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Experimental and theoretical investigations were performed on the propagation behavior of ammonia/hydrogen premixed flames, and an in-depth theoretical analysis of flame instability was carried out. The laminar burning velocity (LBV), burning flux, and net heat release rate (HRR net) were calculated. The instability was measured by flame thickness, thermal expansion ratio, effective Lewis number, and perturbation dimensionless growth rate. The effects of equivalence ratios (0.8, 1.0, 1.2, and 1.4), initial pressures (1, 2, and 3 bar), and hydrogen additions (15, 20, 25, and 30%) on ammonia/hydrogen premixed flames were determined. The findings indicated that as the hydrogen addition rises, both the LBV and burning flux increase, and the peak HRR net rises and shifts towards the low-temperature reaction zone, indicating that the diffusivity and reactivity of hydrogen can enhance the combustion intensity. The increment in the equivalence ratio improves the flame stability. Hydrodynamic instability effect always destabilizes the flame during the combustion process. In rich mixtures, the hydrogen addition improves thermal-diffusion stability, delaying the occurrence of flame instability whereas in lean mixtures, the hydrogen addition thermal-diffusionally destabilizes the flame. • Laminar burning velocity and burning flux of NH 3 /H 2 mixtures were measured. • The intrinsic instability of NH 3 /H 2 flame was investigated. • H 2 promotes the NH 3 /H 2 flame propagation and increases the heat release rate. • The hydrogen addition stabilizes the flame at high equivalence ratio. [ABSTRACT FROM AUTHOR]

The potential of chemiluminescence to develop non-intrusive sensors for the monitoring and control of turbulent ammonia–hydrogen flames is here investigated experimentally. This study looks into the impact of equivalence ratio (0.35 ≤ ϕ ≤ 1.70), NH 3 fuel fraction (0.55 ≤ X N H 3 ≤ 0.90) and Reynolds number (4000 ≤ Re ≤ 7000) on UV, visible and infrared chemiluminescence signatures and NO x emission of NH 3 /H 2 turbulent flames within an atmospheric tangential swirl burner. Chemiluminescence spectroscopy is employed to provide detailed information about the excited species (e.g., NO ∗ , OH ∗ , NH ∗ , NH 2 ∗ , and NO 2 ∗) in both in-flame and post-flame zones. Findings are compared to previous measurements in laminar flames and similar trends are observed. Many chemiluminescence intensity ratios are investigated but none are found to be potential surrogates of equivalence ratio and NH 3 fuel fraction across all the conditions considered. Therefore, a more advanced method based on machine learning is used to infer equivalence ratio and NH 3 fuel fraction from the chemiluminescence intensities of more than just two excited radicals at once. This method referred to as Gaussian Process Regression (GPR) is found to provide predictions of equivalence ratio and NH 3 fuel fraction with an accuracy better than 0.1 and 0.02, respectively, across the whole range of conditions. GPR is also able to predict the measured NO, N 2 O and NO 2 emissions using only measured chemiluminescence intensities, confirming the potential of chemiluminescence sensors coupled with a machine learning-based method for the monitoring and control of practical NH 3 /H 2 flames. • Chemiluminescence dataset of 252 test points for NH 3 -H 2 turbulent flames. • Chemiluminescence helps the detection and control of NOx emissions. • NO, NO 2 and N 2 O emissions are measured in 193 stable NH 3 -H 2 flame cases. • Machine learning used to build correlation between excited species and NO x. [ABSTRACT FROM AUTHOR]

To investigate the impact of ammonia volume fraction (X NH3) and obstacle blockage ratios on the explosion propagation of a hydrogen-ammonia mixture, this work conducts a numerical study of flame propagation in a closed duct under three different obstacle blockage ratio configurations, focusing on flame structure, propagation speed, overpressure development, key free radicals, and evolution process of vortex dynamics. The research reveals that increasing the X NH3 decreases the density gradient behind the flame front, weakens velocity shear across gas layers and flow instability, and inhibits the formation rate and quantity of key free radicals, which dampens flame propagation and explosion overpressure in closed ducts. Moreover, obstacle blockage ratios near the ignition source and at the end significantly influence flame propagation. As the X NH3 increased from 0 to 0.2, the steepest attenuation of the flame propagation speed is observed for the decreasing gradient of obstacle blockage ratio (BR-Down), with a 52% attenuation. The arrangement with an increased blockage ratio (BR-Up) exhibits the highest flame propagation speed and overpressure rise rate under all conditions, reaching 373.62 m/s and 2058.59 MPa/s respectively when the X NH3 = 0. Finally, the vortex distribution within the duct is visualized by the numerical simulation results, clearly illustrating the influence of the X NH3 and the obstacle blockage ratio on the generation and evolution of spatial vortices. The research offers a theoretical foundation for the safety of hydrogen-ammonia mixture explosive combustion, particularly in confined spaces containing obstacles. • Adding ammonia reduces flame speed and explosion pressure in ducts. • Flame propagation is significantly affected by obstacles near the ignition source and end. • Increased ammonia volume leads to more intense vortices and complex flame dynamics. • Faster flame propagation and higher pressure peaks are resulted from higher obstacle blockage ratios. [ABSTRACT FROM AUTHOR]

Understanding mechanisms controlling flame stabilization and pollutant emissions in swirled hydrogen flames at elevated pressures is crucial for advancing hydrogen-powered gas turbines. In this work, a single sector model gas turbine combustor operated with a coaxial dual swirl H 2 /air injector is installed in a high pressure test rig equipped with optical access. Flame stabilization and pollutant emissions of NO, NO 2 , and N 2 O are investigated at atmospheric injection temperature across a wide range of air and hydrogen injection velocities and operating pressures up to 8 bars. Two stabilization modes are identified: flames anchored to the hydrogen injector nozzle and flames lifted above the coaxial injector. It is shown that the air injection velocity required to lift the flame from the hydrogen injector rim increases with rising hydrogen velocity or pressure. However, with the current burner design, the lift-off air velocity reaches a plateau beyond 4 bars, regardless of the hydrogen inlet velocity. N 2 O emissions remain negligible for all operating conditions explored. Except at very lean operating conditions with global equivalence ratios below 0.3, NO 2 emissions are negligible too. It is finally shown that NO emissions scale with the adiabatic flame temperature, residence time in the flame volume, and pressure and that lifted flames typically yield lower NO emissions than anchored flames. The observations presented in this study help identifying critical flow parameters and lay solid foundations for the development of swirled hydrogen burners at elevated pressures. [Display omitted] • Air velocity at flame lift-off increases with pressure and hydrogen velocity. • NO emissions increase with pressure and equivalence ratio. • NO emissions decrease with hydrogen injection velocity and tend to a plateau. • NO 2 and N 2 O emissions are only significant at very low equivalence ratios. • NO x emissions scale with flame temperature, residence time and pressure. [ABSTRACT FROM AUTHOR]

To understand the effect of flame instability on the formation of the tulip flame, this paper modified the multi-phenomena combustion model and used the CFD code GASFLOW-MPI system to perform a numerical simulation of premixed stoichiometric hydrogen/air deflagration without considering the influence of flame stretch rate, Darrieus-Landau (DL) instability and thermal-diffusive (TD) instability based on the consideration of the heat transfer mechanism. The influence of different control factors on the formation of the tulip flame was compared and analyzed, and the accuracy of the numerical simulation was verified by combining it with experimental results. The research results showed that the coupling effect of DL instability and TD instability was the main reason for the formation of tulip flames, and DL instability played a dominant role. Furthermore, the DL instability determined the growth rate of the deflagration pressure amplitude and its fluctuation frequency. Finally, DL instability and TD instability had a more significant effect on flame propagation than heat loss. When either effect of DL instability and TD instability was ignored as a factor, the pressure gradient near the flame front decreased sharply and no vortex was generated. • The reason for the formation of tulip flames was obtained. • The Darius-Landau instability played a dominant role in the instability coupling. • Flame instability have a greater effect on flame propagation than heat loss. [ABSTRACT FROM AUTHOR]

Considering the requirements for developing hydrogen combustion chambers and the application potential of micro-mix combustion, the flame macrostructure and evolution of hydrogen-air multi-microjet flames have been investigated to discover the relationship between flame macrostructure and thermoacoustic instability. A novel burner has been tested under different combustor liner lengths to simultaneously produce stable and unstable combustion under the same operation conditions. A compact conical flame shape without adjacent flame front interference is observed. In comparison with the combustor liner length, the flame temperature variation shows an insignificant effect on the thermoacoustic instability but triggers an oscillation mode transition from low-frequency (∼210–240Hz) to high-frequency (∼400–440Hz) under unstable combustion. Different flame evolutions are revealed for these oscillation modes. DMD analysis and LES simulation show that the high-frequency oscillation mode is mainly controlled by flame front oscillation and flame pinch-off process. However, the low-frequency oscillation mode is characterized by flame extinction on a large scale. Both equivalent ratio and velocity fluctuations contributed to flame and heat release oscillations under low and high flame temperatures. These findings help understand the mechanisms, driving hydrogen-air flame dynamics, and designing hydrogen combustors. • A novel hydrogen micro-mix combustion burner was investigated. • A change in the liner length significantly impacted the combustion instability. • An increase in the flame temperature induced a transition of the oscillation mode. • The relationship between flame evolution and combustion instability was revealed. [ABSTRACT FROM AUTHOR]

