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The influence of microstructure on the response of energetic structural materials (ESMs) under shock conditions remains inadequately quantified, and the energy release process is not thoroughly understood. In this work, taking the classical Ni/Al ESM as an example, the shock response was investigated by the shock compression theory with the microstructure-based chemical reaction kinetics model. This theory mainly refers to the equation of the state of multi-component materials with mixture rule, and the reaction at the particle contact interface is built to form the multiscale thermodynamics model. The physical states of material after shock, including relative volume, temperature, and extent of reaction, were analyzed. The results revealed the effect of the burn velocity, particle size and molar ratio on the shock response. Furthermore, the model facilitates a comprehensive understanding of energy release, the extent of the intermetallic reaction, and the oxidation reaction. Despite the involvement of only a small portion of materials in the oxidation reaction, the energy release proportion was comparable to that of the intermetallic reaction. Additionally, insights into the effect of the microstructure on the energy release revealed by the model matched the tests well. [ABSTRACT FROM AUTHOR]

Reactive burn models for heterogeneous energetic materials (EMs) must account for chemistry as well as microstructure to predict shock-to-detonation transition (SDT). Upon shock loading, the collapse of individual voids leads to ignition of hotspots, which then grow and interact to consume the surrounding material. The sub-grid dynamics of shock-void interactions and hotspot development are transmitted to macro-scale SDT calculations in the form of a global reactive "burn model." This paper presents a physically evocative model, called meso-informed sub-grid source terms for energy localization (MISSEL), to close the macro-scale governing equations for calculating SDT. The model parameters are explicitly related to four measurable physical quantities: two depending on the microstructure (the porosity ϕ and average pore size D ¯ v o i d ), one depending on shock–microstructure interaction (the fraction of critical voids ξ c r ), and the other depending on the chemistry (the burn front velocity V h s ). These quantities are individually quantifiable using a small number of rather inexpensive meso-scale simulations. As constructed, the model overcomes the following problems that hinder the development of meso-informed burn models: (1) the opacity of more sophisticated surrogate/machine-learning approaches for bridging meso- and macro-scales, (2) the rather large number of high-resolution mesoscale simulations necessary to train machine-learning algorithms, and (3) the need for calibration of many free parameters that appear in phenomenological burn models. The model is tested against experimental data on James curves for a specific class of pressed 1,3,5,7-tetranitro-1,3,5,7-tetrazoctane materials. The simple, evocative, and fast-to-construct MISSEL model suggests a route to develop frameworks for physics-informed, simulation-derived meso-informed burn models. [ABSTRACT FROM AUTHOR]

The laminar burning velocity (LBV) and cellular instability of ammonia/syngas/air were investigated using the spherically expanding flame method at an initial pressure of 3 atm and temperatures of 298–448 K. The impacts of equivalence ratio, temperature, and hydrogen ratio on LBV were scrutinized. The results demonstrate that the LBV varies non-monotonically with the equivalence ratio and increases with increasing temperature and hydrogen ratio. Flame instability was analyzed using flame uplift rate, Lewis number, thermal expansion ratio, flame thickness, critical radius, critical Peclet number and logarithmic growth rate of perturbation in combination with the schlieren images. Of these, the flame uplift rate is used to quantify buoyancy instability, a flame instability that has received little attention. At high pressures, lower LBV makes the flame susceptible to buoyancy instability. The flame uplift rate increases with increasing flame radius, suggesting that the buoyancy instability becomes increasingly significant as the spherical flame expands. The diffusional-thermal instability is weaker at the fuel-rich side and enhanced at the leaner side. As the hydrogen ratio increases, the flame instability increases. Hydrodynamic instability is not susceptible to temperature, which is only slightly weakened with increasing temperature. Additionally, the phenomenon of superadiabatic flame temperature (SAFT) in ammonia/syngas/air flames was first found and analyzed its temperature dependence. Increasing the initial temperature suppresses SAFT occurrence. [Display omitted] • LBVs of ammonia/syngas/air mixtures were measured at 298–448 K and 3 atm. • The buoyancy instability was studied in detail using flame uplift rates. • The effect of initial temperature on the flame instability was analyzed in detail. • The phenomenon of SAFT in ammonia/syngas/air flames was first found. [ABSTRACT FROM AUTHOR]

