 and 1.12 ± 0.17 cm/s for four-fold acceleration. The maximum through-plane mean flow velocities were measured as 63.2 cm/s and 84.5 cm/s in ascending and descending aortas, respectively. CONCLUSION The addition of magnitude-difference regularization into conventional CS reconstruction improves the accuracy of image reconstruction using highly undersampled phase-contrast flow MR data.

No abstract available

No abstract available

No abstract available

No abstract available

Conventional phase-contrast (PC) MRI relies on electrocardiogram (ECG)-synchronized cine acquisition and respiration control. It often results in relatively low data acquisition efficiency, and is unable to assess blood flow variabilities. Real-time imaging is a promising technique to overcome these limitations; however, it results in a challenging image reconstruction problem with highly-undersampled (k; t)-space data. This paper presents a novel model-based imaging method, which integrates low-rank modeling with parallel imaging, to enable 4D real-time PC MRI without ECG gating and respiration control. The proposed method achieves an isotropic spatial resolution of 2.4 mm and temporal resolution of 35.2 ms, with three directional flow encodings. Moreover, it is able to resolve beat-by-beat flow variations, which cannot be achieved by the conventional cine-based approach. The proposed method was evaluated with in vivo experiments with one healthy subject and one arrhythmic patient. For the first time, we demonstrate the feasibility of 4D real-time PC MRI.

No abstract available

No abstract available

The pilot flame serves as the primary anchor for global flame stabilization in a centrally staged combustor. In engineering practice, it typically operates in the diffusion mode. The fuel non-uniformity and diffusion kinetics of the pilot flame may have a significant impact on the flow and flames within the combustor. The flame structure and flame–flow interaction in a centrally staged burner featuring a diffusion pilot flame are investigated in the present paper, using high-frequency CH2O planar laser-induced fluorescence (CH2O-PLIF), CH* chemiluminescence, and particle image velocimetry (PIV) measurements. The stratified flame (S-flame) and the lifted flame (L-flame) are identified under two-stage conditions. The S-flame and L-flame correspond to the separated flow and the merged flow of the two stages, respectively. Significant radial oscillation of the pilot stage airflow is also found. Extensive tests demonstrate that the pilot equivalence ratio (Φp) plays an important role in flame mode switching. Silicone droplets with extremely fine sizes are introduced into the pilot fuel to trace its transportation. When the oscillating pilot stage airflow rushes towards the lip in an instant, it can entrain the pilot fuel to reach the inner side of the main stage outlet. With a low pilot fuel supply and relatively low injection velocity, the pilot fuel and the hot radicals are more likely to be entrained and accumulate in larger amounts at the inner side of the main stage outlet. Consequently, the main stage premixed mixture can be ignited at the main stage outlet, forming the S-flame. The flame mode switches from S- to L-flame when the equivalence ratio increases to the point where the corresponding velocity ratio of pilot fuel to air (Vfp/Vap) approaches 1.0, with a reduced entrainment of the pilot fuel and radicals. Simultaneous CH2O-PLIF and flow field results show that when the main stage is ignited downstream, hot products cannot recirculate to the pilot stage outlet, causing the extinction of the pilot flame root. This paper reveals that the fuel diffusion characteristics of the pilot stage can dramatically change the flame structure. To achieve the ideal designed flame shape, the interaction between the pilot fuel and pilot air requires very careful treatment in practical centrally staged combustors.

The present study applies high-speed Mie-scattering spray imaging and flame image velocimetry (HS-FIV) to a jet fuel flame in a small-bore optical compression-ignition engine. The integration of sustainable aviation fuel (SAF) into the jet fuel supply may introduce challenges for stable engine operation due to low fuel reactivity. To resolve this issue, the engine is equipped with an ignition assistant plug providing additional heat into the compressed air. The impact of ignition assistant on the combustion of low reactivity fuel varies with injector tip protrusion, requiring detailed optical analysis to evaluate how the spray targeting changes relative to the piston bowl and thermal distributions. An optical engine is operated on a blend of 40% SAF and 60% F-24, a conventional jet fuel with mil-spec additives. The injector tip protrusion was varied between 1.5 and 4.5 mm below the cylinder head and for each protrusion, the high-speed movies were obtained with 30 cycles for sprays and 100 cycles for flames to address uncertainty concerns. The spray images were post processed via image binarisation to compute the liquid penetration length. An ensemble averaging method was applied to the FIV-derived flow fields to show the in-flame flow structure development while a spatial filtering approach was used for flow turbulence and combustion stability analysis. The spray image results exhibited decreased liquid penetration length and higher vaporisation for deeper injector tip protrusion, indicating higher temperature within the piston bowl. When the ignition assistant plug was activated, the FIV results showed similar overall flow structures and flow magnitude distribution to the plug off condition despite more advanced combustion phasing. However, lower cyclic variation was measured for the plug on condition, indicating more stable combustion as a key benefit of the active energy assistance. Regarding the injector tip protrusion variation, the shortest depth of 1.5 mm showed more retarded combustion phasing and lower peak pressure than those of 3.0 mm despite higher wall bounced-off flow magnitude. Enhanced vaporisation of the tested fuel blend as the injector tip was positioned deeper into the bowl was a likely cause of this observed trend. However, as the injector tip was protruded further to 4.5 mm, the combustion phasing was also more retarded and peak pressure was lower than those of 3.0 mm. Detailed flow field analysis showed lower magnitude flow vectors were observed for 4.5 mm depth as the jet impinged more on the floor of the piston bowl than the wall. The decreased wall bounce-off flow for deeper injector tip protrusion also led to lower flow turbulence measured in the r-θ plane. This outperformed higher fuel vaporisation expected, and thus the injector tip protrusion depth of 3 mm showed the most advanced combustion phasing and lowest cyclic variations.

No abstract available

In this study, we conduct a thorough evaluation of the STGSA-generated skeletal mechanism for C2H4/air. Two STGSA-reduced mechanisms are taken into account, incorporating basic combustion models such as the homogeneous reactor model, one-dimensional flat premixed flame, and non-premixed counterflow flame. Subsequently, these models are applied to more complex combustion systems, considering factors like flame-flow interaction and flame-wall interaction. These considerations take into account additional physical parameters and processes such as mixing frequency and quenching. The results indicate that the skeletal mechanism adeptly captures the behavior of these complex combustion systems. However, it is suggested to incorporate strain rate considerations in generating the skeletal mechanism, especially when the combustion system operates under high turbulent intensity.

No abstract available

No abstract available

Abstract Lean hydrogen/air premixed flame flashback in a turbulent boundary layer over a flat plate is investigated using three-dimensional direct numerical simulation with detailed chemical kinetics. The upstream propagation of the flame takes place in near-wall turbulence and the interaction between the flame and the approaching reactant flow is studied. It is found that backflow regions are always present immediately upstream of flame bulges that are convex towards the reactants, confirming earlier observations. A flashback speed, including the effects of flame displacement speed and flow velocity, is introduced to quantify the flame flashback behaviour. This analysis indicates that the flashback speed is overall positive and it is considerably affected by the presence of the backflow regions. A budget analysis of the pressure transport equation is performed to explain the presence of the backflow regions. It is suggested that the positive dilatation and thermal diffusion terms near the leading edge of flame bulges are the main reasons for the pressure increase, leading to an adverse pressure gradient. The effects of the flame-induced adverse pressure gradient on the structures of the turbulent boundary layer are also investigated. It is revealed that the near-wall mean velocity and skin-friction coefficient are reduced due to the adverse pressure gradient. The coherent vortical structures of the boundary layer turbulence are lifted by the adverse pressure gradient. The analysis of the Reynolds stress component showed that the ejection event is augmented by combustion while the sweep event is attenuated, which facilitates the occurrence of flame flashback.