NOx emission reduction is one of the major challenges when using ammonia/hydrogen blends as an alternative fuel for carbon-free combustion. In this study, chemical reaction pathways of NO formation in planar premixed flames of stoichiometric ammonia/hydrogen/air at atmospheric pressure and reactants temperature of 298 K are investigated under laminar and decaying turbulent conditions using quasi-DNS with detailed chemistry and the mixture-averaged transport model. The sweep parameter, $$\alpha $$ α , is the volumetric ratio of hydrogen to the ammonia/hydrogen mixture. Here, for unstrained laminar conditions, $$\alpha $$ α = 0, 0.4, and 0.6 and for turbulent condition, $$\alpha $$ α = 0.4 within the thin reaction zones region ($$Ka$$ Ka $$ \approx $$ ≈ 34.2) are studied. Under 1D laminar conditions, increasing $$\alpha $$ α enhances NO formation drastically, around 3 times when comparing $$\alpha $$ α = 0 and 0.6. At different $$\alpha $$ α , despite the high activity of the reactions which belong to the Zeldovich pathway, the contribution of this pathway is insignificant in NO formation. However, HNO and N2O pathways have the most significant roles. For instance, at $$\alpha $$ α = 0.4, R85 (NH+NO $$ \Leftrightarrow $$ ⇔ N2O+H) in the N2O pathway and R144 (HNO+H $$ \Leftrightarrow $$ ⇔ NO+H2) in the HNO pathway have major roles in NO formation. Higher NO formation at larger $$\alpha $$ α is found to be due to the increased H and O radicals within the reaction zone as well as the increased reaction rates because of the higher flame temperature. Under the 3D turbulent condition ($$\alpha $$ α = 0.4), it is observed that turbulence does not switch the pathways trends across the flame brush compared to the laminar condition. In the reaction zone, however, it is observed that NO formation is lower (higher) in the regions convex (concave) toward the reactants. The underlying reason for this effect is the preferential diffusion of H2 to the regions of the flame front convex toward the reactants, which consumes NO and also depletes O and OH radicals, which are necessary for NO formation. [ABSTRACT FROM AUTHOR]

Combining experimental research and numerical simulation, this paper studies the effects of obstacle position and blockage ratio on hydrogen flame evolution and explosion characteristic. The mechanism of obstacle-induced turbulence on the hydrogen flame acceleration is revealed. The results show that in a double obstacle tube, When the obstacle spacing is larger, the turbulent combustion intensity and pressure rise rate is relatively higher, and the pressure oscillation is enhanced. The sufficient turbulent combustion promotes the formation of stronger vortex motion at the downstream obstacle. As the blockage ratio increases, the turbulent combustion intensity, flame front speed, and pressure rise rate increases significantly. While obstacles with a high blockage ratio have a blocking effect on the pressure, slightly weakening the pressure oscillation. As the blockage ratio increases, the jet speed at the obstacle increases, the scale and speed of vortex increase, and the vorticity magnitude of unburned and burned area increases. • Effects of obstacle position and blockage ratio on hydrogen explosion are investigated. • A numerical model is developed to analyze the flame evolution and pressure dynamic. • Mechanism of obstacle-induced turbulence on the hydrogen flame acceleration is revealed. [ABSTRACT FROM AUTHOR]

Lifted multi-jet flames of hydrogen and steam-diluted oxygen are studied using direct numerical simulations (DNS). The DNSs have been carried out for laminar and turbulent flame configurations. The simulation data are analyzed to investigate the flame and flame-base structures and the flame-base propagation in a configuration akin to a gas turbine with multiple fuel jets. The laminar flames have a tribrachial structure, although lean premixed and diffusi on flame branches collide downstream owing to preferential diffusion. The turbulent multi-jet flames have a single connected base region, whereas the laminar flames have separate bases, each of which is associated with the corresponding fuel jet. Although turbulent multi-jet flames have substantially different flame-base characteristics from laminar multi-jet flames (and conventional single-jet lifted flames), the propagation speed is similar for both conditions. The results suggest that the well-known flame-base stabilization theory can be straightforwardly applied to the multi-jet configuration, despite the turbulent flame structure's unconventional features. [ABSTRACT FROM AUTHOR]

In order to study the flame retardant performance of corrugated flame arrester core on the explosion characteristics of 9.5% hydrogen/methane/air premixed gas, a self-built experimental platform was used to study the explosive characteristics and propagation rules and the impact of varying thicknesses and different volumes of hydrogen. The results indicated that the corrugated flame arrester core on the explosion flame of premixed gas was observed with two scenarios, quenching and penetration. The flame is dominated by reverse vortex flow after quenching, reversing the axial direction of flame propagation. However, the flame penetration leads to more intense burning reactions as φ increases. And the increase in thickness changes the flame structure forms a spherical flame to a mushroom-shaped flame and finally form a turbulent flame. The velocity of flame decreased significantly when L = 60 mm with φ = 10% and 20% respectively. The quenching effect is more obvious in P2 with hydrogen added, and its better performance with increasing thickness, with 9.76% and 3.70% decrease when L = 40 mm with φ = 10% and 20%, respectively, and with 57.05% and 48.55% decrease when L = 60 mm with φ = 10% and 20%, respectively. [ABSTRACT FROM AUTHOR]

This study investigates the suppression effects of heptafluoropropane (CF 3 CHFCF 3)–CO 2 mixtures on the premixed hydrogen-air explosion. The experiments are conducted in a stainless-steel duct system with the concentrations of CF 3 CHFCF 3 and CO 2 ranging from 1% to 4% and 10%–15% respectively. The hydrogen-air equivalent ratio, Φ is varied from 0.8, 1.0, and 1.2. The "finger" shape flame front of the baseline cases changes to the "reversed slope" shape and is significantly dimmed at the center and end sections of the duct as each suppressant concentration increases. When 4% CF 3 CHFCF 3 -15% CO 2 is used, the P max of the fuel-lean (Φ = 0.8) and stoichiometric (Φ = 1.0) cases are reduced by 46.8% and 57.5% compared to baseline cases. The maximum pressure arrival time, t max increases by 16.2 and 25.8 times. The fuel-rich mixture (Φ = 1.2) cannot be ignited under this mixture concentration. Furthermore, the chemical kinetic simulation shows that the addition of the CF 3 CHFCF 3 –CO 2 mixture noticeably reduces the adiabatic flame temperature and CF 3 CHFCF 3 promotes the consumption of O and OH radicals through dehydrogenation reactions (R1466, R1475). At Φ = 1.2 , CF 3 CHFCF 3 inhibits reactions by consumption of H radicals (R1476). CO 2 mainly acts as a physical inhibitor with kinetic coupling with CF 3 CHFCF 3 , which effectively reduces the explosion enhancement of low-concentration CF 3 CHFCF 3. The results provide theoretical support for explosion mitigation strategies in hydrogen applications. • The addition of CO2 enhances the CF3CHFCF3 suppression effects with reduced use of both suppressants. • The "reverse slope" flame is observed at the 4 % CF3CHFCF3 and the flame speed decreases with CO2. • The mixtures effectively reduce the flame temperature and the peak molar fraction of key radicals. • The undesired reaction enhancement caused by CF3CHFCF3 is mitigated by coupling effects of CO2. [ABSTRACT FROM AUTHOR]

This paper presents a theoretical framework to predict the jet flame length of type III high-pressure hydrogen storage tanks, thereby developing safety discharge strategies. The non-adiabatic tank heat exchange and under-expansion jet models, accounting for friction, predicted venting pressure and discharge time with an error of ∼11.5%. The Froude model accurately predicted the jet flame length during blowdown. The critical tank status under different fire times has little impact on the jet flame length (2%–2.5%). When the TPRD diameter drops from 6 to 1 mm, the flame length shortens from 9.31 to 1.44 m. Friction in the tank valve and discharge pipe extended the discharge time by 2.4 times compared to a frictionless scenario. A nomograph for type III tank (NWP = 35 MPa) discharge time under fire scenarios was proposed, which helped rescuers to determine the discharge time as well as to assist designers in identifying the safe discharge diameter. [Display omitted] • Proposed a theoretical framework model of tank jet flame. • Predicting the tank pressure and discharge time with an error of about 10%. • The froude model accurately predicts the change of jet flame length. • Increased critical pressure minimally affects the discharge time and flame length. • Propose a safe discharge strategies for NWP 35 MPa tanks based on nomogram. [ABSTRACT FROM AUTHOR]

Due to the high costs associated to experimental testing of hydrogen (H 2) explosions, numerical simulations play an essential role in the design of safe industrial systems. In this work, we investigate the use of the Thickened Flame Model (TFM) to perform simulations of H 2 deflagrations. Specific issues arise when considering hydrogen: (i) due to differential diffusion, the flame is very sensitive to stretch; (ii) instabilities of the flame front are generated and lead to an increased flame propagation speed. Both of these phenomena are affected by flame thickening. First, the flame sensitivity to stretch is artificially increased by the thickening. The recently developed Ma-TFM model proposes a correction of diffusive fluxes to reach the correct Markstein length and is selected in this work. Secondly, thermo-diffusive and hydrodynamic flame instabilities are also altered by thickening. By computing the dispersion relationship for a hydrogen/air mixture, we show that the instability scales are proportional to the thickening factor F. A simple subgrid scale (SGS) model, based on the computations of laminar unstable 2-D and 3-D flames, is then considered to take into account the effect of instabilities. The Ma-TFM and SGS instability models are applied together for the first time on a semi-industrial vented lean hydrogen deflagration, leading to promising results. In particular, it is found that considering 3-D unstable flames is necessary to accurately predict the experimental pressure peaks. [Display omitted] • Stretch corrected Thickened Flame Model coupled to a model for flame instabilities. • Thermodiffusive instability scales are proportional to the thickening factor. • Successful simulation of a semi-industrial vented hydrogen deflagration. • 3-D nature of thermo-diffusive instabilities needs to be considered. [ABSTRACT FROM AUTHOR]

