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Chemiluminescence of OH∗ is commonly employed as an optical indicator for the combustion process. It is widely recognized that the generation of OH∗ in hydrogen flames is primarily governed by the reaction H + O + M=>OH∗+M. In this study, the time histories of OH∗ chemiluminescence from the A→X band around 308 nm were recorded behind reflected shock waves for stoichiometric and lean H 2 /O 2 mixtures highly diluted in argon over the temperature range of 1130–3150 K and at the pressure of approximately 0.9 atm. Based on the time histories of OH∗ chemiluminescence, the best-fit reaction rate coefficient for the reaction H + O + M=>OH∗+M was determined as k = 3.17 × 1027exp(-121968 cal mol−1/RT) + 2.25 × 1017exp(-4246 cal mol−1/RT) cm6 mol−2 s−1 over the temperature range studied. Furthermore, by using the current rate coefficient in mechanism predictions, the predicted time histories and relative peak intensities of OH∗ chemiluminescence are in good agreement with experimental results reported in literature. • The time histories of OH∗ chemiluminescence up to 3150K are recorded. • The rate coefficient for H + O + M=>OH∗+M is determined. • The OH∗ sub-mechanism with the fitted rate coefficient has been verified. [ABSTRACT FROM AUTHOR]

To reduce fossil energy consumption and achieve global carbon neutrality, it is essential to develop alternative fuels such as hydrogen (H2) and dimethyl ether (DME). In order to analyze the energy-mass conversion behavior of alternative fuels for power units (including gas turbines) and to explore the principles of efficiency enhancement, the exergy loss characteristics of methane (CH4)/air laminar premixed flames blended with H2/DME were investigated. The effects of blending ratio, unburned gas temperature, operating pressure, and equivalence ratio on the exergy loss fraction were considered. The results indicated that DME and H2 had opposite effects on the total exergy loss fraction of the premixed flame. Elevating the operating pressure was beneficial to reducing the exergy loss caused by chemical reactions and exhaust gas, which more significantly enhanced the exergy efficiency of each fuel mixture compared to increasing the unburned gas temperature. The exergy loss fraction due to the exhaust gas was also lower in the lean flame, suggesting that DME could be a promising alternative to CH4 in this scenario. As the exergy loss fraction due to chemical reaction was minimized at an equivalence ratio of 1.2, a variable equivalence ratio operating strategy was expected to further optimize the exergy efficiency of each fuel mixture. Furthermore, response functions of the total exergy loss fraction, which considered the combined effects of multiple operating parameters, were constructed. These response functions allow the total exergy loss fractions of CH4 premixed flames with high DME blending ratios and moderate H2 blending ratios to be quantified. The results from this study enable a deeper understanding of the energy-mass conversion behavior of systems with added H2/DME during combustion and provide theoretical support for the application of alternative fuels to more complex configurations of industrial burners. [ABSTRACT FROM AUTHOR]

Due to the carbon dioxide (CO2) content in biogas, using biogas is relatively limited in many industrial applications. Hydrogen (H2) is a highly reactive gas that is used extensively to enhance the burning rate of biogas, on the other hand, nitrous oxide (N2O) is a powerful oxidizer that can improve the burning rate of biogas. The laminar burning velocities of biogas-hydrogen-air and biogas-N2O mixtures at various equivalence ratios were investigated experimentally using the spherical flame methodology. It was found that the laminar burning velocities were enhanced in biogas-hydrogen-air and biogas-N2O mixtures. However, the enhanced laminar burning velocities of biogas-N2O showed a more significant increase than biogas-hydrogen-air due to the significant energy released by the N2O decomposition reaction. The flame thickness of the biogas-N2O mixture indicated lower values than biogas-hydrogen-air mixtures, suggesting a higher flame instability of the biogas-N2O mixture than biogas-hydrogen-air mixtures. The Lewis number of biogas-N2O mixture showed lower values than all biogas-hydrogen mixtures, indicating higher diffusive-thermal instability influence on biogas' flame. The reactions of H + O2⇔ OH + O and H + CH3(+M) ⇔ CH4(+M) represented the most significant reactions influencing the laminar burning velocities of biogas-hydrogen mixtures, while the N2O + H⇔N2 + OH, and N2O(+M) ⇔ N2 + O(+M) reactions represented the most important reactions in biogas-N2O combustion. [ABSTRACT FROM AUTHOR]