Abstract Considering the complex phenomenon of in-cylinder flame impingement occurring in internal combustion engines, flame-wall interaction in different impinging distance cases is presented for the non-premixed impinging flames. In this article, the quenching mechanism of impinging flame is focused on using experiments and numerical simulations. The quenching information on the flow and thermal field is obtained in the experiment. Large Eddy simulation is used to provide the details in the flow characteristics. In the wall jet region, multiple vortexes are formed accompanied by strong turbulence and large wall shear stress, which becomes more significant with the increase of the impinging distance. The low-temperature region corresponds to a large wall heat flux and Nusselt number, and this non-positive correspondence between temperature and heat transfer leads to excessive heat loss and weakens the flame stability. The quenching behavior in the impinging flame is analyzed in terms of the flow field and thermal characteristics.

No abstract available

No abstract available

In this paper, simultaneous measurements using particle image velocimetry (PIV)/OH-planar laser-induced fluorescence (OH-PLIF) combined with fuel-PLIF testing are employed to obtain, data on the flow field, OH concentration field, and fuel concentration field of a typical combustor with primary holes under heated and pressurized conditions for the first time. The interaction between the primary jets and swirling flow is analyzed for both non-reacting and reacting operating conditions under variable fuel–air ratios (FARs). Combustion reactions accelerate the internal gases within the combustor, ultimately resulting in distinctly different flow field structures under non-reacting and reacting operating conditions. As the FAR increases in the experiments, the flame stabilization mechanism also changes. At low FAR, combustion reactions primarily occur within the primary recirculation zone (PRZ), with the flame stabilized in the shear layer where the PRZ intersects with the corner recirculation zone. However, as the FAR increases, unburned fuel returns to the PRZ with the primary jets to participate in reactions, while another portion participates in reactions in the secondary zone. At this point, combustion instability occurs. Proper orthogonal decomposition modal analysis reveals that this leads to enhanced fuel pulsation, further complicating the flow and combustion characteristics within the combustor.

The combustion flow field inside a scramjet combustor exhibits complex three-dimensional (3D) characteristics and interacts with the flame evolution. An experimental study on the 3D multi-sectional flow field visualization and the correlation between flow and flame dynamics within a strut-cavity based scramjet combustor is conducted with particle image velocimetry, high-speed photography, and proper orthogonal decomposition. Both non-reacting (Ma 3, 1600 K) and reacting (Ma 3, 2300 K) flows are tested. Four horizontal planes and one vertical plane in the rear of the strut are measured. Results show that the non-reacting flow is characterized by high-speed free stream on both sides (1100–1300 m/s) and a central low-speed recirculation zone (100–300 m/s). The recirculation zone narrows as it approaches the cavity and the vortices inside become more pronounced. The recirculating flow drives the flame to establish from the rear to the front, with an increasing velocity toward the top of the strut. The reacting flow consists of about 2/3 of the low-speed turbulent zone (100–400 m/s) and a high-speed free stream (900–1200 m/s) on one side. The stabilized flame repeatedly deflects horizontally, causing the relative positions of the high- and low-speed regions to switch. The recirculation behind the strut evolves into a large vortex, driving the detonation-induced flame to enter the cavity, bifurcate, and propagate backward to the upstream of the strut. The acquisition of the three-dimensional flow field and its interaction with flame behaviors helps deepen the understanding of scramjet combustion and provides data reference for engine design.

No abstract available

No abstract available

Micron-sized iron particles are promising carbon-free fuels. Efforts have been made to understand the combustion process of pure iron particles. However, it is not well-established how important are the impurities and their influence on combustion. The ignition and combustion characteristics of iron particles with 6.18 wt.% silicon were studied both by online optical diagnostics and offline scanning electron microscope (SEM) and X-ray Diffraction (XRD) analysis in this work. It is found that the ignition delay time is proportional to 𝑑 𝑛 with 𝑛 = 1 . 23 ∼ 1 . 56 , indicating that the oxidation reaction is unlikely solely limited by internal ionic diffusion before ignition. Compared with pure iron particles, the silicon-containing iron particle has a lower peak temperature and less oxidized combustion products. The intensity-rising time in the first intensity peak stage scales (1∕X O 2 ) 𝑛 with 𝑛 = 0 . 80 or 1 . 38 , which implies an external diffusion controlled combustion regime. Silicon in the oxide layer likely reduces the internal diffusion rate of oxygen, which becomes a rate-limiting step after the first intensity peak. Meanwhile, the surface tension also decreases due to the silicon content, which may facilitate the gas bubble nucleation and burst. The particle could partially change back to the external diffusion regime when the oxide layer is broken by the bubble expansion. This mechanism allows the reaction rate to increase and particle temperature to rise again. This explanation is verified by plenty of holes in the combustion products as well as the fact that the rising time in the large peak stage is proportional to (1∕X O 2 ) 𝑛 with 𝑛 = 0 . 37 or 0 . 32 . With the increase in the temperature of the ambient gas flow, evaporation was observed. The high ambient temperature is hypothesized to be a key factor to trigger the evaporation of silica.

The formation of soot and NOx in ammonia/ethylene flames with varying ammonia ratios was investigated through experimental and numerical analysis. The spatial distribution of the soot volume fraction and NOx concentrations along the flame central line were measured, and the mechanism of soot and NOx formation during ammonia/ethylene co-combustion was analyzed using CHEMKIN 17.0. The experimental results indicated that the soot volume fraction decreases with an increase in ammonia ratio, with the soot peak concentration occurring in the upper region of the flame. The distribution of NOx is complex. In the initial part of the flame, a higher concentration of NOx is generated, and the lower the ammonia ratio, the higher the concentration of NOx. As the combustion process progresses, the concentration of NOx initially decreases and then subsequently increases rapidly, with higher ammonia ratios leading to higher concentrations of NOx. The addition of ammonia results in a decrease in CH3, C2H2, and C3H3, and an increase in CN concentration. This leads to a transformation of carbon atoms within the combustion system, reducing the available carbon for soot formation and suppressing its generation. A higher ammonia ratio increases the likelihood that NH3 will be oxidized to N2, as well as increasing the probability that any generated NO will undergo reduction to N2 through the action of the free radicals NH2 and NH.

No abstract available

The design of fuel injection schemes is crucial for improving the combustion performance of high-Mach number scramjet. To clarify the feasibility of the coaxial jet injection scheme, high fidelity Large Eddy Simulation of the supersonic coaxial jet flame is conducted. The simulations are in good agreement with the experimental results in terms of time-averaged velocity, temperature, and species distribution. Auto-ignition phenomenon and the characteristics of partially premixed flame are well captured. The introduction of co-flow air increases the vorticity magnitude close to the injection port and downstream near-wall region, which results in a 400 K rise in the time-averaged temperature on the downstream near-wall region and a 4% increase in the proportion of premixed combustion near the injection port. Moreover, the instantaneous distribution of hydroxy radical indicates that the spanwise width of the windward reaction shear layer is reduced utilizing the coaxial jet scheme. Chemical kinetic analysis is applied to reveal the propagation mechanism of partially premixed flames. Thermal explosion is the chemical explosion mode for all coaxial jet flame front, which are dominated by a high-temperature reaction path. The low-temperature reaction path mainly exists in the transverse jet injection port, downstream near-wall region of the single transverse jet and co-flow lifted flame base. These significant findings provide valuable insights for the potential engineering application of the coaxial jet injection scheme to a high Mach number scramjet.