Hydrogen has drawn great attention in recent years as a carbon-free fuel. The turbulent burning velocity ( S T ) is an important parameter for the design and modeling of hydrogen-fueled engines given the high propagation speed of hydrogen flames. It has been well documented that S T of hydrogen flames can be dramatically increased by thermo-diffusive effects which are sensitive to thermodynamic conditions. Previous studies have mainly focused on the pressure effect on S T of lean hydrogen flames, while the temperature effect has been largely ignored. In the present study, the turbulent burning velocity for a lean hydrogen/air mixture over a wide range of temperatures (300–641 K) and pressures (1–15 atm) is investigated through direct numerical simulations of statistically planar turbulent premixed flames. Results show that the variation of normalized turbulent burning velocity ( S T / S L , where S L is the laminar flame speed) with temperature and pressure is mainly controlled by the variation of the stretching factor I 0 . While S T / S L is only marginally dependent on temperature at the atmospheric pressure, it exhibits a decreasing trend with temperature at an elevated pressure (10 atm). This is associated with different temperature dependencies of flame surface area enlargement at the two different pressures, despite the monotonically decreasing trends of I 0 with temperature at both pressures. In addition, under engine-relevant conditions where the temperature and pressure increase simultaneously, the promotion effect of pressure is found to be largely canceled out by the suppression effect of temperature, leading to only a slight increase in I 0 and S T / S L . The observed trends are further explained through detailed flame dynamic analysis. Furthermore, I 0 at different temperatures and pressures is found to correlate very well with the enhancement of fuel consumption rate in the critically strained laminar flames. The present study elucidates the strong impact of temperature on S T of lean premixed hydrogen/air flames at elevated pressures and provides new insights into the modeling of S T , especially for engine-relevant conditions. [ABSTRACT FROM AUTHOR]

The benefits of using hydrogen as a fuel for rocket engines dictate the necessity of a deep understanding of possible scenarios of hydrogen ignition and subsequent combustion. Safety requirements on launch sites and hydrogen transportation and storage facilities should be elaborated based on a thorough investigation of hydrogen flame development. The paper aims to provide a detailed numerical analysis of the hot wall ignition of hydrogen-air mixtures with compositions near low flammability limits. It is shown that the development of ultra-lean flames initiated by a hot spot on the solid wall possesses several features compared to ultra-lean flames ignited by a point energy source. Three modes of flame development are observed: stable flame column with bow-shaped flame structure on top, individual flame kernel, and unstable flame column with multiple kernels generation. Effects of mixture composition, along with the impact of hot spot size, are analyzed. Column tip acceleration mechanism is determined. All of the combustion modes obtained are grouped in the diagram, providing information on the limits of various combustion modes depending on the mixture concentration and hot spot size. • Ultra-lean hydrogen-air flame initiated by a hot spot on a wall is analyzed. • Effects of mixture composition and size of igniting spot are evaluated. • Individual flame kernel, stable flame column and puffing modes are observed. • Mechanism of flame acceleration in flame column mode is identified. [ABSTRACT FROM AUTHOR]

To ensure the safe utilization of hydrogen-enriched natural gas (HENG), it is essential to explore effective explosion suppressants to prevent and mitigate potential explosions. This study experimentally investigates the impact of ultrafine water mist containing K2CO3 additives on the explosion characteristics of methane/hydrogen/air premixed combustion. The influence of varying K2CO3 concentrations on pressure rise rates and flame propagation was analyzed across different hydrogen blending ratios. The results demonstrate that the addition of K2CO3 to ultrafine water mist significantly enhances its suppression effects. The peak overpressure decreased by 41.60%, 56.15%, 64.94%, and 72.98%, the flame speed decreased by 30.66%, 70.56%, 46.72%, and 65.65%, and the flame propagation time was prolonged by 25%, 20.83%, 22.92%, and 18.75%, respectively, for different hydrogen blending ratios, showing a similar trend. However, the suppression effectiveness diminishes under high hydrogen blending ratios and low K2CO3 concentrations. Further analysis using thermogravimetric infrared spectroscopy and chemical kinetics simulations revealed that the heat release rate and the generation rate of active free radicals significantly decrease after the addition of K2CO3 to the ultrafine water mist. The recombination cycle of KOH → K → KOH, formed by reactions (R211: K + OH + M = KOH + M) and (R259: H + KOH = K + H2O), continuously combines active free radicals (·O, ·OH) into stable product molecules, such as H2O. However, at low K2CO3 concentrations, reaction R211, which suppresses laminar combustion sensitivity and consumes a larger quantity of active free radicals, does not dominate, leading to a reduced suppression effect of K2CO3 ultrafine water mist. Several factors during the reaction process also adversely affect the performance of K2CO3-containing ultrafine water mist. These factors include the premature onset of laminar flame instability at low K2CO3 concentrations, the increased flame-front propagation speed due to the addition of hydrogen to methane, which shortens the residence time of K2CO3 in the reaction zone, and the turbulence caused by unvaporized droplets. [ABSTRACT FROM AUTHOR]

With the gradual improvement of the hydrogen energy industry chain and the continuous expansion of application scenarios, the hazardous consequences brought about by hydrogen combustion and explosion need to be emphasized. In this paper, compounded superfine dry powder fire extinguishing media (CSFDP) and flame retardant compounded ultrafine dry powder fire extinguishing media (CSFDP-MgAl-LDHs) were prepared, respectively. On this basis, a hydrogen deflagration experimental platform was constructed to investigate the inhibition efficiency of different extinguishing media. The results showed that the inhibition rates of CSFDP-MgAl-LDHs on the peak overpressure of hydrogen explosion were 68.57%, 68.43% and 81.72%, which were significantly higher than the corresponding inhibition rates of CSFDP (28.28%, 53.23% and 54.85%) when the hydrogen release pressure was 2.0, 4.0 and 6.0 MPa, respectively. Furthermore, under a release pressure condition of 2.0 MPa, the suppression rates of jet flame front propagation speeds under the action of CSFDP and CSFDP-MgAl-LDHs reached 15.99% and 23.65%, respectively. In addition, the pyrolysis properties of CSFDP-MgAl-LDHs were explored and the chemical reaction mechanism between them and the reactive radicals within the hydrogen jet flame was predicted by thermogravimetric analysis, differential scanning calorimeter and fourier transform infrared spectroscopy, respectively. Ultimately, the physicochemical synergistic inhibition mechanism exerted by CSFDP-MgAl-LDHs during pure hydrogen deflagration was revealed. This study demonstrates broad prospects and value in the areas of hydrogen energy safety and sustainable development, as well as providing a basis for the establishment of safety standards and regulations in the hydrogen energy industry. • A novel fire extinguishing agent containing flame retardants is prepared. • The suppression of pure hydrogen deflagration is taken as the research object. • The physicochemical inhibition mechanism of compounded media is predicted. [ABSTRACT FROM AUTHOR]

Pre-chamber spark ignition system enhances the combustion stability and efficiency of lean burn engines. Pre-chamber spark ignition system performance directly relies on the characteristics of the turbulent flame jet, which are governed by the pre-chamber geometrical parameters and nozzle hole configurations. Based on the existing literatures, limited research on nozzle hole numbers and patterns is noted with passive pre-chamber, and thus, the impact of these nozzle hole configurations on the combustion and flame development characteristics for hydrogen-methane-air mixtures is investigated in a constant volume combustion chamber. In the current study, single-layer nozzle hole configurations with three, four, six, and eight nozzle holes and double-layer nozzle hole configurations with six and eight nozzle holes are designed and tested in an optically accessible combustion chamber under various equivalence ratio conditions. The results indicated that the combustion characteristics deteriorated with an increase in nozzle holes, except with the single-layer four nozzle hole configuration. Compared to the single-layer eight nozzle hole configuration, the ignition delay and combustion duration for hydrogen-methane-air mixtures with single-layer four nozzle hole configuration was reduced by 31.9% and 30.7% under lean conditions. A reduction in flame jet penetration length, area and velocity and increased adjacent flame jet interaction was noticed for the single-layer configuration with more nozzle holes, and thus, a different nozzle hole pattern was studied in the current study. The double-layer configurations with more nozzle holes showed better combustion and flame development characteristics than single-layer configurations due to their unique flame structure, uniform and faster flame jet distribution, and reduced adjacent flame jet interaction. The ignition delay and combustion duration for hydrogen-methane-air mixtures with the double layer eight nozzle hole configuration were reduced by 30.5% and 40.7% compared to the single-layer eight nozzle hole configuration. [Display omitted] • Combustion characteristics deteriorated with an increase in pre-chamber nozzle holes. • SL-H4 configuration exhibited better combustion and flame development characteristics. • Increased jet interaction and reduced jet penetration was noted for more nozzle holes. • DL-H8 configuration improved the combustion process with a unique flame structure. [ABSTRACT FROM AUTHOR]