This study investigates the residual tensile behaviour of hybrid Carbon Glass fibers reinforced thermoplastic PEEK laminates after they were exposed for 5 min to a hydrogen/oxygen flame. This flame results in a severe thermal aggression characterized by a wall temperature ranging from 900 to 1270 °C and with different heat fluxes (from 200 to 800 kW/m2). The thermally-induced damages were examined by means of microscopic observations and micro CT analyses. The results show that the mass loss linearly depends on the measured heat flux for a 5 min exposure. Depending on the fire testing conditions, the mechanical properties in tension (stiffness and strength) are totally degraded after exposure to the highest heat fluxes (600 and 800 kW/m2) but the retention of the tensile properties is moderate (about −35 to −60% decrease in strength and stiffness, respectively) after exposure to a 200 kW/m2 heat flux. The residual tensile properties of CG/PEEK laminates follow master curves representing the correlations between the mass loss and the changes in the tensile properties regardless the heat flux. These master curves provide a relevant design rule for composite parts to be used under critical service conditions (H 2 /O 2 flame exposure). • The influence of a H 2 /O 2 flame on the behaviour of composite materials was studied. • Wall temperature ranges from 900 to 1270 °C for heat fluxes from 200 to 800 kW/m2. • The mechanical properties are totally degraded after exposure to 600–800 kW/m2. • The changes in the tensile properties are correlated to the mass loss. • Master curves provide a criterion to design composite parts under H 2 /O 2 flame. [ABSTRACT FROM AUTHOR]

The combustion technology of hydrogen enriched natural gas can directly apply hydrogen energy to household gas, playing an important role in promoting the development of hydrogen energy. This study designed a domestic burner with porous media for hydrogen enriched natural gas. The effect of hydrogen blending ratio on the primary mix process in the injector was analyzed numerically, and an experiment was conducted to investigate the flame morphology, thermal efficiency and pollutant emissions of the hydrogen-enriched natural gas. The results show that there is a risk of backfire inside the injector when the hydrogen blending ratio exceeds 45%, and the pre-mixing uniformity decreases as the hydrogen doping ratio increases. Corresponding to variable hydrogen doping ratios, two combustion modes appear in the porous medium region, immerged combustion and surface combustion. When a combustion mode transition occurs, CO emissions increase rapidly, while thermal efficiency and NOx emissions show a downward trend. Additionally, the critical hydrogen doping ratio of mode transition drops as the porous density increases. Overall, taking the environmental characteristic and efficiency into considerations, the maximum hydrogen blending ratio should be not exceed 35%. [Display omitted] • A domestic burner with porous media for hydrogen enriched natural gas was designed. • The effect of hydrogen blending ratio on the primary mix process was analyzed. • The combustion performance and emissions of the domestic burner was investigated. • The transition of combustion mode induced by hydrogen doping ratio was observed. [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 addition has been widely used to improve the combustion performance of low-reactivity fuels such as NH 3. In this study, a series of two-dimensional NH 3 /H 2 flames in mixing layers were investigated numerically. The aim is to assess and interpret the effects of H 2 addition on the structure and dynamics of ammonia flames. Detailed chemistry and transport models were considered in the simulations. It was shown that for pure hydrogen or ammonia flames, a classical tribrachial structure was observed. The non-premixed branch in the tribrachial flame of NH 3 combustion features high heat release rate (HRR), while the two premixed branches are weak. The mixture fraction corresponding to the maximum HRR of NH 3 combustion is significantly lower than the stoichiometric mixture fraction. When blending ammonia with hydrogen, a tetrabrachial flame structure consisting of two premixed branches and two non-premixed branches was observed, where one non-premixed branch corresponds to the reaction of NH 3 , while the other is formed due to the reaction of H 2. It was suggested that the formation of the tetrabrachial flame is related to the preferential diffusion of hydrogen. After decreasing artificially the diffusivity of hydrogen to suppress the preferential diffusion effect, the flame exhibits only three branches with the merging of the two non-premixed branches. By analyzing the flame dynamics at the leading edge, it was found that the flame speed (S d ∗) is negatively correlated with the flame stretch (κ) and scalar dissipation rate (χ), with S d ∗ decreasing almost linearly with κ or χ , consistent with previous studies of methane and hydrogen flames. As the H 2 concentration is increased, the values of S d ∗ increase due to the enhanced reactivity. • Effect of H 2 addition on NH 3 flame structure and dynamics in a mixing layer is studied. • Tetrabrachial flame of NH 3 /H 2 combustion is identified for the first time. • Influence of preferential diffusion of H 2 on the structure of NH 3 flame is analyzed. [ABSTRACT FROM AUTHOR]