No abstract available

Flash-back characteristics of lean-premixed syngas swirl flames were investigated using simultaneous OH planar laser-induced fluorescence and stereoscopic particle image velocimetry at a repetition rate of 10 kHz. The syngas consisted of carbon monoxide, carbon dioxide, and hydrogen. A stable burning condition was first reached. While keeping the flow rates of air and other fuel components fixed, the hydrogen flow rate was increased incrementally until the upstream-propagating flame suddenly flashed from the combustion chamber back into the plenum and quenched. There existed a condition at which appropriate changes in air/fuel flow rates could prevent the flame from irreversible flash-back; these conditions defined the recoverable operation limits. Spectral proper orthogonal decomposition results revealed a transition in flow characteristics from the precessing vortex core instability to Kelvin–Helmholtz (K–H) instability under recoverable conditions with increasing hydrogen, closely related to flow symmetry. A linear trend was observed between the bulk velocities under critical conditions and the corresponding laminar flame speeds, indicating a strong correlation between flow instability transition and flash-back limits.

An afterburner encounters two primary features: high incoming flow velocity and low oxygen concentration in the incoming airflow, which pose substantial challenges and contribute significantly to the deterioration of combustion performance. In order to research the influence of oxygen content on the dynamic combustion characteristics of the afterburner under various inlet velocities, the effect of oxygen content (14–23%) on the field structure of reacting bluff body flow, flame morphology, temperature pulsation, and pressure pulsation of the afterburner at different incoming flow velocities (0.1–0.2 Ma) was investigated in this study by using a large eddy simulation method. The results show that two different instability features, BVK instability and KH instability, are observed in the separated shear layer and wake, and are influenced by changes in the O2 mass fraction and Mach number. The oxygen content and velocity affected the oscillation amplitude of the downstream flow. As the O2 mass fraction decreases, the flame oscillation amplitude increases, the OH concentration in the combustion chamber decreases, and the flame temperature decreases. Additionally, the amplitude of the temperature pulsation in the bluff body flame was primarily influenced by the temperature intensity of the flame and BVK instability. Moreover, the pressure pulsation is predominantly affected by the dynamic characteristics of the flow field behind the bluff body. When the BVK instability dominated, the primary frequency of the pressure pulsation aligned with that of the temperature pulsation. Conversely, under the dominance of the KH instability, the temperature pulsation did not exhibit a distinct main frequency. At present, the influence of oxygen content and incoming flow rate on the combustion performance of the combustion chamber is not clear. The study of the effect of oxygen content on the combustion characteristics of the combustion chamber at different incoming flow rates provides a reference for improving the performance of the combustion chamber and enhancing the combustion stability.

The combustion performance can be promoted by the shear layer, recirculation zones, and precessing vortex cores in the swirling flow field. The swirling flow promotes the mixing between air and fuel, but may also jeopardize the combustion stability. In order to study the interaction between the swirling flow structure and the combustion reaction, large eddy simulation (LES) is employed to simulate the turbulent combustion process of a typical swirling kerosene spray burner. A flamelet generated manifold (FGM) chemistry table built with three-component surrogate kerosene skeletal mechanism is coupled with LES to describe the combustion reaction. The FGM-LES approach is validated by comparing the velocity and temperature statistics against the experimental data. The obtained instantaneous LES snapshots of the kerosene flame are then analyzed using proper orthogonal decomposition (POD) and wavelet transform to investigate the time–frequency characteristics. The influence of swirling flow structures on the combustion reaction field is discussed. The results show that the first POD mode with the highest energy contribution is characterized by a low-frequency signal at 2.46 Hz. The second and third modes correspond to the double helix structure in the flow field, while the fourth and fifth modes correspond to the large vortex core structure in the central recirculation zone. The signals of combustion intermediate product hydroxyl species show higher amplitudes near the 78.62 Hz associated with the second and third modes, suggesting the two modes have a significant influence on the turbulent combustion characteristics of the swirling kerosene spray flame.

ABSTRACT This study investigated the flow and flame structure of a bluff-body stabilised CH4-H2 flame using the large eddy simulation (LES) and the flamelet progress variable (FPV) model. The performance of three subgrid-scale (SGS) LES models, i.e. the standard Smagorinsky model (SSM), the wall-adapting local eddy viscosity (WALE) model, and the dynamic Smagorinsky model (DSM) in predicting turbulent and reactive flow characteristics was assessed. Parameters, namely y+, Pope index (M), vorticity field, and resolved turbulent kinetic energy (kt), were analysed to determine the predictive capability of the three models. The results demonstrated that DSM and WALE successfully resolved 80% of kt for chosen grid configurations, whereas SSM exhibited discrepancies. The overall trends captured by all three models matched well with the experimental trend with DSM and WALE outperforming SSM in predicting reacting flow fields in the recirculation zone, while SSM produced better predictions in the regions away from the recirculating zone.

The effects of oxygen concentration in oxidizer flow with a low speed of 0.1–0.3 m/s on a co-current flame spread over a thin liquid fuel bed at microgravity is numerically studied. The soot model is based on the Laminar Smoke Point (LSP) concept, which was used to reproduce the behaviour of a non-premixed, heavily sooting laminar flame. The results including flame patterns, soot emissions, temperature, and liquid burning rate are examined. Pyrolysis rate of liquid fuel significantly increases by increasing forced flow velocity and oxygen concentration, favouring flame length and soot formation. The flame behaviour at very low strain rates depends on both radiative heat loss and combustion efficiency, which are affected by oxygen concentration. The reactive boundary layer is significantly lifted along the pyrolysis surface due to lack of oxygen in the growing boundary layer, and the 3D effects are of importance due to thermal expansion. The ratio between the flame stand-off distance and the boundary layer thickness converges toward unity, however, the soot resides within the boundary layer. Compared to a heptane flame, a dodecane flame has lower pyrolysis rate and more effective oxygen transport ensures intensive combustion. A high oxidizer flow velocity results in a longer flame, and a reduction in flame standoff distance from the flat plate.

No abstract available

No abstract available

No abstract available

Two models based on the deep learning-based convolutional neural network (CNN) and the re-parameterized convolutional neural network (RepCNN) were designed to reconstruct the flame in the combustor. Experiments were performed on a ground-pulse combustion wind tunnel at a fixed inlet Mach number of 2.5 and different pressures to inject hydrogen to obtain the relevant datasets. The results showed that both models could reconstruct the image of the flame in the combustor based on pressures of the upper and lower walls as well as the pressure at which hydrogen was injected. The average structural similarity index between the reconstructed image of the flame and its actual/original image was 0.9553, the average peak signal-to-noise ratio was 34.201, and the average correlation coefficient was 0.9819. The speed of reconstruction of the image using the RepCNN model improved by 40.7% at the cost of a slightly lower accuracy compared with the CNN model, and it took only 2.85 ms to reconstruct the image of a single flame. The lightweight feature of the RepCNN provides an important foundation for monitoring the model to reconstruct the image of the flame in real time. The work here simplifies requirements on the hardware for ground wind tunnel tests and provides a new idea for examining the characteristics of the flame in small combustors.