In the context of the growing use of hydrogen for decarbonization in combustion, understanding NO x emissions from hydrogen burners is crucial. This study focuses on the impact of partial premixing on NO x production in hydrogen flames using a coaxial injector with swirl. The goal is to expand conventional jet scaling laws to encompass additional physical phenomena. The flexible injection system employed allows the stabilization of various flame types depending on the operating conditions, and double-front flames are obtained in several configurations. Observing the limitations of known scaling laws, particularly in coaxial flows with low-velocity differences or swirl, we conduct experiments measuring NO x and flame volume. Two Large Eddy Simulations explore distinct flame configurations to deepen our understanding about coaxial jet and recirculating flames, especially in the presence of a secondary premixed front due to rich premixing. Our investigation delves into the relationship between NO production and strain rate, considering turbulent and strain-related timescales. Notably, the presence of a premixed flame front alters the velocity profile on the shear layer, and the observed relationship between strain and turbulence in jet flames does not hold for recirculating flames. Thanks to this new insight, we propose a novel formulation for NO x scaling in coaxial jets, leveraging existing literature on coaxial jet modeling. Additionally, we propose a simple model to represent the piloting strain rate for NO emission in recirculating flames. Intricate effects of both swirl and partial-premixing are then included in the derived NO x scaling laws. [Display omitted] • Nitrogen oxide (NO x) emissions are measured in many swirled hydrogen flames. • Large-eddy simulations are used to enlighten the formation mechanisms. • Scaling laws for NO x emissions are derived to work for different flame topologies. [ABSTRACT FROM AUTHOR]

Blending a low ratio of hydrogen into traditional hydrocarbon fuels can reduce the modification in structure of existing combustors. However, even a low hydrogen ratio can increase the propensity of flashback due to the extremely high reactivity of hydrogen. Swirl number is a key parameter of swirl combustors which determines the flow structure. To design reliable gas turbine combustors for hydrogen-blended fuels, the study in the effect of swirl strength on flashback characteristic and the underlying mechanism is necessary. In this study, the flashback characteristics of premixed 30% H 2 /70% CH 4 /air flame in a bluff-body swirl burner is investigated via both experiment and large-eddy simulations with the flamelet-generated-manifold method (FGM-LES). The flashback mode and flame-flow interaction are analyzed based on the LES results. It is found that the increase in swirl strength will increase the propensity of flashback and promote the flashback speed of premixed 30% H 2 /70% CH 4 /air flame in the bluff-body swirl burner. The turns from Mode II (small-scale flame bulges leading flashback) to Mode I (large-scale swirling flame tongue leading flashback) during flashback of hydrogen-blended flames due to the production of stronger reversed flow by swirling flow. It is also found that stronger swirl strength will result in a smoother flame front and narrower strain rate distribution range which suppresses the local hydrogen enrichment due to preferential diffusion of hydrogen, thus suppressing the flame propagating speed. However, this effect is not dominant in flashback process compared to the promotion of reversed flow by increased swirl strength. • Efficient FGM tabulation including heat loss and differential diffusion for premixed hydrogen/methane flame is applied. • The flashback processes of premixed hydrogen-blended flame in experiment were reproduced well through FGM-LES. • The interaction between swirl strength and preferential diffusion of hydrogen was investigated. [ABSTRACT FROM AUTHOR]

Hydrogen is regarded as a promising energy source for the future due to its clean combustion products, remarkable efficiency and renewability. However, its characteristics of low-ignition energy, a wide flammable range from 4% to 75%, and a rapid flame speed may bring significant explosion risks. Typically, accidental release of hydrogen into confined enclosures can result in a flammable hydrogen–air mixture with concentration gradients, possibly leading to flame acceleration (FA) and deflagration-to-detonation transition (DDT). The current study focused on the evolutions of the FA and DDT of homogenous/inhomogeneous hydrogen–air mixtures, based on the open-source computational fluid dynamics (CFD) platform OpenFOAM and the modified Weller et al.'s combustion model, taking into account the Darrieus–Landau (DL) and Rayleigh–Taylor (RT) instabilities, the turbulence and the non-unity Lewis number. Numerical simulations were carried out for both homogeneous and inhomogeneous mixtures in an enclosed channel 5.4 m in length and 0.06 m in height. The predictions demonstrate good quantitative agreement with the experimental measurements in flame-tip position, speed and pressure profiles by Boeck et al. The characteristics of flame structure, wave evolution and vortex were also discussed. [ABSTRACT FROM AUTHOR]

Cryogenic compressed hydrogen (CcH2) technology combines the advantages of high pressure and low temperature to achieve high hydrogen storage density without liquefying the hydrogen, which has broad application prospects. However, the safety concerns related to cryogenic hydrogen need to be carefully addressed beforehand. In the present work, cryogenic hydrogen jet flames are experimentally investigated for various release pressures and initial temperatures. The flame length and thermal radiation flux were measured for horizontally releasing with nozzle diameters of 0.5–2 mm, temperatures ranging from 93 to 298 K, and initial pressures of 2–10 MPa. The results show that the flame length is dependent on the nozzle diameter, stagnation pressure and temperature. At a given pressure, the flame length, size and total radiant power increase with decreasing temperature, which is attributed to the lower jet flow velocity and higher density of low-temperature hydrogen. The normalized flame length Lf/D is correlated with the pressure ratio and temperature ratio. The correlation can be used to predict the flame length at various hydrogen pressures and temperatures. The normalized flame length of the cryogenic hydrogen jet flame is greater than that of the room-temperature hydrogen jet flame. The radiative heat flux of the flame can be predicted by the mass flow rate of the jet flow. [ABSTRACT FROM AUTHOR]

To reveal the dynamic and hazardous characteristics of a typical hydrogen-rich gas explosion in confined space the structural distribution of overpressure is studied in detail by CFD, as well as the evolutionary behaviors of explosion flames at varying hydrogen concentrations. Results indicate that indoor and outdoor venting overpressure curves exhibit multi-peak characteristics, primarily comprising seven types of peak values: P b , P mfa , P ext , P crowd , P reflected wave , P hel , and P superposition wave , with P crowd and P reflected wave dominating indoor overpressure, while P mfa and P ext forming the main pressure peaks in the outdoor flow field. With an increasing hydrogen concentration, the various peak pressures gradually rise, and their arrival times tend to shorten. The range and formation time of the expansion wave, Mach disk, and precursor wave in the outdoor flow field also vary with the hydrogen ratio. The peak overpressure hazards and their damage distance exponentially increase as the hydrogen ratio rises. The peak and average velocity of the venting flames both increase gradually with the hydrogen ratio, with instantaneous flame velocity and flame acceleration being notably influenced by flame shape. Findings from the study offer a scientific foundation for evaluating and managing explosion risks in industrial facilities utilizing hydrogen-blended natural gas. • Large-scale hydrogen-blended natural gas (HBNG) vented explosion is studied. • Characteristics of flow field of HBNG vented explosion are explored. • Evolution process of typical seven overpressure peaks is investigated. • Flame propagation velocity of HBNG vented explosion is illustrated. • Coupling effect of flame velocity and its acceleration on flame shape is explained. [ABSTRACT FROM AUTHOR]

This study explores the complex mechanisms driving flame acceleration (FA) in hydrogen/air mixtures with transverse stratification, using high-fidelity 3-D Large Eddy Simulations (LES) in a confined, obstructed channel based on the GraVent explosion setup. A systematic methodology for LES of stratified hydrogen deflagrations is presented, showing reasonable agreement with experimental data. The impact of stratification on FA across different mixtures is analyzed, comparing each stratified mixture to an homogeneous counterpart for an equivalent amount of hydrogen leakage. A striking finding is that stratification does not always enhance FA. The analysis of the LES results reveals that FA in stratified mixtures is strongly influenced by channel geometry and obstructions, driven by two primary mechanisms: Flow Contraction (FC) and Flame-Vortex Interaction (FVI). These mechanisms show significant asymmetry, with greater amplification in regions of higher reactivity. Large gradients in burning velocity along the flame front also contribute to pronounced flame surface elongation, leading to larger flame surfaces that influence FA to varying degrees. Ultimately, stratified FA operates through two modes: a dominant small-surface mode in rich gas regions that strongly drives acceleration, and a large-surface mode in lean mixtures, which initially contributes less but becomes more significant as the flame expands. This highlights the need to perform both global and local analysis for understanding these flames. [Display omitted] • First 3D LES investigation of flame propagation in stratified hydrogen/air mixtures. • Excellent agreement between LES predictions and experimental data. • New insights on the role of mixture stratification on flame acceleration mechanisms. [ABSTRACT FROM AUTHOR]

To explore the combustion characteristics of ammonia‒hydrogen‒air flame, explosion venting experiments of ammonia‒hydrogen‒air mixtures with different equivalence ratios (ϕ), in the range of 0.6–1.7, are carried out in a 2 m long duct. The results show that the pressure‒time histories inside the duct have a three-peak structure (P b , P out , and P ext), which is caused by the burst of the vent cover, venting of burned mixtures, and counterflow flame generated by the external explosion, respectively. P out evolves into three pressure peak types with increasing ϕ. The pressure peak value P e is generated at the pressure measuring point outside the duct because of the external explosion. The change in P e with ϕ is consistent with P out and P ext , all of which increase first and then decrease with increasing ϕ and reach a maximum value at ϕ = 1.3. This study provides basic theoretical support for the promotion and application of ammonia mixed fuel. • The effects of the equivalence ratio, ϕ , on duct-vented ammonia‒hydrogen‒air deflagration were studied. • Counterflow flame generated by the external explosion was observed in all experiments. • The pressure‒time histories inside the duct under each operating condition present had a three-peak structure. • The maximum overpressure inside and outside duct first increased and thereafter decreased. [ABSTRACT FROM AUTHOR]