Hydrogen has a strong effect on the tendency of fuels to produce soot. The aim of this study is to investigate the effect of hydrogen addition on the formation of soot and its structural evolution in ethylene/oxygen inverse diffusion flames. Laser spectroscopy was used to measure the spatial distribution of soot and Raman spectroscopy was utilized to characterize the nanostructure of soot. Additionally, the influence of hydrogen addition on the soot formation pathway was analyzed using ReaxFF molecular dynamics simulations. The results indicate that increasing hydrogen addition (α) progressively delays soot formation and narrows the flame radius. At 40% hydrogen, soot peak reduction was approximately 64.7%. Raman spectra revealed an increase in both graphite particle size and amorphous carbon content during soot growth. During the evolution of soot nanostructures, the initial particles dominated by sp carbon chains are gradually reorganized to form sp2 aromatic carbon structure, which is consistent with the simultaneous increase of soot concentration and graphite crystal size. It is worth noting that the crystal size of these graphite structure particles is concentrated in the range of 1.6–2.3 nm, indicating that they are in the transition stage from amorphous to microcrystalline. However, at the flame's end, both particle size and amorphous carbon content decreased simultaneously, attributable to oxidation by OH radicals. The study confirms the influence of soot formation and oxidation processes on nanostructures within flames. Raman spectroscopy also indicated a nonlinear change in soot oxidation reactivity along the axial direction of the flame, initially governed by gradual graphitization as flame temperature rises. Molecular dynamics simulations elucidated the nucleation and growth mechanisms of initial soot from polycyclic aromatic hydrocarbons (PAHs). These simulation results show that the increase of hydrogen content in the gas phase reduces the aggregation ability of polycyclic aromatic hydrocarbons and delays the formation of initial soot particles (C41+). With the growth of soot, its morphology gradually evolved from the initial dense, curved, and rotating spindle to a planar structure. The main chemotactic effect of this hydrogen participation is to hinder the dehydrogenation reaction of unsaturated hydrocarbons (H + C m H n → C m H n-1 + H 2), thereby inhibiting the formation of soot precursors, resulting in a decrease in the overall soot concentration. • The microstructure changes of soot in different regions of flame were obtained. • The effect of increasing hydrogen concentration on initial soot generation was obtained. • The effect of hydrogen doping on the morphology change and reaction path of soot formation was obtained. • The main factors affecting soot formation by hydrogen mixing were obtained. [ABSTRACT FROM AUTHOR]

The issue of spontaneous ignition has become a bottleneck for hydrogen applications. This comprehensive review delves into the intricate mechanisms governing the spontaneous ignition of pressurized hydrogen released through tubes. Beginning with an exploration of fundamental concepts, including the properties of hydrogen and various theoretical frameworks, the study meticulously examines the development of diffusion ignition theory. Methodologically, both experimental and simulation approaches are scrutinized for their contributions to understanding this complex phenomenon. Moving deeper into the core of the investigation, the review dissects the spontaneous ignition mechanisms occurring inside pipes, unraveling the influence of different initial conditions as well as the impact of varying pipe structures. Subsequently, the review shifts its focus to the flame morphology at the pipe exit, analyzing combustion behavior, shock wave dynamics, flow structures, and the formation of jet flames, while exploring influential factors affecting combustion. Through a comprehensive synthesis of findings, this review provides valuable insights into the spontaneous ignition dynamics of pressurized hydrogen within tube systems, offering a foundation for future research and engineering applications in safety and energy sectors. • Introduced theories of self-ignition and development of diffusion ignition theory. • Summarized ignition mechanisms of pressurized hydrogen released inside pipes. • Summarized flame morphology at pipe exit based on vortex position. • Identified deficiencies in reviewed studies and areas needing further research. [ABSTRACT FROM AUTHOR]

This paper presents a Large Eddy Simulation (LES) study investigating jet flame ignition characteristics to achieve stable ammonia combustion for novel applications in gaseous ammonia/hydrogen-fueled engines with an active prechamber. The study investigated three cases with initial global equivalence ratios of 0.6, 0.8, and 1.0. Two main ignition modes were proposed, and five stages of coupled flame propagation processes were identified. The oscillation of supersonic hydrogen jet flame was investigated. The modes of flame-vortex interaction including vortex ring formation, along with the mechanism of highly reactive zone formation, were proposed. The effects of reactivity and turbulence transition on piloted ammonia flame were investigated to attain stable ammonia combustion and achieve a high heat release rate. The results showed that oscillations in the jet flame were strengthened at low global equivalence ratios with high hydrogen mass ratios. Enhancing propagation of initial and secondary vortex rings broadened highly reactive zones. Vorticity dissipated more rapidly at low equivalence ratios, leading to a stratified structure and enhanced piloted spherical flame. The Unsteady Flamelet Ignition Prediction (UFIP) sub-model of combustion was verified for the piloted ammonia flame, enabling a more comprehensive discussion of flame propagation tendencies. • The transient characteristics of jet flames ignited by H 2 /NH 3 mixtures were analyzed using LES. • The behavior of stratified flames at various equivalence ratios was investigated. • Various modes of turbulent jet flame ignition were proposed according to different stages. • The mechanism of vortex evolution to form the reactive zone and pilot flame was proposed. [ABSTRACT FROM AUTHOR]