Ammonia (NH 3), as an ideal zero-carbon fuel, plays a positive role in mitigating global warming. By cofiring NH 3 with highly reactive small molecule fuels such as hydrogen (H 2), carbon monoxide (CO) and methane (CH 4), the reactivity of NH 3 combustion can be significantly enhanced. In this work, a detailed kinetic mechanism for NH 3 /CH 4 /H 2 /CO containing 146 species and 1099 reactions was developed and extensively validated with selected experimental data from the literature on laminar burning velocity (LBV), ignition delay time (IDT), and species concentrations measured in burner-stabilized flames (BSF), plug flow reactors (PFR) and jet-stirred reactors (JSR), covering temperatures of 273–2000 K, pressures of 0.053–40 atm and equivalence ratios of 0.1–2. In addition, the detailed mechanism was simplified to include 53 species and 353 reactions using three simplification methods. Kinetic modeling analysis showed that the chemical and transport effects of H 2 , CO and CH 4 are the main factors enhancing NH 3 combustion, which rising with the increasing initial temperature and decreasing ammonia blending ratio. Among the four reactants, H 2 is consumed first, followed by CH 4 and NH 3 , with CO reacting last. By modifying the Metghalchi-Kech power law equation, it was found that a clear linear relationship exists between LBV and the sum of the maximum mole fractions of O, H, OH, and NH 2 radicals. [Display omitted] • A kinetic mechanism for NH 3 /CH 4 /H 2 /CO was proposed and simplified. • Validation range includes T 0 of 273–2000 K, P 0 of 0.053–40 atm and Ф of 0.1–2. • Chemical kinetics of LBV for NH 3 /H 2 , NH 3 /CO and NH 3 /CH 4 blends were studied. • The correlation between LBV and free radicals was investigated. [ABSTRACT FROM AUTHOR]

For net‐zero carbon emissions, hydrogen/ammonia blends have drawn considerable attention for the application in industrial combustion devices. Various chemical mechanisms have been developed to describe the oxidation and combustion of hydrogen/ammonia mixtures at certain conditions. A comprehensive evaluation and comparison of the performance of these mechanisms is thus of high interest, especially in terms of their application for particular computational studies. Thus, this work aims to compare the existing chemical mechanisms in terms of their performance for the combustion of hydrogen/ammonia mixtures over a wide range of experimental conditions. In addition to previous literature studies, the model performance is evaluated by using two different methods for the assessment of prediction accuracy. Besides the conventional measure of point‐wise differences between model and data, the curve‐matching method is also applied, which quantifies the dependence of model response on physical conditions additionally, by comparing the similarity between the curve shapes of the predicted and measured results. Extensive experimental data are taken into account in the model evaluation, including 136 datasets obtained from various facilities in the past 10 years. Nineteen mechanisms are compared, which were published in recent five years. It is revealed that these models give strongly different numerical results for combustion targets, such as laminar burning velocities, ignition delay times, and species concentrations. The chemical mechanisms of Zhang et al. (2021), Han et al. (2023), Mei et al. (2019), Li et al. (2019), and Stagni et al. (2020) show relatively satisfactory performance over the entire investigated domain. Moreover, it is found that the estimated prediction accuracy of chemical mechanisms is highly sensitive to model evaluation methods. [ABSTRACT FROM AUTHOR]

In this work, combustion characteristics of premixed n-dodecane/air mixtures are investigated at elevated mixture temperatures through laminar burning velocity measurements at atmospheric pressure and elevated temperatures using an externally heated diverging channel method. The experiments were conducted up to high-temperature conditions of 610 K at varying equivalence ratios (ϕ = 0.7–1.3) to mimic the conditions of practical application devices. With an increase in the mixture temperature, the laminar burning velocity increases because of the rise in reactant enthalpy. The peak laminar burning velocity is observed at ϕ = 1.1 for all the mixture temperatures, except at 600 K, where the maximum laminar burning velocity is observed for stoichiometric conditions. The minimum temperature exponent exists for a slightly rich mixture condition (ϕ = 1.1). Out of all mechanisms, the laminar burning velocity predictions using the Lawrence Livermore National Laboratory model are consistent with the present data at all mixture temperatures. Normalized sensitivity analysis is analyzed using the Lawrence Livermore National Laboratory mechanism to understand the key reactions affecting the laminar burning velocity of the n-dodecane/air mixture. The chain branching reaction R16: H + O2 <=> O + OH has a major influence on the enhancement of the laminar burning velocity. Reaction pathway analysis is carried out at stoichiometric conditions for mixture temperatures of 400 K and 600 K, where it is observed that the elemental flux of the reaction, converting CO to CO2, gets reduced by 42.91% at the high-temperature condition. [ABSTRACT FROM AUTHOR]