Ammonia's distinctive chemical makeup, coupled with its ability to combust fully without carbon emissions, position it as a promising carbon-free fuel but requires careful management to mitigate NO x emissions. In practice, supplementing ammonia with hydrogen addresses its slow combustion rate and high ignition energy requirements. Meanwhile, the introduction of a bluff body (BB) within the pipe can alter the way of flame propagation. This study examines the combustion dynamics of the premixed ammonia/hydrogen/air flame in a pipe with a variable cross-section. The experimental findings demonstrate that increasing both equivalence ratio (Φ) and hydrogen volume fraction (α H 2 ) significantly alters the flame front velocity (FFV) and maximum overpressure (P max), while also enhancing the symmetry of the flame across variable cross-section. The addition of BB creates a vortex in the flame, the formation of which intensifies the combustion of the flame behind the BB, which can promote the generation of Helmholtz oscillations, elevate the amplitude of the pressure, improve FFV, and shorten the flame propagation time in the pipe. With the increase of Φ and α H 2 , the time is shortened less. When Φ = 0.8, α H 2 = 40%, the flame propagation time was shortened by 709 ms. When Φ = 1.2, α H 2 = 60%, the flame propagation time was shortened by 5.67 ms only. In addition, P max and maximum flame front velocity (FFV max) are more sensitive to α H 2 than Φ. After the addition of BB, the increase amplitude of P max and FFV max is increased. However, when Φ = 1.0 and Φ = 1.2, the amplitude of FFV max increase decreases with the increase of α H 2 . This study can provide some suggestions for the application of NH 3 /H 2 fuels in variable cross-sections as well as under obstacles. • Ammonia-hydrogen premixed flames were studied. • The BB makes the propagation time is shortened by 709 ms at Φ = 0.8, α H 2 = 40%. • The BB promotes the generation of Helmholtz oscillations. • The addition of BB creates a vortex in the flame, the formation of which intensifies the combustion of the flame behind the BB. • The BB makes the propagation time is shortened by only 5.67 ms at Φ = 1.2, α H 2 = 60%. [ABSTRACT FROM AUTHOR]

Studying the synergistic effects of dilution gases and hydrogen on the laminar combustion characteristics and NO X -emission concentrations of methane/air can contribute to achieving efficient and clean natural-gas combustion. This study quantitatively analyzes the impact of N 2 and CO 2 as dilution gases on the laminar combustion characteristics and NO X emissions of methane/air under different equivalence ratios. The virtual gases FN 2 and FCO 2 are introduced to distinguish between the physical and chemical effects of the dilution gases. Subsequently, the effect of hydrogen addition on the laminar combustion characteristics and NO X emissions of a methane/5% dilution gas/air mixture is investigated. Finally, the synergistic effects of the dilution gases and hydrogen on the laminar flame speed and NO X -emission concentrations of methane/air under various blending conditions are discussed. The results indicate that CO 2 exhibits a more substantial reduction in laminar flame speed, flame temperature, key radical concentrations, and NO X -emission concentrations than N 2 , primarily because of its physical effects. N 2 has minimal chemical effects, but marginally increases NO X emissions. The addition of hydrogen increases the laminar flame speed and key radical concentrations of the methane/dilution gas/air mixture. however, significant differences in the NO X -concentration trends with increasing hydrogen-blending ratios are observed for the three equivalence ratios (Φ = 0.7,1.0,1.4). Selecting appropriate blending ratios of dilution gases and hydrogen can simultaneously enhance the laminar flame speed of methane/air while reducing the NO X -emission concentrations. This study provides valuable insights into the optimization of the blending ratio of hydrogen-enriched natural-gas engines coupled with exhaust-gas recirculation. • Effects of N 2 and CO 2 on combustion characteristics of CH 4 /air are compared. • Physical and chemical effects of N 2 and CO 2 are distinguishedd. • Coupling H 2 with dilution gases enables clean and efficient combustion of CH 4. • Optimal blending ratios of dilution gases and hydrogen are discussed. [ABSTRACT FROM AUTHOR]

Hydrogen is a promising zero-carbon fuel, and its application in the micro-combustor can promote carbon reduction. The structural design of micro-combustors is crucial for combustion characteristics and thermal performance improvement. This study investigates the premixed combustion characteristics of hydrogen/air in a micro-cylindrical combustor with double ribs, using an orthogonal design method to assess the impact of various geometric parameters on thermal performance. The results indicate that the impact of rib height, rib position, and inclined angle is greater than rib width and their interactions, while their influence decreases in that order. Increased rib height improves mean wall temperature and exergy efficiency due to an expanded recirculation region and increased flame–wall contact, but negatively affects temperature uniformity and combustion efficiency. Although double ribs enhance performance, placing them too close may reduce heat transfer due to the low-temperature region between the ribs. When the double ribs are positioned at the axial third equinoxes of the micro-combustor, the highest mean wall temperature is achieved. Meanwhile, with a rib height of 0.3 and an inclined angle of 45°, the micro-combustor achieves optimal thermal performance, with the mean wall temperature increasing by 61.32 K. [ABSTRACT FROM AUTHOR]

Although research interest in pressure gain combustion remains high, information on the deflagration-to-detonation transition (DDT) process in actual multiple-cycle pulse detonation combustors is sparse, especially in terms of initial pressure studies, a crucial parameter in design and operation of such combustors. To develop microscale detonation combustion technology and gain a better understanding of the impact of initial pressure, an experimental study on flame acceleration of hydrogen-oxygen premixed gas is conducted using a micro spiral channel with a Tesla turbine at the outlet, which can realize precise initial pressure regulation in an open-ended pulse detonation combustor. The initial pressure consists of turbine wind pressure (p T) and intake residual pressure (p ir). p T increases the back pressure at combustor's outlet, and p ir is derived from the channel static pressure and flow velocity during the gas intake phase by bringing the ignition timing close to the moment when the solenoid valve closes. The flames are recorded by high-speed photography and the flame propagation characteristics under different initial pressures are compared. Results demonstrate that different combinations of p ir and p T yield three distinct DDT modes (Mode1: low p ir ∩ low p T ; Mode2: low p ir ∩ high p T ; Mode3: high p ir ∩ any p T), with p ir significantly outperforming p T in shortening DDT distance (L DDT , Mode3＜Mode2＜Mode1) and mitigating the detonation wave velocity deficit (DWVD, Mode3＜Mode2≤Mode1). High-pressure intake and timely ignition after valve closure (to trigger Mode3) is a very efficient way to generate detonation waves, which could be an ideal operation method for microscale pulse-detonation-based devices with hydrogen energy. [Display omitted] • DDT distance for 2H 2 –O 2 mixtures in a millimeter-scale spiral channel is investigated. • Initial pressure regulation in an open-ended pulse detonation combustor is realized. • The impact of initial pressure and initial flow field is compared. • Intake process flow field can promote DDT and reduce detonation velocity deficits. • Generating detonation waves within a coin-sized space. [ABSTRACT FROM AUTHOR]

Large eddy simulations (LES) with flamelet and presumed filtered density function closure are used to simulate turbulent premixed and partially premixed hydrogen flames. Different approaches to model differential diffusion are investigated and compared. In particular, two existing models are extended to the LES framework to correct the resolved diffusive flux of the controlling variables due to differential diffusion. A lean premixed turbulent hydrogen flame in a slotted burner configuration is simulated first to compare the capability of the considered models in capturing local mixture fraction redistribution, super-adiabatic temperatures and thermo-diffusive instabilities. Results show that both models describe the formation of cellular burning structures. Next, a partially premixed lifted hydrogen flame in vitiated hot coflow is simulated to gain insight on the relevance of differential diffusion modelling at a higher turbulence level, a different combustion mode and in the presence of a complex stabilisation mechanism. Good predictions of the turbulent mixing and temperature fields are observed. Moreover, results show that the flame lift-off height has an appreciable sensitivity to the differential diffusion model. When differential diffusion is included only in the thermochemistry database, only mild effects on the predicted temperature fields, mixing and flame height are observed. On the contrary, a considerable shift of the flame base is observed when corrections are applied in the LES at the resolved level, depending on what controlling variables are considered. Further analyses reveal how the corrections of diffusive fluxes in the thermochemistry and at the LES level affect differently the flame burning mode, whose details are given throughout the paper. [ABSTRACT FROM AUTHOR]

Hydrogen, which emits large amounts of heat and has harmless products, is considered a potential alternative fuel. However, its wide flammability range and low ignition energy make it hazardous during storage and transportation. Therefore, in the event of a hydrogen leak in a confined space, a certain amount of inert gas can be added to inhibit combustion. Nonetheless, the addition of inert gas can also lead to an increase in the initial pressure as well. The aim of this research is to analyze the characteristics of laminar flame propagation and the inherent instability of laminar flames composed of 30% H 2 -air under various initial pressures and different types or quantities of inert gas dilution. In the experimental setup, hydrogen and air are pressurized into a 20 L spherical chamber to create the required mixed gas, following which inert gases such as CO 2 , N 2 , and He are added to achieve the desired pressure levels. Initially, the study focuses on elucidating the impacts of different variables on parameters such as flame propagation speed, flame stretching rate, un-stretched flame propagation speed, Markstein Length, and un-stretched flame burning speed. Additionally, it delves into investigating the effective Lewis number, density ratio, and flame thickness under varying conditions to understand the mechanisms influencing inherent flame instability. The findings indicate that increasing p 0 or augmenting the amount of dilute gas mixing both exacerbate flame front instability and reduce the critical radius of the flame. Among the three admixed species, CO 2 addition exhibits the most significant effect. [Display omitted] • Experiments study on the inherent flame instability of H 2 -air with combined effects of p 0 and inert gas dilution. • For p 0 ≥ 150 kPa, CO 2 addition≥ 20%, or N 2 addition≥ 25%, the stretching rate enhances flame propagation speed. • Inert gas dilution alters the structure of the H 2 -air premixed flame front. [ABSTRACT FROM AUTHOR]