For a rapid transition to a carbon-neutral transport sector, hydrogen internal combustion engines could be one measure. To support the development, suitable simulation models are needed. This paper provides enhancements to an existing combustion modeling approach for lean homogeneous hydrogen–air mixtures with online reaction kinetics and combines swirl charge motion with a spark ignited combustion process. These include a real gas equation of state, a wall heat loss model that is suitable for hydrogen and a flow model. The model is implemented in Python using Cantera with a custom solver interface. The model was successfully validated through experimental measurement data from an engine test bench. Additionally, some insights into the validity of the model assumptions are given by comparing model results to three-dimensional computational fluid dynamics simulation data. • Model for a spark-ignited combustion process and swirl-based charge motion. • Solver interface for connection of thermodynamics handled by Cantera and flow model. • Successful model validation using experimental data. • Analysis of flame sphericity with 3D CFD simulation results. [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]

To improve energy savings and emission reduction in industrial heating furnaces, this study investigated the impact of various molar fractions of hydrogen on natural gas combustion and compared the results of the Non-Premixed Combustion Model with the Eddy Dissipation Combustion Model. Initially, natural gas combustion in an industrial heating furnace was investigated experimentally, and these results were used as boundary conditions for CFD simulations. The diffusion flame and combustion characteristics of natural gas were simulated using both the non-premixed combustion model and the Eddy Dissipation Combustion Model. The results indicated that the Non-Premixed Combustion Model provided simulations more consistent with experimental data, within acceptable error margins, thus validating the accuracy of the numerical simulations. Additionally, to analyze the impact of hydrogen doping on the performance of an industrial gas heater, four gas mixtures with varying hydrogen contents (15% H2, 30% H2, 45% H2, and 60% H2) were studied while maintaining constant fuel inlet temperature and flow rate. The results demonstrate that the Non-Premixed Combustion Model more accurately simulates complex flue gas flow and chemical reactions during combustion. Moreover, hydrogen-doped natural gas significantly reduces CO and CO2 emissions compared to pure natural gas combustion. Specifically, at 60% hydrogen content, CO and CO2 levels decrease by 70% and 37.5%, respectively, while NO emissions increase proportionally; at this hydrogen content, NO concentration in the furnace chamber rises by 155%. [ABSTRACT FROM AUTHOR]

The research has examined the effects of CO 2 dilution on ignition characteristics in H 2 /air mixtures. An ignition kernel was initiated by a laser-induced plasma in H 2 /air/CO 2 mixtures with a bulk velocity of 6.5 m/s. An infrared camera was used to measure radiation intensity emitted from H 2 O and ignition probabilities. A high-speed schlieren image system was utilized to measure ignition kernel areas and ignition time. High-speed chemiluminescence was performed to examine OH* intensities. Numerical simulations were conducted using GRI 3.0 mechanisms to calculate reaction rates and laminar flame speeds. The ignition probability is highest for the undiluted mixtures and decreases with increasing the CO 2 dilution fraction at constant adiabatic temperatures. A prolonged ignition time is observed at elevated CO 2 dilution concentrations. Increasing the CO 2 mole fraction decreases the ignition kernel growth, the integrated OH* intensity, and the integrated radiation intensity for each adiabatic temperature. CO 2 reacts with H radicals and decreases the production of OH*, OH, and H 2 O. The CO 2 dilution in the H 2 /air flames significantly contributes to the reduced reaction rate and flame speed, resulting in the low ignition probability. The high ignition probability is found at the large integrated OH* intensity in all cases. The ignition probability is high at the large integrated radiation intensity under the constant adiabatic flame temperatures. The OH* intensity is a critical parameter to estimate the ignition probability in the diluted lean hydrogen flames. • The effects of CO 2 dilution on the ignition process have been studied in lean premixed H 2 /Air flames. • The ignition probability decreases with increasing the CO 2 dilution fraction. • An increase in the CO 2 dilution fraction in the mixtures reduces OH* intensity and radiation intensity emitted from H 2 O. • The presence of CO 2 in the mixtures decreases reaction rates and flame speed. [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]