Effects of swirl divergence cup and the central bluff body on premixed flame response with external excitation are experimentally investigated. Flame transfer functions (FTFs) associated with different swirlers are measured in 50–450 Hz. The corresponding flame and flow responses are examined with the help of chemiluminescence images and Particle Image Velocimetry (PIV) method. Results show that FTF gain curves of swirlers with different divergence cups are characterized by alternating regions with first a minimum and then a maximum value as the excitation frequency increases. Increasing the divergence cup may greatly reduce the corresponding FTF minimum gain. Dynamic mode decomposition and proper orthogonal decomposition analysis indicate that flames with large divergence cup angles are dominated by the flame angle oscillations at the minimum gain point, while the flame with zero cup presents both the flame angle oscillations and vortex shedding. PIV results indicate that vortical structures located at the outer shear layer (OSL) could induce high-flame response, while the impacts of vortical structures located at inner shear layer are much weaker. Increasing the divergence cup could largely weaken the strength of vortical structures at OSL. In addition, effects of the central bluff body on flame response are significant. The flame in the swirler without the central bluff body is mainly governed by flame angle oscillations, and the elongated flame induced by the swirler with a large body is almost not sensitive to acoustic excitations. These results are useful for the understanding of flame response mechanisms in premixed swirling combustion.

To implement four‐dimensional‐flow MRI using phase‐contrast balanced steady‐state free precession (bSSFP) at 0.6 T using a free‐running three‐dimensional (3D) radial trajectory and referenceless background phase correction.

Light scattering from soot particles in hydrocarbon flames presents a significant source of background noise in particle image velocimetry (PIV) measurements. To mitigate this effect, the relative scattering intensities of 200 nm PIV seed particles and soot particles in a hydrocarbon-air flame were optimized for a 90∘ viewing angle. A polarization control assembly-comprising a Pockels cell, vertical polarizer, and half-wave plate-was implemented to vary the incident light polarization angle relative to the scattering plane. The observed trends in scattered irradiance agree with Lorenz–Mie theory predictions for the ratio of scattered light from a single PIV seed particle to that from a single soot particle, across a range of polarization angles. Experimental single-shot PIV images qualitatively illustrate the resulting improvements. Reduced soot-scattered light led to decreased velocity vector uncertainty and a lower incidence of vector dropouts in the processed PIV fields.

Coherent anti-Stokes Raman scattering (CARS) is a powerful nonlinear spectroscopic technique widely used in biological imaging, chemical analysis, and combustion and flame diagnostics. The adoption of pulse shapers in CARS has emerged as a useful approach, offering precise control of optical waveforms. By tailoring the phase, amplitude, and polarization of laser pulses, the pulse shaping approach enables selective excitation, spectral resolution improvement, and non-resonant background suppression in CARS. This paper presents a comprehensive review of applying pulse shaping techniques in CARS spectroscopy for biophotonics. There are two different pulse shaping strategies: passive pulse shaping and active pulse shaping. Two passive pulse shaping techniques, hybrid CARS and spectral focusing CARS, are reviewed. Active pulse shaping using a programmable pulse shaper such as spatial light modulator (SLM) is discussed for CARS spectroscopy. Combining active pulse shaping and passive shaping, optimizing CARS with acousto-optic programmable dispersive filters (AOPDFs) is discussed and illustrated with experimental examples conducted in the authors’ laboratory. These results underscore pulse shapers in advancing CARS technology, enabling improved sensitivity, specificity, and broader applications across diverse scientific fields.

Planar infrared visualization of species in flames is challenging due to the severe thermal radiation background and relatively weak fluorescence quantum yields from ro-vibration transitions. In this express, we report imaging of molecular species in a flame via an absorption-based coherent optical method, namely infrared polarization spectroscopy (IRPS). Single-shot, planar imaging of hydrogen fluoride (HF) has been achieved in a premixed CH(4)/O(2) Bunsen flame, being seeded with a small amount of SF(6). The HF molecule was excited through a rovibrational transition at around 2.5 µm, which belongs to the fundamental vibration band. High spatial resolution was guaranteed using an orthorgonal pump-probe geometry, and an effective suppression of thermal background emission was achieved owing to the coherent nature of the demonstrated two-dimensional IRPS. Other advantages, e.g. high temporal resolution and species-specificity, are also features of this laser-based technique, which make it suitable for imaging of non-fluorescent but infrared active gaseous molecules in harsh environments.

High-temperature furnaces and coal-fired boilers are widely employed in the petrochemical and metal-smelting sectors. Over time, the deterioration, corrosion, and wear of pipelines can lead to equipment malfunctions and safety incidents. Nevertheless, effective real-time monitoring of equipment conditions remains insufficient, primarily due to the interference caused by flames generated from fuel combustion. To address this issue, in this study, a through-flame infrared imager is developed based on the mid-wave infrared (MWIR) radiation characteristics of the flame. The imager incorporates a narrowband filter that operates within the wavelength range of 3.80 μm to 4.05 μm, which is integrated into conventional thermal imagers to perform flame filtering. This configuration enables the radiation from the background to pass through the flame and reach the detector, thereby allowing the infrared imager to visualize objects obscured by the flame and measure their temperatures directly. Our experimental findings indicate that the imager is capable of through-flame imaging; specifically, when the temperature of the target exceeds 50 °C, the imager can effectively penetrate the outer flame of an alcohol lamp and distinctly capture the target’s outline. Importantly, as the temperature of the target increases, the clarity of the target’s contour in the images improves. The MWIR through-flame imager presents considerable potential for the real-time monitoring and preventive maintenance of high-temperature furnaces and similar equipment, such as detecting the degradation of refractory materials and damage to pipelines.

Background suppression (BS) is recommended in arterial spin labeling (ASL) for improved SNR but is difficult to optimize in existing velocity‐selective ASL (VSASL) methods. Dual‐module VSASL (dm‐VSASL) enables delay‐insensitive, robust, and SNR‐efficient perfusion imaging, while allowing efficient BS, but its optimization has yet to be thoroughly investigated.

An electro-optical shutter (EOS), comprising a Pockels cell located between crossed-axis polarizers, is integrated into a nanosecond coherent anti-Stokes Raman scattering (CARS) system. The use of the EOS enables thermometry measurements in high-luminosity flames through significant reduction of the background resulting from broadband flame emission. A temporal gating ≤100 ns along with an extinction ratio >10,000:1 are achieved using the EOS. Integration of the EOS enables the use of an unintensified CCD camera for signal detection, improving upon the signal-to-noise ratio achievable with inherently noisy microchannel plate intensification processes previously employed for short temporal gating. The reduction in background luminescence afforded by the EOS in these measurements allows the camera sensor to capture CARS spectra at a broad range of signal intensities and corresponding temperatures, without saturation of the sensor, thus enhancing the dynamic range of these measurements.

Background-Oriented Schlieren (BOS) is a powerful technique for flow visualization. Nevertheless, the widespread dissemination of BOS is impeded by its dependence on scientific cameras, computing hardware, and dedicated analysis software. In this work, we aim to democratize BOS by providing a smartphone-based, open-access scientific tool called “Pocket Schlieren”. Pocket schlieren enables users to directly capture, process, and visualize flow phenomena on their smartphones. The underlying algorithm incorporates consecutive frame subtraction (CFS) and optical flow (OF) techniques to show the density gradients inside a flow. It performs on both engineered and natural background patterns. Using pocket schlieren, we successfully visualized the flow produced from a burning candle flame, butane lighter, hot soldering iron, room heater, water immersion heating rod, and a large outdoor butane flame. We have also demonstrated the flow visualization in liquid medium. It is able to detect minuscule refractive index variations up to the order of ~ 10–3. Pocket schlieren promises to serve as a frugal yet potent instrument for scientific and educational purposes.