This study employs Cantera code to investigate the laminar burning velocity of different ammonia–hydrogen mixtures. Suitable models were selected from recent literature, and the one with the lowest root mean square error (RMSE) against experimental data was identified through the error function method. Bao mechanism shows an RMSE value of 4.71 at atmospheric pressure for ammonia–hydrogen mixtures, while the Otomo mechanism exhibits an RMSE of 2.11 under high‐pressure conditions. Additionally, sensitivity analysis was conducted to highlight critical reactions within each mechanism, emphasizing distinctions between different pressures. This approach aims to choose the proper mechanism to reduce computational and experimental costs in the early stages of ammonia–hydrogen research. [ABSTRACT FROM AUTHOR]

As a carbon-free hydrogen-carrying fuel, ammonia can effectively alleviate the problem of CO 2 emissions and has great application potential in marine engines. The low laminar burning velocity and calorific value, and the narrow flammability limit range make the combustion of pure ammonia difficult. Ammonia/methanol mixed combustion is a feasible method to improve ammonia combustion and mitigate CO 2 emissions. It is difficult to accurately predict the laminar burning velocity (LBV) and ignition delay time (IDT) of ammonia/methanol combustion simultaneously by the existing chemical kinetic mechanisms. This study aims to develop a detailed chemical kinetic mechanism that can simultaneously predict the LBV and IDT. The new-developed mechanism was fully validated based on experimental data. Furthermore, it was employed to investigate the effects of methanol blending ratios and equivalence ratios on the LBV and IDT in NH 3 /CH 3 OH mixed flames. Besides, chemical kinetic analyses were conducted to elucidate the influence of methanol mixing ratios and equivalence ratios on LBV and IDT. Reaction pathway analysis revealed that the NH 3 and CH 3 OH chemistry interacted by the sharing of H, OH and O radicals in fuels' decomposition, and chemical interactions of N-containing radicals with the C 1 species. The reactions involving C-containing species can notably improve the LBV of NH 3 /CH 3 OH mixed combustion. At low equivalence ratio, LBV increases in proportion to the fuel proportion. Conversely, at high equivalence ratio, the effect of oxygen concentration on LBV becomes the dominant factor. • A chemical kinetic mechanism of NH 3 /CH 3 OH was developed. • CH 3 O, CH 2 OH, H and OH radicals facilitate the NH 3 oxidation. • C-containing species notably improved LBV. • The effect of equivalence ratio on IDT is smaller than methanol blending ratio. [ABSTRACT FROM AUTHOR]

The deflagration-to-detonation transition (DDT) poses significant risks in the oil, gas, and nuclear industries, capable of causing catastrophic explosions and extensive damage. This study addresses a critical knowledge gap in understanding the DDT of ethane–air mixtures on a large scale, amid increasing industrial utilization and production of ethane. A novel computational framework is introduced, utilizing the finite-volume code named Morris Garages, which incorporates reactive compressible Navier–Stokes equations, adaptive mesh refinement, and correlations of turbulent burning velocities. This model integrates the most recent data on laminar and turbulent burning velocities for premixed ethane–air mixtures, simulating flame acceleration and DDT within a two-dimensional large-scale setting, measuring 21 m in length and 3 m in height, with obstacles mimicking pipe congestion. Two mixture scenarios, lean and near-stoichiometric, are analyzed to evaluate the effects of equivalence ratios on flame propagation and DDT. The simulations, validated against large-scale experimental data from Shell, show reasonable agreement and provide critical insights into the onset conditions of DDT, such as temperature, pressure, flame speed, and turbulent kinetic energy. Furthermore, the ξ–ε detonation peninsula diagram is utilized to explore autoignition and detonation behaviors in ethane–air mixtures. [ABSTRACT FROM AUTHOR]

The determination of the characteristics and main combustion properties of fuels is necessary for post-implementation in different applications. Among the most important combustion properties of a fuel are the combustion products, flame temperature and laminar burning velocity. Therefore, this paper describes the step-by-step development and coding of a MATLAB application that can determine 12 combustion products, flame temperature and laminar burning velocity in order to understand the logic of calculus procedure, so any user would be able to make improvements of new functionalities (add more fuels, add more combustion products, etc.). The numerical procedure and methods (Gaussian elimination, Taylor Series and Newton–Raphson) parallel with their implementation as code lines for the development of the application are carried out using flow charts. In addition, simulations in Ansys Chemkin were performed and included in the application as part of the results comparison. It was found that: (1) The MATLAB Application codification and development were successfully explained in detail, (2) the functions and execution sequence are described by using flow charts and code extract, (3) the application is available to everyone for modifications, (4) the application can only be used for hydrocarbons fuels, (5) the application execution time registered was less than 8 s. [ABSTRACT FROM AUTHOR]