Hydrogen fuel cell vehicles (HFCVs) may cause fires in the event of an accident. When the high-pressure hydrogen storage cylinders are exposed to fire for a long time, there will be a risk of catastrophic consequences such as leakage, jet fire and explosion. Fire test is the main method to study the safety performance of hydrogen storage cylinders. During the firing process, the wall and internal temperature of the hydrogen storage cylinder rise gradually, and the thermally activated pressure relief device (TPRD) of the cylinder reaches a certain temperature and then releases high-pressure hydrogen to prevent the cylinder from bursting. As the hydrogen storage cylinder is in the fire environment, the release of high-pressure hydrogen will be ignited to form a jet fire and cause damage to personnel and equipment. In this paper, CFD software Fluent was used to numerically simulate the local fire test of the hydrogen storage cylinder with high-temperature air as the fire source. The variation law of cylinder wall temperature, gas temperature and pressure inside the cylinder, and the activation of TPRD were analyzed. At the same time, on the basis of the fire test simulation, a high-pressure hydrogen jet fire simulation numerical model caused by the TPRD action was established. The velocity field, temperature field and hydrogen concentration distribution of the jet fire were obtained, and the overpressure generated by the jet fire and the flame length size were analyzed. The results show that the high-pressure hydrogen released by TPRD was ignited under the fire condition to produce a jet flame and generated an overpressure in the jet direction, and the influence range of the overpressure was smaller than the influence range of the jet flame. • The internal temperature, pressure and wall temperature variation laws of cylinders were analyzed. • Hydrogen jet fire flow field changes after TPRD activation were simulated. • The influence range of overpressure in the direction of the jet is smaller than that of the jet flame. • The length of the jet flame is affected by the initial injection pressure and the TPRD discharge diameter. [ABSTRACT FROM AUTHOR]

To analyze the influence of different ignition positions on the propagation of deflagration characteristics and the disaster mechanism in the hydrogen deflagration process in the tunnel, this study compares and verifies the experimental data of tunnel model deflagration based on the CFD code GASFLOW-MPI. A combustion velocity model based on the accelerated separation effect is used, and the effect of heat transfer on the consequences of deflagration is considered. The propagation laws of pressure waves, fluid velocity and temperature in the tunnel are analyzed, and it is found that the changing trends of shock waves and fluid velocity in the tunnel are consistent, but completely opposite in the tunnel outside. The effects of different ignition positions on the hydrogen deflagration characteristics are compared and analyzed. The results show that when ignition occurs at different heights of the center of the premixed region, the impact on the deflagration characteristics is relatively small, while when ignition occurs at different horizontal distances in the premixed region, the impact on the deflagration characteristics is relatively large; the peak overpressure of side ignition is far lower than the case where ignition occurs in the center of the premixed region, but the maximum flow velocity is much higher than the center ignition. The impact on temperature is the smallest. In general, in areas close to the hydrogen/air premixed zone, side ignition results in more severe consequences, whereas in areas far from the premixed zone and outside the tunnel, central ignition leads to more severe consequences. This study provides theoretical support for understanding the disaster-causing mechanisms of hydrogen/air premixed gas deflagration in tunnels under different ignition positions. • Deflagration behaviors of hydrogen-air mixtures in the tunnel were studied. • Heat transfer and flame instability are taken into account in numerical simulations. • The deflagration consequences of different ignition positions are discussed. • Side ignition causes the least peak overpressure in the tunnel. [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]

Iron ore pellet production processes produce large amounts of CO2 emissions when burning fossil fuels. One example is the grate-kiln process, where normally a coal flame is used to indurate pellets. To mitigate the emissions, coal used in rotary kilns can be replaced with for example hydrogen. However, the fuel is mixed with secondary process air, and replacing a solid fuel with a gaseous one changes the mixing characteristics. This demands different means of injecting the fuel. A coaxial jet can be used to control the mixing of fuel and secondary air, as well as the flow field. The aim is to control the mixing of hydrogen and secondary air to achieve a hydrogen flame that is similar to the reference coal flame. This study numerically investigates different coaxial jet configurations. Steady-state simulations of a simplified model of the real kiln are performed using a Reynolds Stress turbulence model. Results show that decreasing the momentum flow ratio between the outer and inner jet, Mjet, to a certain value delays the spread of the hydrogen jet and thus gives a longer jet. Further decreasing this ratio gives an even longer jet, but has the side effect of producing recirculation of hydrogen. [ABSTRACT FROM AUTHOR]