Statement of the problem. The issue of automation and optimization of air supply regulation in radiant heater combustion systems is being investigated. The aim of the work is to develop an algorithm for regulating the air flow to increase energy efficiency, improve the stability of the combustion process and reduce emissions of harmful substances. The main objectives of the study are to develop a flowchart for controlling the air supply in burners and a description of all key control stages, as well as to analyze the mathematical dependencies describing the combustion process, including calculations of thermal efficiency and flame temperature. Results. During the work, a block diagram of the process of regulating the air flow in the burners of radiant heaters was developed. The analysis of the key stages of air supply control has been carried out and their impact on gorenje efficiency has been assessed. A mathematical model of the combustion process is constructed, describing the dependence of thermal efficiency on the air supply rate and the excess oxygen coefficient. Conclusions. The study showed that automation of the airflow control process contributes to improving the energy efficiency of systems by maintaining optimal combustion conditions. It has been found that the turbulence of the air flow improves the mixing of fuel with air, which leads to complete combustion and a reduction in emissions of pollutants. However, excessive turbulence can reduce the flame temperature and increase heat loss.

No abstract available

Integrated flameholder is widely used in advanced combustors. The unsteady characteristic of integrated flameholder combustion flow fields is critical. In this study, experiments and numerical simulation were carried out for the inspection of methods and analysis of their unsteady characteristic. By comparison of experimental and numerical results in two cases, the relative error is found to be about 10%.The comparative results denote that the large eddy simulation-transported probability density function method has positive performance in the calculation of pressure fluctuation in integrated flameholder combustion fields. After evaluating the numerical methods, the velocity and temperature field were analyzed as the basis of further unsteady analysis. Then, the fast Fourier transform (FFT) and empirical mode decomposition (EMD) were applied to analyze the velocity fluctuation and pressure fluctuation in the combustion flow field. The distribution and tendency of fluctuation frequency and amplitude are related to various factors. The velocity fluctuation frequency is low near the central axis, and the velocity fluctuation amplitude is high on both sides of the central axis; the pressure fluctuation frequency is low on either side of the recirculation zone, and the pressure fluctuation amplitude gradually increases downstream as the flame develops. Besides, the result of unsteady analysis demonstrates that FFT is suitable for the analysis of fluctuation amplitude, and the combination of EMD and FFT can give more information on the fluctuation frequency from complex nonlinear data.

ABSTRACT The hydrogen-air premixed combustion flow in the upward swirling combustor with different guide ring angles (from 75° to 115°) was investigated. An increase in the guide ring angle α enhances the velocity difference between its inner and outer sides, leading to an expansion of the central recirculation zone at the trailing edge of the guide ring. This prolongs the mixture residence time in the high-temperature zone, consequently increasing NO emission. The outlet NO emission without guide ring is 0.1159 ppm, lower than the values with most guide ring angles. The outlet velocity without guide ring is 56.92 m/s, the average velocity at the central cross-section is 15.51 m/s, both fall between the values at 75° and 115°, and are slightly higher than those at α = 85°. As α increases from 75° to 115°, on the central section of the combustor, the average flame stretch rate (κ) dropped from 439.81 S−1 to 424.52 S−1, the average temperature escalates from 1149.17 K to 1461.27 K, while the outlet temperature distribution factor (OTDF) decreases from 0.13 to 0.042. The regions with high flame stretch rates in the central recirculation zone and with low flame stretch rates in the downstream are observed to expand. However, the average flame stretch rate in the central section of the combustion chamber without guide ring is 450.05 S−1, higher than that with guide ring.

The supersonic combustion flow experiment conducted by the German Aerospace Center (DLR) employs a strut-based jet as a flame stabilization device, yielding comprehensive measurements including pressure, velocity, temperature and root mean square of turbulent fluctuating velocity. This dataset serves as a critical validation case for flamelet model evaluation. Previous RANS studies predominantly adopted two-dimensional (2D) simulations to simplify mesh generation. However, the DLR combustor configuration with 15 circular fuel injection holes inherently suggests three-dimensional (3D) flow characteristics. This study systematically compares 2D and 3D RANS approaches while varying the number of modeled fuel injectors (1-3 holes) to investigate geometrical dimensionality effects. Results reveal that under non-reacting conditions, congruence between the 3D computational solutions of single- and triple-injector configurations is observed, rendering fuel injector quantity irrelevant in 3D simulations. For reacting cases, weak inter-injector interactions cause minor discrepancies between the 3D computational results of single- and three-injector configurations. In both non-reacting and reacting flows, fuel injected from circular injectors initially exhibits three-dimensional effects. Turbulent mixing causes the spanwise gradients of flow parameters to diminish. Beyond 15 mm downstream of the injector exits, both non-reacting and reacting flows transition to quasi-2D states. Consequently, the flow distributions within the combustor predicted by 2D RANS have satisfactory agreement with 3D RANS results. Thus, 2D RANS can provide grid simplification advantages while maintaining reasonable fidelity.

ABSTRACT The influence of multiple direct injection nozzle arrangements with hexagonal, rhombus, and circular shapes on the combustion flow in gas turbines was numerically studied, and the combustion flow characteristics of swirl combustion combined with multiple direct injection combustion were analyzed, such as velocity and temperature distribution, vortex structure, NOX (nitrogen oxides) emission, flame stretch rate, etc. The result shows that for these different nozzle arrangements, the difference in velocity and temperature distribution is not obvious, but the difference in vortex structure is large. The hexagonal arrangement can form several counter-rotating vortex pairs (CVP) near the side wall, the rhombus arrangement can form several deflection vortex structures outside the center swirl, and the circular arrangement forms the least vortex structure. Furthermore, the circular arrangement is most effective in reducing the non-uniformity of the outlet temperature, OTDF is 0.00548, and it has a higher flame stretch rate, which means that it has a higher combustion intensity. However, the hexagon arrangement has the higher NOX emission, reaching 3.16 ppm.

The impact of the swirl number on the flow field of a single-stage swirl combustor is investigated using the particle image velocimetry technology. The variations in recirculation zone size, pulsating region, turbulent distribution, vorticity, and Reynolds stress within the combustor are summarized through quantitative analysis of the flow field. Experimental results indicate the following: (1) Under the same air mass flow rate, the length of the recirculation zone in the combustion state is shorter than that in the cold state. (2) The length of the recirculation zone and the axial vortex spacing display a decreasing trend as the swirl number increases, while the width of the recirculation zone demonstrates an increasing trend. (3) For the single-stage swirl combustor, the primary pulsating region is at the swirling jet area at the exit of the swirl. As the swirl number increases, the standard deviation of radial velocity fluctuations and turbulent kinetic energy also increase. (4) The strong shear region of the single-stage swirl combustor can be divided into inner and outer shear layers based on the vorticity distribution and the Q criterion. The vortices in the inner and outer shear layers exhibit opposite orientations according to the vorticity distribution. Overall, the research results can provide basic experimental data for numerical simulation of swirl combustion.