The flame characteristics of turbulent premixed jet flames fuelled with stoichiometric hydrogen/methane mixtures are studied using Direct Numerical Simulation (DNS). The study investigates four cases with 0, 10, 50, and 80% hydrogen by volume while maintaining the jet Reynolds number at 10,300. The addition of hydrogen leads to reduced wrinkling as the flame moves from the thin-reaction zone regime towards the corrugated flamelet regime. This study underlines the importance of using appropriate non-dimensionalisation parameters for generalising the findings. Specifically, the flame thickness defined by Peters (2000) which use the conditions at the point of the maximum heat release rate is found suitable to non-dimensionalise curvature and describe the variation of the displacement speed as a function of curvature. • High fidelity simulations for 0-80% hydrogen fuelled turbulent flames. • A significant post-flame oxidation zone was found. • A flame thickness definition collapsed the curvature and displacement speed relation. [ABSTRACT FROM AUTHOR]

The study investigates the effect of tube shrinkage rate on the explosion characteristics of premixed hydrogen-air in a confined space using Large Eddy Simulation (LES). The wrinkle factor of the flame surface density model is dynamically modeled based on the Fureby and Muppala (FurebyM) model. The numerical results from this model can be better verified with previous experimental results. The results indicate that a lower shrinkage rate results in longer flame propagation time from the reducer region. The peak flame front velocities were 68.6 m/s, 83.6 m/s, 96.4 m/s, and 103.6 m/s for tube shrinkage rates of 0.25, 0.5, 0.75, and 1.0, respectively. There is also a negative correlation between explosion overpressure and varying shrinking rates. The split tulip flame forms at a shrinkage rate of 0.25. Analysis of the flame structure and velocity flow field suggests that this phenomenon may be related to the formation of multiple fish-tail vortex structures on the left side of the flame front. Moreover, the effect of different shrinkage rates on the flame structure and the velocity flow field before the propagation of the premixed flame to the reducer region is relatively insignificant for tube structures with different shrinkage rates. This study provides a theoretical basis for protection against confined space explosions with premixed hydrogen-air mixtures. • The wrinkle factor in the flame surface density model is dynamically modeled and the accuracy of the model is verified. • The effect of tube shrinkage on the deflagration characteristics of hydrogen-air was investigated. • The flame structure evolves into a split tulip flame for a shrinkage of 0.25. [ABSTRACT FROM AUTHOR]

The present study deals with the flames of ammonia–hydrogen blends that form a prospective fuel for power generation in practical devices. In particular, the spatial flame dynamics of the bluff body stabilized laminar and stratified ammonia–air/hydrogen–air mixtures with acoustic excitation are considered. For this, we consider a radially staged burner consisting of two concentric tubes namely, the inner and outer tubes with the bluff body housed by the former. Two annulus passages are formed between (i) the inner and outer tube, and (ii) the inner tube and bluff body in which premixed hydrogen–air and ammonia–air mixtures are supplied, respectively. The stratification is controlled by varying the upstream location of the inner tube about the burner exit, termed mixing length taking the values namely, 20 mm (stratified), 60 mm, and 100 mm (unstratified). Time-resolved images of the flames are obtained using OH* (320 ± 10 nm) and NH2* (632 nm ± 10 nm) filters. For the unforced stratified flames, the flames are narrower and the spatial separation between OH* and NH2* region increases towards rich NH3/air mixtures (ϕ NH3–air) compared to the unstratified flames. In addition, the mean spatial OH* is relatively localized for the unforced stratified flames. For any operating condition of the flames, the forced flame response is significant for frequencies < 120 Hz. The rich stratified flame exhibits a higher response at a frequency of ∼ 40 Hz compared to the unstratified flame having a flatter response. The forced flames accompany lobed spatial patterns in OH* and NH2* for richer ϕ NH3–air. The flame oscillation registers a non-sinusoidal oscillation for the rich stratified flame compared to flames of any other condition. The main effect of stratification is the alteration of flame shape resulting in a low-frequency response for which regions of significant flame flapping coincide with regions of peak flame OH*. [ABSTRACT FROM AUTHOR]

High-speed microjet hydrogen–air diffusion flames are investigated computationally. The focus is on the prediction of the so-called bottleneck phenomenon. The latter has been previously observed as a specific feature of the present flame class and has not yet been investigated computationally. In the configuration under consideration, the nozzle diameter is 0.5 mm and six cases with mean nozzle injection velocities (U) between 306 m/s and 561 m/s are considered. The flow in the nozzle lance is analyzed separately to obtain detailed inlet boundary conditions for the flame calculations. It is confirmed by calculation that the phenomenon is mainly determined by the transition to turbulence in the initial parts of the free jet. The transitional turbulence proves to be the biggest challenge in predicting this class of flames, as the generally available turbulence and turbulent combustion models reach the limits of their validity in transitional flows. In a Reynolds-Averaged Numerical Simulation framework, the Shear Stress Transport model is found to perform better than alternative two-equation models and is used as the turbulence model. By neglecting the interactions between the turbulence and chemistry (no-model approach), it is possible to predict the morphology of the bottleneck flame and its dependence on U qualitatively. However, the position of the bottleneck is overpredicted for U < 561 m/s. The experimental flames in the considered U range are all attached to the nozzle. This is also predicted by the no-model approach. The Eddy Dissipation Concept (EDC) used as the turbulence combustion model predicts, however, lifted flames (with increasing lift-off height as U decreases). With the EDC, no bottleneck morphology is observed for U = 561 m/s. For lower U, the EDC results for the bottleneck position are generally closer to the measurements. It is demonstrated that accuracy in predicting the bottleneck position can be improved by ad hoc modifications of the turbulent viscosity. [ABSTRACT FROM AUTHOR]

In this paper, the evolution of an explosion of hydrogen/ammonia/air flames is studied by experiments and numerical simulations. A two-dimensional (2D) model is utilized, employing the detached eddy simulation (DES) method along with the thickened flame model. Stoichiometric hydrogen/ammonia/air with ∂ = 25% and 50% (ammonia blending ratio in fuel) is considered. The results indicate that the numerical simulations accurately replicate the experimental findings. Both the experimental observations and numerical simulations reveal the formation process of the "tulip" flame. With increasing ∂, both the flame front speed and overpressure decrease. A change in ∂ can alter the contribution of intermediate reactions, thereby influencing the explosion behavior. • The explosion behavior of ammonia/hydrogen/air was investigated. • A detached eddy simulation with a thickened flame model is employed. • The 2D simulation can accurately predict the explosion of ammonia/hydrogen/air. • The effect of the ammonia blending ratio on the explosion reaction kinetics is analyzed. [ABSTRACT FROM AUTHOR]

Hydrogen is often stored and transported in a high-pressure state, posing a risk of jet flames in the event of accidental leakage. Firewalls are commonly installed around high-pressure hydrogen storage and transportation equipment to protect surrounding facilities and personnel from the fire hazards caused by hydrogen leaks. By combining experimental and numerical simulation methods, this study investigated the effects of varying hydrogen release pressure, nozzle shape, nozzle-wall distance, and barrier height on the behavior of high-pressure hydrogen jet flames. A predictive model for the flame extension length after contact with the wall was proposed. The study found that when the wall height is sufficient to cause complete flame deflection, the temperature distribution near the wall exhibits a strong regularity. The temperature of the barrier wall below the nozzle height is higher than that above the nozzle height, indicating that enhanced thermal protection is needed in this region for practical applications. [Display omitted] • The flame deflection angle increases with the height of the wall barrier. • The models for the extension length of the flame behind the wall are proposed. • Reducing nozzle-wall distance and increasing height of wall can reduce the horizontal extent of high-temperature zone. • The temperature of the barrier walls below the nozzle height is higher than that above. [ABSTRACT FROM AUTHOR]

With the rapid development and application of hydrogen in the chemical and energy industry, the safety of hydrogen industrial facilities has been widely concerned. In this study, the safety of hydrogen accidently released from a high-pressure tube storage system (tube diameter:10 mm, storage tank volume:4 L) was explored experimentally. The combustion behavior of the transient hydrogen jet and explosion overpressure under self-ignition and electric spark induction were investigated. Besides, the explosion overpressure and jet flame evolution laws of the hydrogen cloud induced by the release of unsteady turbulent hydrogen jets into the open environment were analyzed. The results revealed that the peak value of hydrogen self-ignition explosion overpressure was higher than that of electric spark ignition overpressure. Compared with spark ignition, hydrogen self-ignition flame structures can be maintained for longer time. However, spark ignition at low release pressures was more likely to cause hydrogen ignition and explosion compared to self-ignition. Finally, the combustion behavior and potential hazards of the ignited free transient hydrogen jet under different ignition modes were analyzed. This study can provide a reference for the prevention and control of hydrogen explosion accidents in practice. • Hydrogen jet explosion can be induced by spark within 5D of the nozzle axis. • Comparing with spark ignition, self-ignition explosion had higher overpressure peak. • Self-ignition process was stable, but spark ignition was sensitive to the spark position. [ABSTRACT FROM AUTHOR]