Flame stability and pollutant emission performances of a porous burner fueled with ammonia/hydrogen blend were investigated numerically in this study. In this regard, a 2D solver based on finite volume method was developed in order to simulate reacting flow and heat transfer modes through a porous medium. Combustion characteristics in terms of porous structure, equivalence ratio and fuel component were studied and the pollutant emission trends were discussed. Flame stability limit was observed to be extended with increasing equivalence ratio regardless to ammonia fraction. Results proved that addition of ammonia in the fuel blend reduced the flame stability limits and thermal flame thickness but led to the enhanced NO emission. Increasing equivalence ratio led to a decrease in thermal flame thickness under fuel-lean conditions while it caused that the thickness of flame zone to be increased slightly under fuel-rich conditions. It was found that flame stability limits were extended as the mean pore diameter of the porous medium increased. The maximum amounts of NO concentration were achieved at stoichiometric conditions while, NO emission increased as equivalence ratio increased at fuel-lean conditions and it decreased as equivalence ratio increased at rich combustion regimes. [ABSTRACT FROM AUTHOR]

Hydrogen-fueled gas turbines play one of the key roles in future carbon-neutral energy structure. Due to hydrogen's high flame speed and extreme flame temperature, design of burners applying hydrogen as fuel is of challenge. In this work, we conduct direct numerical simulations to investigate a non-premixed steam-diluted oxygen/hydrogen combustion, which is formed by inclined impinging fuel and oxidizer jets injected by sub-millimeter nozzles. This configuration inherently prevent flashback and has a higher mixing efficiency enhanced by jet impingement, thereby leveraging advantages of both conventional premixed and non-premixed configurations. The flame is lifted flame held at the jet impinging position far away from the wall boundary. The most intensive mixing process occurs at the outer sides of the two branches of hydrogen jet after impinging, forming the flame base and resulting in an edge-flame configuration. A larger jet inclination angle leads to more effective bulk turbulence mixing because of the increased mixing volume, thereby enhancing combustion completeness. Simultaneously, it results in lower wall heat flux due to gentler upstream recirculation of burnt products. Chemical explosive mode analysis is conducted to analyze the combustion procedure from ignition to burnt out. The flame is held by the recirculation of high-temperature burnt products upstream of the impingement position, and its structure is primarily formed by the simultaneous ignition of a widespread explosive mixture enhanced by intense turbulence. • DNSs are conducted to investigate the combustion of impinging fuel and oxidizer jets. • Edge-flame configuration is formed by impinging fuel and oxidizer jets. • Fuel and Oxidizer are intensively mixed near the oxidizer jet branches. • Inclination angle of jets affects the statistical thermophysical performance. • Flame is held by hot product recirculation. [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]

The addition of hydrogen in a dual-fuel mode based on natural gas and diesel offers great potential for improving engine efficiency and reducing emissions. Therefore, detailed numerical simulations have been used to explore the effects of soot formation characteristics of natural gas-diesel dual-fuel engines doped by hydrogen at elevated pressure. Numerical simulations are compared with available experimental data, the effects of H 2 addition on soot formation in laminar CH 4 + n-heptane, H 2 + CH 4 + n-heptane, FH 2 + CH 4 + n-heptane diffusion flames at 2, 4, 6 and 8 atm are simulated, FH 2 is added to separate the H 2 chemical effects. The results show that the dilution and thermal effects of H 2 inhibit soot formation, while the chemical effects of H 2 promote soot formation under different pressures. The addition of H 2 increases the concentration of H and OH radicals, which promotes the formation of C 2 H 2 and benzene (A1). By simplifying the reaction path and analyzing the production rate of A1, the mechanism of H 2 addition to promote A1 formation is illustrated. The concentrations of polycyclic aromatic hydrocarbon (PAH) pyrene (A4), benzo(ghi)fluoranthene (BGHIF) and benzo(a)pyrene (BAPYR) are inhibited at higher pressures that result in lower inception and condensation rates. Therefore, the increase of hydrogen abstraction acetylene addition (HACA) rate is the main reason for the increase of soot formation under chemical effects. While the O 2 oxidation rate decreases and the OH oxidation rate increases, that is due to the decrease of O 2 concentration and the increase of OH concentration in the soot formation area under chemical effects. • The dilution, thermal and chemical effects of H 2 addition on soot formation in methane/n-heptane flame were investigated at high pressures. • Key specie A1 formation pathways and related reactions were confirmed when H 2 addition. • The changes of soot formation and oxidation mechanisms with H 2 addition in different pressure were explained. [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]