No abstract available

Femtosecond laser electronic excitation tagging (FLEET) is a molecular tagging velocimetry technique that can be applied in combustion flow fields, although detailed studies of its application in combustion are still needed. We report the applicability of FLEET in premixed CH4–air flames. We found that FLEET can be applied in all of the combustion areas (e.g., the unburned region, the burned region and the reaction zone). The FLEET signal in the unburned region is significantly higher than that in the burned region. This technique is suitable for both lean and rich CH4–air combustion flow fields and its performance in lean flames is better than that in rich flames.

The phase of a MRI signal is used to encode the velocity of blood flow. Phase unwrapping artifacts may appear when aiming to improve the velocity‐to‐noise ratio (VNR) of the measured velocity field. This study aims to compare various unwrapping algorithms on ground‐truth synthetic data generated using computational fluid dynamics (CFD) simulations.

A micro gas turbine swirl combustor with protruded bluff body is proposed. Compared with no bluff body, the protruded bluff body can significantly improve the combustion performance. When the bluff body height is 40 mm and the length is 30 mm, the average outlet NO emission is reduced by 14.43%, the pressure loss is decreased by 5.96%, and the outlet temperature distribution factor (OTDF) is dropped by 31.93%. The influence of different bluff body lengths (15–30 mm) and heights (10–50 mm) on the combustion flow are numerically analyzed further. The results show that the change of bluff body structure will affect the performance of combustor. In the range of 15–25 mm, with the increase of the bluff body length, the scope of central recirculation zone becomes narrower, velocity near the inlet and pressure loss decrease, the OTDF and field synergy angle β both become larger. When the bluff body length grows from 15 mm to 30 mm, the average outlet NO emission increases by 36.74%. The range of the central recirculation zone is gradually widen, and the average reaction rate grows up with the increase of bluff body height. The synergistic effect becomes better, and the heat transfer capacity is enhanced. When the bluff body height grows from 30 mm to 50 mm, the average outlet NO emission reduces from 35.84 ppm to 25.65 ppm.

To investigate the impact of different vortex structure on the combustion stability in a chamber, this paper evaluates the performance of a vortex combustion chamber coupled with a swirler and a guide ring. The simulated cases include different guide ring angles of 95°, 105°, 115°, 125° and 135°, in addition to a control case without coupling the guide ring. The distributions of velocity, vorticity, temperature, outlet temperature distribution factor (OTDF), reaction rate, turbulent kinetic energy (k), synergy angle (β), and NO emission are numerically analyzed. Results show that the coupling of the swirler and guide ring changes the vortex structure and significantly influences the chamber performance. The increase of the guide ring angles (α) enhances the interaction between the swirl flow and the direct jet, which intensifies heat and mass transfer, as well as turbulence. The application of the guide ring induces external entrainment of the jet, leading to the formation of a large central recirculation zone compared to that without a guide ring. With increasing the α from 95° to 135°, both average outlet velocity and tangential velocity (Vt) at the coaxial line position decrease, while the flame length decreases by 286 mm. And the OTDF variation among all cases is less than 0.065. The β of the central section decreases from 76.18° to 70.17°. Furthermore, combustion chamber without the guide ring exhibits higher temperature and OTDF.

Using sparse sensor data to reconstruct the global combustion flow state under supersonic combustion conditions is a novel problem, which is of great significance for breaking through the measurement and diagnosis technology of high-speed aircraft in extremely complex environments. However, the problem is particularly challenging when the number of sensors is extremely scarce. At present, the common end-to-end learning model usually has extremely strong generalization problems when reconstructing the combustion flow in the whole space, especially for the sparse pressure measurement system common in the actual ground wind tunnel test. To this end, this study adopts an encoder–decoder structure to construct an inversion method for turbulent combustion flow driven by sparse observation data. The channel aggregation algorithm is used to aggregate the sparse observation information obtained by sparse pressure sensors in different spaces, and the scale factor is used to adjust the numerical scale of the aggregated channel to prevent local channel information from being covered. In order to fully express the obtained feature information, the feature fusion attention network is used to enhance the detailed features of complex flow areas. The effectiveness of the proposed model is demonstrated on a ground test dataset acquired at multiple jetting backpressures. The results show that the proposed model can still effectively predict the global flow field, given 14 sparse observation points.

Compared to longitudinal ventilation, there are few studies on fire source development under semi-transverse ventilation. This work studied the influence of semi-transverse ventilation on the combustion characteristics of fire sources in a scaled tunnel. The burning rate and heat transfer feedback during pool fire combustion were revealed under different longitudinal and transverse ventilation velocities. The results showed that transverse ventilation had little influence on combustion characteristics, and the burning rate was more obviously affected by longitudinal ventilation. The heat convection feedback increased monotonically with the increase of the longitudinal ventilation, which led to the increase of the total heat feedback on the fuel. The heat radiation feedback changed little, and the heat conduction feedback decreased monotonically with the increase of the longitudinal ventilation velocity. By aid of a Fire Dynamics Simulator, it was found that the flame tilted downstream and was in the flow line of the lower cold air flow coming from upstream and the upper hot smoke flow outgoing in the downstream direction. The transverse ventilation of 2 m/s or lower hardly affected the combustion field of the fire source. Therefore, semi-transverse ventilation is preferable to longitudinal ventilation from the point of view of limiting fire expansion.

Abstract The pressure effects on the mixing fields of non-reacting and reacting jets in cross-flow are studied using large eddy simulation (LES). A hydrogen jet diluted with 30 % helium is injected perpendicularly into a cross-stream of air at four different pressures: 1, 4, 7 and 15 bar. The resulting interaction and the mixing fields under non-reacting and reacting conditions are simulated using LES. The subgrid scale combustion is modelled using a revised flamelet model for the partially premixed combustion. Good agreement of computed and measured velocity fields for reacting and non-reacting conditions is observed. Under non-reacting conditions, the mixing field shows no sensitivity to the pressure, whereas notable changes are observed for reacting conditions. The lifted flame at 1 bar moves upstream and attaches to the nozzle as the pressure is increased to 4 bar and remains so for the other elevated pressures because of the increasing burning mass flux with pressure. This attached flame suppresses the fuel–air mixing in the near-nozzle region. The premixed and non-premixed contributions to the overall heat release in the partially premixed combustion are analysed. The non-premixed contribution is generally low and occurs in the near-field region of the fuel jet through fuel-rich mixtures in the shear layer regions, and decreases substantially further with the increase in pressure. Hence, the predominant contributions are observed to come from premixed modes and these contributions increase with pressure.

Abstract This study investigates the impact of shockwaves induced by various combustor wall geometries – wedge, wavy wall, circular bumps, and triangular bumps – on scramjet combustor performance. Using CFD simulations in ANSYS Fluent 23.1 with RANS equations and the SST turbulence model, key parameters such as velocity, static pressure, temperature, turbulence intensity, and combustion efficiency are analyzed. Circular bumps exhibit superior performance, achieving higher turbulence intensity (42 %), enhanced fuel-air mixing, and minimized static pressure losses (8 %), optimizing supersonic flow dynamics. This geometry ensures better combustion efficiency, reducing unburned fuel and maximizing heat release. While wedge and triangular geometries improve mixing and flow stability, they are less effective than circular bumps. The wavy wall structure provides a balanced performance. The findings highlight the potential of circular bumps in advancing scramjet combustor designs, offering valuable insights for hypersonic propulsion system enhancements.