As a promising strategy to mitigate carbon emissions, hydrogen enrichment of conventional fuels is gaining increasing interest, but can lead to increased propensity to damaging thermoacoustic instability. This paper demonstrates the ability of machine learning algorithms to reliably predict a key flame behavior relevant to thermoacoustic stability prediction – the flame's nonlinear response to upstream acoustic forcing. Using computational simulations of a laminar premixed flame under difference hydrogen enrichment and equivalence ratio conditions, training data for machine learning are generated. The machine learning models are then used to predict the flame response under new test conditions, including extrapolating into new hydrogen enrichment regimes. The effect of algorithm choice, data structure, data size, and extrapolation distance on the performance of different machine learning models is investigated with particular attention to performance with sparse training data. It is found that a type of neural network known as the multi-layer perceptron (MLP) model outperforms the Gaussian process and random forest models in both interpolation and extrapolation tasks. A physics-informed preprocessing strategy, involving extracting the time delay of the flame response (which can be deduced from geometry and flow parameters) prior to applying machine learning, is found to remarkably improve the accuracy of machine learning models, especially in extrapolation tasks. When the extrapolation into new hydrogen content levels is strong enough, the MLP model no longer performs well. However, for small and intermediate extrapolation, the MLP model shows a commendable level of prediction, even with small data sets, fulfilling needs relevant to thermoacoustic stability prediction. Extending the MLP model to experimental data from turbulent flames, the gain fall-off frequency and phase lag are accurately predicted but not the secondary periodic fluctuations in gain. • AI-assisted prediction of flame response in new hydrogen enrichment regimes. • A physics-informed strategy extracting the time delay improves model performance. • The effects of algorithm choice, data structure and data size are studied. • MLP model shows good predictions in small extrapolation tasks with sparse data. • Extending to turbulent flames, the MLP model captures fall-off frequency in gain. [ABSTRACT FROM AUTHOR]

Once the thermal pressure relief device (TPRD) of a high-pressure hydrogen tank activates in a fire, released hydrogen could be ignited, leading to a coupled fire induced by the interaction of the hydrogen jet flame and surrounding fire. This could result in unpredictable casualties and property losses. However, the physics and flame behavior of such fires are still unknown. In this study, a series of experiments were conducted on the coupled fire involving a hydrogen jet flame and a heptane pool flame at various pool diameters and horizontal distances from the nozzle to the pool. Theoretical analysis is conducted first, and it is found that the presence of a hydrogen jet flame enhances radiation and conduction to the fuel. The flame behavior would be dominated by the competition between the buoyancy flux of the pool flame and the momentum flux induced by the hydrogen jet flow. As a result, the heat release rate of a coupled fire is higher than that of a single pool flame. The heat release rate increases initially but then decreases as the distance from the nozzle increases. The flame reaches its maximum value at a horizontal distance of 0.35 m, as well as the total flame length. In addition, the heat release rate of a coupled fire increases with the increase in the pool diameter. Compared to a single pool flame, the coupled fire has a lower flame height. Finally, prediction models for the total flame length and height of the coupled fire are proposed based on the ratio of buoyancy flux from the pool flame and the momentum flux caused by the hydrogen jet flow. The results can provide a guidance for fundamental research on standardizing hydrogen storage safety. • The existence of hydrogen jet flame strengths the radiation and conduction to the fuel. • The convection transitions to forced convection induced by the momentum. • The heat release rate of a coupled fire is higher than that of the single pool flame. • Compared with a single pool flame, the coupled fire has a lower flame height. • Prediction models of the total flame length and height of the coupled fire are proposed. [ABSTRACT FROM AUTHOR]

One of the important choices for achieving decarbonization goals on a large scale is the extensive use of H 2. However, the highly reactive nature of H 2 often leads to challenges such as flashback, excessive NO x emissions, and combustion instability when traditional combustion methods are directly applied. Therefore, developing safe and efficient H 2 combustion technologies are currently important research directions in gas combustion. It holds significant importance for addressing the coordinated development of future energy and the environment. In recent years, hydrogen-enriched methane has been widely applied in gas turbines with the aim of reducing carbon emissions. Ammonia is also regarded as an important carrier of hydrogen. The blending of ammonia with hydrogen has been widely recognized and accepted for simultaneous use. The use of H 2 O as a diluent is another technology capable of achieving stable H 2 combustion and reducing NO x emissions. Syngas, primarily composed of CO and H 2 , serves as the main fuel for Integrated Gasification Combined Cycle technology. These are collectively referred to as hydrogen hybrid fuels, and their combustion chemical properties are all influenced by hydrogen. In recent years, significant efforts have been made to deepen the understanding of flame structures of pure hydrogen and these hydrogen hybrid fuels through advanced numerical simulations and experimental diagnostics. The utilization of Planar Laser Induced Fluorescence techniques for flame structure characterization has significantly advanced the understanding of flame-flow-acoustic coupling mechanisms in phenomena such as flashback processes, NO x emission mechanisms, and combustion instability. This has also contributed to the development and validation of advanced numerical models. This paper categorizes hydrogen and various hydrogen hybrid fuels according to different combustion application scenarios and purposes. It discusses the application of Planar Laser Induced Fluorescence technology in flashback, NO x emissions, and combustion instability aspects across different hydrogen hybrid fuels. This brief review aims to provide useful references for researchers and engineers designing and conducting foundational experiments on flame structures and combustion instability of hydrogen and hydrogen hybrid fuels. • Thoroughly summarize the applications of PLIF in studying the combustion characteristics of hydrogen fuel. • Provide insights into the application of laser diagnostic in the field of turbulent combustion of hydrogen fuels. • A detailed discussion is provided on the complex flow-flame interactions in unstable combustion of hydrogen fuel. [ABSTRACT FROM AUTHOR]

The deflagration evolution of premixed CH 4 /H 2 /air mixtures in sudden expansion duct was investigated by explosion experiments. Based on the previous research on different obstacle widths and spaces, the effect of various hydrogen blend ratios and the fuel equivalence ratios on deflagration evolution were further investigated. The results showed that the flame inversion is pronounced for hydrogen blend greater than 40% and the fuel equivalence ratio no less than 1, which is closely related to the increased flame propagation speed. Besides, the results showed that the overpressure in the chamber increases as the hydrogen blend ratio increases. Moreover, the maximum pressure (P red) and the maximum pressure rise rate ((dP/dt) max) are observed at the fuel equivalence ratio for the low hydrogen blend ratio, however, for the high hydrogen blends, the maximum values are observed at slightly higher fuel equivalence ratios. Furthermore, a model was developed to predict the maximum overpressure under conditions of sudden expansion and half-open ducts. • The effect of sudden expansion duct on the deflagration evolution was investigated. • The equivalence ratio for obtaining the maximum pressure is close to 1.2 at high hydrogen blend ratio. • As The hydrogen blend ratio increases, the second explosion pressure becomes high. • The addition of obstacles and hydrogen can promote the flame to inverse. • The pressure prediction model suited for the sudden expansion duct is corrected. [ABSTRACT FROM AUTHOR]

The mechanism of flame front propagation in NH 3 /air and NH 3 /H 2 /air steady, laminar premixed flames is examined. Since the process is characterised by a state of chemical non-equilibrium, the analysis focuses on the explosive mode that is introduced by chemical kinetics. The chemistry expressed in this mode is the one that tends to lead the system away from equilibrium and sustains the chemical non-equilibrium state. The algorithmic tools of Computational Singular Perturbation method are employed, so the analysis is not hindered by the size of the detailed chemical kinetics mechanism employed. Under engine-relevant conditions and a stoichiometric mixture, it is shown that in the NH 3 /air case the flame front propagation is driven by reaction ▪ far from the front and by reaction ▪ closer to the front; the latter assisted by reaction ▪. These reactions are mainly responsible for the heat released, by effectively feeding the most exothermic reactions, which are OH-consuming. The ensuing chemical activity in the neighbourhood of maximum heat release rate generates upstream diffusion of heat, NH 2 , NO, H and H 2 , which initiate the chemical activity ahead of the flame front. This mechanism of front propagation is promoted by H 2 addition in the mixture, by reinforcing the action of these three reactions and by activating another OH-producing reaction ▪. A preliminary investigation of lean mixtures indicated that this flame front propagation mechanism is also present in the case of a pure ammonia fuel. However, when H 2 is present in the initial mixture, significant changes are observed that relate to the prevailing lower temperatures and the decreased upstream diffusion of heat. These findings provide novel insights with direct implications for controlling and optimising NH 3 and NH 3 /H 2 flames planned for engine applications. The approach proposed here can also be extended for analysing flame propagation mechanisms across a more diverse spectrum of fuel mixtures and flame configurations, offering invaluable support to technologies pivotal in the ongoing energy transition efforts. [Display omitted] • Flame propagation mechanism of NH 3 /air & NH 3 /H 2 /air revealed using timescale analysis. • NH 2 ＋NO → NNH + OH drives the chemical dynamics far from the flame front. • H＋O 2 → OH＋O and H 2 ＋OH → H 2 O＋H dominate close to the flame front. • H 2 addition mainly reinforces the influence of the above 3 reactions. • H 2 addition also reinforces reaction O ＋H 2 → OH ＋H. [ABSTRACT FROM AUTHOR]