In this paper, we apply a framework based on decomposition techniques to study the synchronization of flow velocity with acoustic pressure and heat release rate in swirl flames. The framework uses the extended proper orthogonal decomposition to identify regions of the velocity field where velocity and heat release fluctuations are highly correlated. We apply this framework to study coupled interactions associated with period-1 and period-2 type thermoacoustic instability in a technically premixed, swirl-stabilized gas turbine-type model combustor operated with hydrogen-enriched natural gas. We find the structures in the flame surface and the heat release rate correlated with the dominant coherent structures of the flow field using extended POD. We observe that the correlated structures in the flow velocity, flame surface and heat release rate fields share the same spatial regions during thermoacoustic instability with period-1 oscillations. In the case of period-2 oscillations, the structures from flame surface and heat release rate field are strongly correlated. However, these structures contribute less to the coherent structures of the flow field. Using the temporal coefficients of the dominant POD modes of the flow velocity field, we also observed 1:1 and 2:1 frequency locking behaviour among the time series of acoustic pressure, heat release rate and the temporal coefficients of the first two dominating POD modes of velocity field during the state of period-1 and period-2 oscillations, respectively. These frequency-locked states, which indicate the underlying phase-synchronization states, correlate with coherent structures in the flow velocity field. [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]

Practical combustors in furnaces, industrial heaters, and gas turbine engines face a sudden loss of flame due to rich blowout (RBO) at an extremely rich fuel-air mixture or lean blowout (LBO) at an extremely lean fuel-air mixture. Thus, the stability limits of the combustion regime are governed by RBO and LBO. In the present study, we focus on the dynamics of swirl-stabilized premixed combustion near lower and higher flammability limits, where the possibilities of LBO and RBO, respectively, are observed. Near RBO and LBO, we employ metrics from statistics and dynamical systems theory to characterize the transition in flame dynamics. We observe that the range of frequencies obtained using Fourier Transform near RBO and LBO is alike. The emergence of dominantly low-frequency oscillation near those blowout limits is investigated using Hilbert Transform. The mean frequency of the combustion system gradually reduces near both blowout limits due to behavioral oscillations prominently observed. The scaling property of flame oscillation is examined using the Hurst exponent, and we observe a decreasing trend of the parameter as combustion approaches both LBO and RBO. Therefore, for the gradual reduction of the Hurst exponent near both flammability limits, we can use the parameter as a precursor for both RBO and LBO. [ABSTRACT FROM AUTHOR]

System failure in large-scale electrolyzer and fuel cell installations may cause the formation of explosive H2–air–steam mixtures. Detonation properties (e.g., detonation cell size) and flame dynamics features (e.g., flame acceleration, runup distance, and deflagration-to-detonation transition "DDT") of these mixtures were investigated experimentally and numerically to gain a more in-depth understanding of the hazards of H2–air–steam under conditions pertinent to PEM-based electrolyzers and fuel cells (temperatures between 50 °C and 80 °C and pressures between 20 and 40 bar). While our results confirm the findings of previous studies in terms of the cooling effects of steam on detonation, we found that operating pressures between 20 and 40 bar counteract the effect of steam, making the H2–air–steam mixture more detonable. This is particularly evident from the experimental data on detonation cell size and runup distance at pressures greater than 20 bar. [ABSTRACT FROM AUTHOR]

Green hydrogen is an important future energy source, which offers a vast potential to implement the decarbonization of the marine sector and advance broad shift to clean-energy alternatives globally. There are various advantages of offshore floating photovoltaics (FPVs) technology for hydrogen production; however, hydrogen storage in FPVs-based hydrogen production system faces several challenges. It is found that the major barrier concerning the system under investigation is related to safety. The current study aims to present an applicable offshore FPVs-based hydrogen production system, which involves both the FPV section and the hydrogen production section based on a project in China. A numerical 3D model is performed to investigate the characteristics of accidental damage through potential hydrogen storage device failure during system operation. The hydrogen release process of an FPVs-based hydrogen production system is presented with different offshore wind conditions, and the parameters for understanding the motion state and hydrogen release mode of hydrogen are also analyzed. The study further explores the dynamic development of hydrogen dispersion from a hydrogen production platform, including a momentum-dominated region, a horizontal spreading region, and a vertical buoyancy region. In addition, the influence of hydrogen explosive flame on thermal damage evaluation is illustrated, and thermal hazards under different offshore wind conditions are also discussed. The current study contributes to a better understanding of failure analysis of the FPVs–hydrogen production system and elaborates on damage evolution of hydrogen storage integrated with the system. The study also concentrates on marine environmental synergistic limits considering thermally damaged mechanical properties. [ABSTRACT FROM AUTHOR]