No abstract available

This study comprehensively investigates the turbulence flow within a four-fan stirred combustion furnace using experimental and numerical approaches. To analyze the impact of turbulence on fuel combustion, a prerequisite is to accurately obtain the control rules before fuel combustion. This study established a high-frequency sampling method using a hot-wire anemometer to quickly test the homogeneous and isotropic turbulence (HIT) region and employing a continuous laser combined with a high-speed camera to achieve particle image velocimetry measurements of the central two-dimensional flow field. A steady simulation combining the realizable k–ε model with a multiple reference frame was performed to further analyze the three-dimensional flow field. The results show that the adopted method has high accuracy. Within a central spherical region of approximately 40 mm, the flow field exhibited HIT characteristics, with turbulent fluctuation velocity urms varying linearly with fan speed ω (urms = 0.000 814 ω). Integral eddy length remains around 14 mm, while smaller-scale Taylor and Kolmogorov eddy significantly decreased with increasing ω. Full-field simulations reveal that the highest turbulence kinetic energy occurred at the intersection of adjacent fan flows. Additionally, the study explored the effect of a 4 mm glass bead on flow field, revealing a rebound effect on mean velocity and a reduction in urms near the bead. The boundary layer thickness decreases at higher fan speed but remains on the order of particle radius 2 mm. These findings provide a foundation for future research on the role of turbulence lack of mean flow on combustion.

No abstract available

No abstract available

No abstract available

Efficient combustion is essential for various applications, including gas turbines, industrial furnaces, and propulsion systems. The interaction between axial and tangential velocities significantly influences combustion stability, flame shape, and pollutant formation. However, a comprehensive understanding of these effects in a cylindrical vortex combustor remains elusive. The primary of this study is to analyse how axial and tangential velocities impact combustion characteristics within the Cylindrical Vortex Combustor (CVC). Specifically, we aim to determine the optimal velocity combination that ensures stable combustion, minimal emissions, and efficient energy release. The CVC geometry was modelled, and the simulations were conducted for various axial and tangential velocity combinations by Computational Fluid Dynamics (CFD) simulations using ANSYS Fluent software were employed to explore the behaviour of the vortex combustor under varying conditions. The Navier-Stokes equations, energy equation, and species transport equations were solved. Air inlet conditions included a mass flow rate of 40 mg/s and for fuel inlet was set 3.0 to 4.0 mg/s through an equivalence ratio (j) ranging from 0.5 to 1.5. The numerical findings indicate higher axial velocities enhance mixing and promote stable combustion and this velocity component excessive lead to flame blowout. The tangential velocities influence vortex strength and flame stability and will enhances recirculation and flame anchoring. Due to these conditions, an optimal balance between axial and tangential velocities yields efficient combustion. Understanding the interplay between axial and tangential velocities is essential for designing efficient vortex combustors. The findings provide valuable insights for optimizing combustion systems, reducing emissions, and improving overall performance.

No abstract available

No abstract available

Currently, the steelmaking process uses a pulverized coal injection (PCI) system that serves as the heat source and reductant for ironmaking (blast furnace and FINEX) where system uses expensive high-grade coal and high operating costs. Hydrogen steelmaking is currently being developed to achieve carbon-free operation. To achieve a soft-landing during this phase of rapid change, the use of biomass and inexpensive, thermal coal, and coke dust is necessary. Research on their combustion characteristics is necessary to apply these alternative fuels to PCI. Therefore, this study analyzed the combustion characteristics of ignition delay, devolatilization, and char combustion using a laminar flow reactor visualization equipment that simulates blast furnace (BF) and FINEX PCI tuyere, using flame image data processing. The ignition time were generally longer in BF than in FINEX, and the char combustion length and time also showed the same trend due to the high oxygen rate which indicate under 2 ms on ignition delay, under 16 ms on char combustion. Also, the volatile cloud was qualitatively shown in the image to be highest in thermal coal and biomass with high volatile matter. Based on the correlation and theoretical calculation with proximate analysis and the results, ignition delay time had a combined effect of volatile matter and moisture except coke dust, and char combustion time affected unburned carbon. The combustion chemical characteristics were discussed with chemical percolation devolatilization (CPD) model parameter. Through SEM image and BET analysis, the surface area has been increased more than 10 times after combustion. Consequently, the biomass and high moisture thermal coal could cofired within 10 % and coke dust could be cofired within 9 %, respectively.

Engine power and exhaust emissions result from the in-cylinder's fuel combustion process. The test rig for combustion visualization is one of the modern and specialized systems with high precision requirements. This research aims to design and fabricate a Constant Volume Combustion Chamber (CVCC) to investigate the combustion characteristics under various operating conditions such as fuel injection pressures, ambient pressures, ERG ratios, etc. This design concept depends on the diesel engine operating conditions with compression ratios of up to 26, fuel injection pressure in the range of 1600 bar; be the ability to combine the Z-type Schlieren optical method and high-speed camera to collect images of the combustion process. The strength of the structure was analyzed by ANSYS software with conditions of combustion pressure at 100 bar and 200 bar. This design was fabricated and experimented with the combustion conditions at ambient pressure of 42 bar, oxygen concentration of 21% after the pre-combustion phase, and fuel injection pressure of 800 bar, 1000 bar, and 1200 bar, respectively. The results revealed that the combustion chamber satisfied the durable requirement criteria, with the safety factor of 8.6 and 4.3 respectively under the mentioned simulating conditions. The leakage rate is negligible with 0.01% at an applied compression pressure of 80 bar in CVCC, approximately. The CVCC system can perform and record combustion process parameters at conditions similar to operating the diesel engine.

No abstract available

A deep learning network based on MVSNet multi-view 3D reconstruction network (IM-MVSNet) is proposed in this paper for the reconstruction of the surface of laminar flame, which could suppress the influence of background noise on the reconstruction data when reconstructing the surface of laminar flame and improve the reconstruction accuracy of the flame surface at the same time. The network obtains high-quality segmented images by image segmentation of the reference frames and neighboring frames of the input sampled images to remove the background noise during the time of sampling, then reconstructs the multi-view images in 3D to build a 3D point cloud of the laminar flame surface, finally obtains the reconstructed laminar flame surface. The comparison of computational results using different reconstruction models shows that the 3D reconstruction network proposed in this paper could effectively reduce the point cloud noise of the reconstructed flame surface, improve the reconstruction accuracy of the flame surface, and provide a novel technical means for combustion research.

Background-oriented Schlieren tomography (BOST) is widely used for 3D reconstruction of turbulent flames. Two major concerns are associated with 3D reconstruction. One is the time asynchrony within the data acquisition of the high-speed camera. The other is that the ray tracing process requires significant computational consumption. This study proposes a ray tracing optimization method based on the k-d tree. The study results show that the average search nodes for each ray are only 0.018% of 3D flame with 3.07 million grid nodes. In addition, a parameter estimation method of the unknown azimuth power spectrum function is proposed. First, a typical Sandia turbulent jet diffusion flame dataset was built and validated accordingly, with experiments. The algorithm’s applicability to the 3D reconstruction of temperature and density fields is discussed on this basis. The root-mean-square error (RMSE) of the cross-section density for 3D reconstruction is below 0.1 kg/m3. In addition, the RMSE of the cross-section temperature is below 270 K. Finally, an uncertainty analysis of the flame reconstruction based on a physical model is performed by optimizing the ray tracing method. For the time asynchronous variance of 1 ms, the density uncertainty of the 3D reconstruction is below 1.6 × 10−2 kg/m3, and the temperature uncertainty is below 70 K. The method can provide an essential basis for the design of BOST systems and the 3D reconstruction of turbulent flames.