Even in the laminar regime, lean H 2 -air flames are prone to intrinsic Thermo-Diffusive (TD) instabilities which contribute to flame acceleration. Applying the Thickened Flame (TF) strategy to simulate such flames prompts two erroneous behaviors: (1) amplification of the flame sensitivity to stretch due to thickening; (2) inability to capture TD cells onset and development. A new paradigm is proposed based on 2D spherical flames simulations: (1) the flame response to global stretch is corrected by the Stretched-Thickened Flame model, extended here for lean hydrogen mixtures; (2) a TD efficiency function is introduced, encompassing both subgrid wrinkling and subgrid stretch effects due to TD instabilities. These corrections are introduced in the diffusion and reaction terms of the Navier–Stokes equations, in a manner similar to the TF model. The new model accurately reproduces the propagation of these flames, both the early stages governed by global stretch effects and the TD-dominated phase, without adding any significant computational cost. • The Thickened Flame (TF) model is shown to inaccurately simulate lean H2-air flames. • A model is proposed to correct its deficiencies with such flames. • The model accounts both for global stretch effects and flame front instabilities. • The Stretched-TF model solves the amplification of stretch effects by thickening. • A subgrid model reproduces the thermo-diffusive instabilities dynamics. [ABSTRACT FROM AUTHOR]

A possible way to introduce hydrogen into the energy matrix would be to enrich traditional fuels like natural gas and propane with hydrogen. Therefore, the study of the flame propagation behavior of these hydrogen enriched fuels becomes important to future industrial applications. In the present work the aim was to experimentally study the effect of hydrogen enrichment of propane and natural gas. However, in order to achieve more generality, the studied mixtures were characterized through the Lewis (Le), Zeldovich (Ze) and the reaction Damköler (Da°) numbers. A total of six mixtures were considered, in which the Ze varied from 4.94 to 8.45, the Le varied from 0.71 to 1.36 and the Da° varied from 14.56 to 36.11. In addition, the obtained results were compared to previous work to assess the effect of the experimental boundary conditions on the flame propagation velocity. The experimental results show that, for mixtures with similar Le numbers, the lower the Ze number the higher the flame propagation velocity. The maximum propagation velocities obtained in the first propagation section of the module are between 18.60 m/s and 57.11 m/s. Therefore, these flames can be considered as weakly accelerating flames. Moreover, the maximum flame propagation velocities showed a good correlation with the product Le × Ze. Regarding the presence of the flame arrester housing without an arrester element, it was observed that it causes an increase in flame propagation velocity. However, when compared to a previous work in which a flame arrester element was inside the housing, the velocity was higher for the present work. • Hydrogen enrichment of mixtures containing propane and gas natural were considered in this study. • The influence of the Lewis and Zeldovich numbers was investigated. • The flame propagation velocity was investigated in terms of these dimensionless numbers. • The results show that lower Zeldovich numbers promote higher propagation velocities for similar Lewis numbers. • Through comparison with published results the effect of experimental boundary conditions was assessed. [ABSTRACT FROM AUTHOR]

This study developed a one-step chemical reaction model for hydrogen combustion and investigated the effect of heat loss on premixed hydrogen/air deflagration in closed pipelines using numerical simulations with the CFD code GASFLOW-MPI. The simulation results were compared and verified with experimental data. The numerical simulations accurately predicted the flame front structure, pressure, and flame tip position during the deflagration of premixed hydrogen/air. Furthermore, a comparison of adiabatic simulation and heat transfer simulation results revealed that large-scale vortices within the turbulent flow were closely linked to heat loss during the tulip flame stage. Simultaneously, heat loss reduced pressure and flame tip position by diminishing the flame wrinkling factor caused by thermodiffusive instability. Finally, convective heat transfer identified as the most critical factor contributing to heat loss during the deflagration of premixed hydrogen/air in closed pipelines. Specifically, during the tulip flame stage, heat loss was mainly due to convective heat transfer (73.8%∼76.3%), with radiation playing a minor role (23.7%∼26.2%). Therefore, accurate prediction of characteristic parameters during the deflagration of premixed hydrogen/air requires numerical simulations that account for both convective and radiative heat transfer. • Heat transfer simulation can better predict the evolution of hydrogen deflagration. • Heat loss can cause a reduction in pressure and flame tip position. • Large-scale vortices within the turbulent flow are closely linked to heat loss. • Thermodiffusive instability is closely related to heat loss. • Convection heat transfer plays a crucial role in the heat loss. [ABSTRACT FROM AUTHOR]

Biogas is an efficient and environmentally friendly gaseous fuel. To ensure the safety of biogas applications, highly effective extinguishing technologies for suppressing biogas flames should be developed. In this paper, the extinguishing performance and kinetic mechanism of 2-BTP were studied experimentally and numerically. The results indicate that 2-BTP has excellent inhibition effect on biogas flames, but it depends on the ratio of hydrogen and oxygen in the reactants. For hydrogen-rich and oxygen-poor flames, 2-BTP shows efficient inhibition effect. However, it shows a little combustion promotion effect for hydrogen-poor and oxygen-rich flames at the addition of certain concentrations. The H and OH radical concentration variation and their production/consumption reactions are analyzed subsequently, and the simulation computation shows that some competing reactions involving Br-containing species lead to these results. The purpose of this research is to gain an in-depth understanding of the potential and effectiveness of 2-BTP in suppressing biogas fires, which can lead to better forecasting and optimization of its application, with the goal of enhancing the efficiency of fire extinguishment and minimizing possible adverse effects. • 2-BTP is particularly effective to slow the burning velocities of stoichiometric and fuel-rich biogas flames. • The effect of 2-BTP on combustion promotion or inhibition depends on the amount of hydrogen and oxygen in the flame. • The hydrogen contained in 2-BTP and the reaction HBr + O → Br + OH support the combustion of lean flame to some extent. [ABSTRACT FROM AUTHOR]

Ammonia, as a renewable energy with broad future application prospects, can alleviate its deficiencies in combustion stability as well as NOx emission by swirl premixed combustion technology. To investigate the impacts of swirler configuration parameters on ammonia premixed combustion, vanes numbers, vanes angles, and inner/outer diameter ratios of the swirler are varied. The combustion characteristics and NOx emissions in the combustion chamber are analyzed using the ammonia combustion mechanism model proposed by Stagni, which is done by large eddy simulation and Flamelet generated manifold combustion model. The results indicate that a stronger inner recirculation and lower NO 2 /N 2 O emissions were obtained when the number of vanes is 10. Increasing the vanes angles has several benefits, including expanding the flame's main combustion region, enhancing the heat release rate and improving turbulence in the flow field. The highest combustion efficiency and the lowest NO emissions are achieved at a vanes angle of 35°. Additionally, a moderate inner/outer diameter ratio of 0.447 can ensure appropriate inner and outer circulation intensity and NOx emissions. Based on the investigation results, the references for swirler parameters design in premixed ammonia combustion are provided. • NH 3 /air premixed flames were modeled by LES and FGM model. • Effects of vanes number, angle, and inner/outer diameter ratio were analyzed. • Linked thermal performance and pollutant emissions to flow field features. [ABSTRACT FROM AUTHOR]

Understanding flame-acoustics interaction (FAI) that can potentially trigger thermoacoustic instabilities is of critical importance to mitigate the serious side effects of pressure oscillations. While FAI has been studied extensively in premixed flames, comparatively less analysis has been conducted to explore FAI mechanisms in non-premixed flames, especially under high-pressure conditions. This study aims to numerically investigate FAI in high-pressure hydrogen/air counterflow diffusion flames, with special attention on the impact of pressure on FAI. To this end, a fully compressible unsteady counterflow solver combined with Navier-Stokes Characteristic Boundary Conditions (NSCBC) is employed to capture the reflection of the acoustic waves at the boundaries. The results show that for all ambient pressures ( $ 1\leq P_a\leq 60 $ 1 ≤ P a ≤ 60 bar) considered here, real-gas effects on flame structures are negligible and that increasing $ P_a $ P a would lead to a power-law dependence of density gradient on $ P_a $ P a with an exponent of 1.5. Furthermore, all tested cases show acoustic growth with time under fully reflecting boundary conditions, which is found to be pressure dependent. The flames become less resilient to FAI when the pressure (or density gradient) is increased. The results from the spectral analysis show that for all ambient pressures, there is only a single dominant frequency at the low-density fuel side and two dominant frequencies at the high-density oxidiser side. This indicates that the pressure oscillation is more sinusoidal at the H $ _2 $ 2 side than that at the air side. Moreover, the phase space portraits of pressure signals indicate that the dynamics of the system are becoming periodic and approaching a limit cycle. It is found that the integrated heat release rate and the pressure fluctuations are partially in phase, which is responsible for the growth of high-frequency acoustic waves at both fuel and oxidiser sides. On the other hand, the significant density gradient contributes to the growth of low-frequency acoustic waves at the oxidiser side due to the high degree of acoustic reflectivity at the density boundary. [ABSTRACT FROM AUTHOR]

Ultra-lean hydrogen–air flames propagating in narrow gaps, under the influence of cold walls and high preferential diffusion, can form two distinct isolated structures. They exhibit either circular or double-cell shapes and propagate at different speeds, with the latter roughly doubling in size and traveling speed to the former. Hydrogen mass diffusivity, convective effects, and conductive heat losses are the physical mechanisms that explain the alterations in morphology and propagation speed. In previous experiments, Veiga-López et al. [Phys. Rev. Lett. 124, 174501 (2020)] found these clearly distinguished flame structures for different combinations of equivalence ratio, channel gap, and the effect of gravity on the dynamics of upward and downward propagating flames in a vertical chamber. Present observations in horizontal channels show the simultaneous appearance of these two stable structures, which arise under identical experimental conditions and conform the first evidence that multiplicity of stable solutions coexists in real devices. To explain the observations, we performed numerical simulations using the simplified model, which shows that symmetry-breaking details during the ignition transient explain the concurrent emergence of the two stable configurations. This discovery urges the need to implement additional engineering tools to account for the possibility of the formation and propagation of isolated flame kernels at different speeds in hydrogen-fueled systems. [ABSTRACT FROM AUTHOR]