Ammonia (NH 3)-hydrogen (H 2) blends which are obtainable by NH 3 cracking can be an eminent carbon-free fuel. To provide a valuable database for the practical application of cracking-based NH 3 -H 2 fuel, the effects of pressure on the combustion characteristics of premixed NH 3 -H 2 (-nitrogen (N 2))/air swirl flames are investigated in terms of H 2 mole fraction in the NH 3 -H 2 blend and fuel-equivalence ratio. Extinction mechanisms at fuel-lean and rich flames are the same for both normal and elevated pressures, though only the blowoff is observed for elevated pressure. With increasing pressure, fuel-rich limits are generally extended but fuel-lean limits are reduced, NO x emissions decrease and the condition of the maximum NO x emissions moves towards fuel-richer condition. Flame visualization exhibits the shifted main reaction zone upstream, the increased flame wrinkling and the reduced OH and NO intensities with increasing pressure. Dilution effects of N 2 at elevated pressure become remarkable compared with those at normal pressure. [Display omitted] • For premixed NH 3 –H 2 (-N 2)/air flames at elevated pressure, only blowoff is observed. • Different fuel-lean and rich limit tendencies with increasing pressure are found. • NO x emissions decrease with increasing pressure. • With increasing pressure, the main reaction zone shifts upstream. • Dilution effects of N 2 are more dominant at elevated pressure than normal pressure. [ABSTRACT FROM AUTHOR]

The microjet flame structure of a hydrogen-oxygen mixture exhausting into air is studied experimentally. A cylindrical metallic micronozzle with a diameter of 0.5 mm and a parabolic velocity profile at its cross-section is used to form a microjet of the hydrogen-oxygen mixture. It is found that adding oxygen to the hydrogen flow leads to a significant change in the dimensions of the laminar combustion zone and the structure of the flame jet. As the microjet exhaustion velocity increases, it is necessary to decrease the oxygen content in the mixture to ensure flame stability. It is shown that adding oxygen to the hydrogen flow transforms the spherical shape of the laminar combustion zone into a narrow and elongated cylinder-shaped flame region. As the microjet exhaustion velocity increases, the longitudinal size of the laminar combustion zone grows. • The combustion of high-velocity microjet of H 2 +O 2 mixture in air is studied. • Increasing of O 2 fraction in the H 2 +O 2 mixture reduces microjet burning stability. • Adding O 2 to H 2 causes the flame to change shape and become more elongated. • The nozzle's flame coverage significantly affects the H2+O2 microjet combustion. [ABSTRACT FROM AUTHOR]

The present study explores the characteristics of reacting flow in a scramjet combustor with struts, focusing particularly on implementing different injection strategies. A three-dimensional DLR scramjet combustor is utilised to assess the impact on the system, incorporating multiple injections and varying injection angles on the triangular wedge. The analysis considers three injectors with parallel, upward and downward injections at angles of 15° and 30°. The numerical investigation is conducted under a constant total pressure of 7.82 bar, a temperature of 340 K, and an airspeed of Mach 2 at the inlet. The results highlight the significance of injector location and shape in promoting flame stabilization. Furthermore, injection angles play a crucial role in mitigating shockwave intensity. The numerical analysis involves a steady-state Reynolds-averaged Navier- Stokes equation with the shear stress transport k--ω turbulence model. The obtained results were analyzed by examining the critical variables such as Mach number, static pressure and combustion efficiency across the combustor. Based on the computational results, injecting fuel upward not only increases the overall pressure loss but also enhances the subsonic regime downstream of the strut, which leads to better mixing and combustion efficiencies. This is primarily due to shockwave generation from the edges of the strut and the interactions with the fuel stream shear layers. [ABSTRACT FROM AUTHOR]

This work proposes a modified dynamic flame thickened model for laminar premixed hydrogen/air flames and describes flame simulations conducted for validation. The model is implemented on the open-source OpenFOAM v9, and four flame configurations are simulated: 1D laminar premixed flames, axi-symmetric stagnation-point flames, axi-symmetric Bunsen flames, and 2D bluff body flames. The 1D flame simulation reconfirms the validity of the thickened flame models. The axi-symmetric stagnation-point flames are simulated to find the optimal modification factor, a modeling parameter. The axi-symmetric Bunsen flame simulations are conducted for validation, and the results showed that the flame speed and stretch rate are well maintained. The 2D bluff body flame simulations are then conducted, and the results confirmed that the proposed model can produce stretched laminar flame speeds that are similar to those of the non-thickened reference cases, especially at low equivalence ratio conditions. Finally, the 2D bluff body flame simulations showed that the proposed model has reasonable prediction accuracy compared with the reference data. [ABSTRACT FROM AUTHOR]