BACKGROUND AND OBJECTIVE Compressed sensing (CS) has gained increased attention in magnetic resonance imaging (MRI), leveraging its efficacy to accelerate image acquisition. Incoherence measurement and non-linear reconstruction are the most crucial guarantees of accurate restoration. However, the loose link between measurement and reconstruction hinders the further improvement of reconstruction quality, i.e., the default sampling pattern is not adaptively tailored to the downstream reconstruction method. When single-contrast reconstruction (SCR) has been upgraded to its multi-contrast reconstruction (MCR) variant, the identical morphologic information as a priori source could be integrated into the reconstruction procedure. How to measure less and reconstruct effectively by using the shareable morphologic information of various contrast images is an attractive topic. METHODS An adaptive sampling (AS) based end-to-end framework (ASSCR or ASMCR) is proposed to address this issue, which simultaneously optimizes sampling patterns and reconstruction from under-sampled data in SCR or MCR scenarios. Several deep probabilistic subsampling (DPS) modules are used in AS network to construct a sampling pattern generator. In SCR and MCR, a convolution block and a data consistency layer are iteratively applied in the reconstruction network. Specifically, the learned optimal sampling pattern output from the trained AS sub-net is used for under-sampling. Incoherence measurement for single-contrast images and the combination of sampling patterns for multi-contrast data are guided by the SCR/MCR sub-net. RESULTS Experiments were conducted on two single-contrast and one multi-contrast public MRI datasets. Compared with several state-of-the-art reconstruction methods, SCR results show that a learned sampling pattern brings the quality of the reconstructed image closer to the fully-sampled reference. With the addition of different contrast images, under-sampled images with higher acceleration factors could be well recovered. CONCLUSION The proposed method could improve the reconstruction quality of under-sampled images by using adaptive sampling patterns and learning-based reconstruction.

BACKGROUND AND OBJECTIVE The quality of quantitative differential phase contrast reconstruction (qDPC) can be severely degenerated by the mismatch of the background of two oblique illuminated images, yielding problematic phase recovery results. These background mismatches may result from illumination patterns, inhomogeneous media distribution, or other defocusing layers. In previous reports, the background is manually calibrated which is time-consuming, and unstable, since new calibrations are needed if any modification to the optical system was made. It is also impossible to calibrate the background from the defocusing layers, or for high dynamic observation as the background changes over time. The background mismatch reduces the experimental robustness of qDPC and largely limits its applications. To tackle the mismatch of background and increases the experimental robustness, we propose the Retinex-qDPC. METHODS In Retinex-qDPC, we replace the data fidelity term of the previous cost function for qDPC inverse problem, by the images' edge features yielding L2-Retinex-qDPC and L1-Retinex-qDPC for high background-robustness qDPC reconstruction. The split Bregman method is used to solve the L1-Retinex DPC. We compare both Retinex-qDPC models against state-of-the-art DPC reconstruction algorithms including total-variation regularized qDPC, and isotropic-qDPC using both simulated and experimental data. RESULTS Retinex qDPC can significantly improve the phase recovery quality by suppressing the impact of mismatch background. Within, the L1-Retinex-qDPC is better than L2-Retinex and other state-of-the-art qDPC algorithms. CONCLUSIONS The Retinex-qDPC increases the experimental robustness against background illumination without any modification of the optical system, which will benefit all qDPC applications.

For a coal-fired boiler, it is always a difficult problem to reconstruct the internal temperature field. Due to the complex environment inside the furnace, the traditional reconstruction methods are not suitable for the furnace, so the current mainstream methods are based on the characteristics of the furnace internal flame image to reconstruct the temperature field. But the flame image inside the furnace is easily affected by external factors, especially the background. Therefore, it is very important to detect the edge of the flame image and extract the separate flame image for the subsequent temperature field reconstruction. For this purpose, a new edge detection method is proposed in this paper. First, a new rule is designed for color image to gray image. The rule can improve the gray difference between flame target and background area. Then the threshold segmentation algorithm is used to preprocess the flame gray image, and a new weighted edge image calculation method is designed to capture more details of the flame edge. Finally, an adaptive double threshold algorithm is used to remove the false edge in the flame edge. The simulation results show that the method is effective and can detect the continuous flame edge very well. It is of great practical value for the temperature field reconstruction in the furnace.

In order to address the issue of low segmentation accuracy in the weak flame region of waste incineration flame images and the potential loss of texture details at the flame edge, this study proposes an algorithm for segmenting incinerator flame images based on multi-step image enhancement. The algorithm consists of several steps. Firstly, a single-scale Retinex algorithm is employed to enhance the details of the noise-reduced image. Subsequently, an adaptive gamma correction method based on the inverse color transform is proposed to enhance the contrast between the image foreground and background, in conjunction with the inverse color transform algorithm. Finally, a 2D-Otsu algorithm is utilized to segment the image and extract the target flame region. Experimental results demonstrate that the proposed incinerator flame image segmentation algorithm based on multi-step image enhancement achieves superior segmentation performance, effectively preserving more detailed information in the weak flame region.

Background Oriented Schlieren (BOS) is an optical technique that visualizes and quantifies refractive index gradients in transparent media, used extensively in studying compressible flows, heat transfer, and combustion. The refractive index field can be converted to a density field using the Lorentz-Lorenz or Gladstone-Dale equations. Recently, the tomographic version of BOS has gained attention for its application in these fields. Sipkens et al. proposed the ARAP method for tomographic BOS, which improves reconstruction accuracy by accounting for light rays converging at the camera pinhole, a common setup with entocentric lenses. This study aims to assess the limitations of tomographic BOS reconstruction using numerical simulations of an axisymmetric hydrogen Bunsen flame, comparing the results with experimental BOS measurements. It has been observed that the constant composition assumption (i.e., assuming a unique value of the Gladstone-Dale constant independently from composition) may introduce some limited errors, whereas the implementation of the ARAP algorithm has a negligible impact on the computation of temperature distribution.

The necessity of minimizing NOx emissions drives the pursuit of ultra-lean premixed combustion in aeroengines and gas turbines, characterized by susceptibility to combustion instabilities. To tackle this issue, swirling flow design is widely incorporated into lean premixed combustor design, enhancing flame stability, and shortening flame length. This study utilizes the tomographic background-oriented Schlieren (TBOS) to reconstruct the spatial distribution of the refractive index gradient of lean premixed turbulent swirl flames with an aeroengine combustor configuration. A parametric study of the TBOS reconstruction quality is conducted, and the results reveal that view sparseness primarily degrades the reconstruction quality compared to the specific iterative algorithm used. The classic visual hull approach is explored to address this challenge, highlighting the significance of visual hull size. Furthermore, to improve the reconstruction quality, a posterior support constraint method is proposed, involving the removal of voxels of nearly constant refractive index within the central volume surrounded by flames. Results demonstrate that implementing this posterior support constraint further improves the reconstruction quality of lean premixed turbulent swirl flames. Finally, the robustness of this posterior support constraint method is validated by introducing high-level noise to the light deflection data, showcasing the potential of combining the dedicated designed visual hull and proposed posterior support constraint in addressing the view sparseness challenge for TBOS measurements.