This study evaluates the laminar burning velocity of oxyfuel syngas mixtures. Experimental measurements were performed in standard conditions (298 K and 1 atm), varying the oxyfuel syngas composition (H2/CO/CO2/O2) and equivalence ratio for lean mixtures. The experimental setup uses the heat flux method, validated with CH4/air and H2/CO/air measurements. The results were compared with six chemical kinetic mechanisms developed for syngas, aiming at evaluating their predictive capabilities. The mixtures presented a tendency to the appearance of cellular instability in the flame front that was prevented to a certain extent. The CO2 effects on the flames were explored through numerical analysis, isolating the thermodynamic properties, transport properties, thermal radiation, and chemical effects. The CO2 or N2 effect as bath gas in the oxidizer is also evaluated in terms of the laminar burning velocity and mixture's effective Lewis number. Limitations due to cellular instability were studied, analyzing the influence of fuel dilution and composition on the onset of cellular instability at the flames. The results showed a good prediction of the experimental data by the evaluated kinetic mechanisms. The expected reduction in the laminar burning velocity with CO2 dilution is primarily due to the effect of carbon dioxide's thermodynamic properties, followed by its transport properties (when compared to N2 dilution), chemical effects, and heat losses by radiation. Cellular instability is attenuated with high dilution levels due to the reduction of hydrodynamic instability. At the same time, the increase of the H2 concentration in the mixture leads to more unstable flames due to both hydrodynamic and thermal-diffusive instability. A laminar burning velocity threshold above which the measurements were not possible due to the effects of cellular instabilities was identified and analyzed in detail. [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]

Values of laminar burning velocity obtained by the constant volume method suffer from the inaccuracies introduced by the semi-analytical burned gas mass fraction model and flame stretch. A cylindrical combustion chamber with full optical access was developed in the present study, enabling the photography of the flame until the flame touches the inner wall of the combustion chamber. This enables direct visualization of the flame, thus eliminating the dependency on burned gas mass fraction models. The values of laminar burning velocity obtained by simultaneous measurement of flame radius and pressure-time data using the constant volume method are not stretch-free. This paper also develops a methodology for stretch correction based on the determined laminar burning velocity. Experiments were performed on mixtures of methane and air at initial pressures (0.8–1.05 bar), initial temperatures (293–320 K) and equivalence ratios (0.8–1.2). Stretch-corrected laminar burning velocity for lean, stoichiometric, and rich methane-air flames has been obtained in the present study, and the results obtained are validated against available experimental data and 1-D flame computations. It is expected that the proposed method significantly reduces the inaccuracies in laminar burning velocity obtained by the constant volume method. [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]

In this paper, the effects of equivalence ratio (0.8-2.0) and temperature (30°C-120°C) on ethanol, acetone, and, ethyl acetate vapors explosion characteristics through experimental and numerical studies were investigated. The explosion overpressure and flame propagation velocity were recorded through the pressure transducer and high-speed camera. The results showed that the flame propagation velocity, peak explosion overpressure, and peak growth rate of explosion overpressure increased first and then decreased with the increase of equivalence ratio. The cracks on the flame surface enhanced with the increase of the equivalence ratio. As the initial temperature increased, peak explosion overpressure, the flame propagation velocity, and peak growth rate of explosion overpressure gradually increased. The sensitivity analysis of laminar burning velocity indicated that with the change of equivalence ratio and initial temperature, the shared elementary reactions that increased the reactivity were H + O2 < = > O + OH, HCO + M < = > H + CO + M, and CO + OH < = > CO2 + H, and the shared elementary reaction that reduced the reactivity was H + OH + M < = > H2O + M. The main factor affecting laminar burning velocity was the mole fraction of H and OH radicals. [ABSTRACT FROM AUTHOR]

The combustion characteristics of n-decane+air mixtures are experimentally investigated through laminar burning velocity measurements at 1 atm pressure and higher initial temperatures using an externally heated diverging channel (EHDC) method. Up to 610 K mixture temperature over an equivalence ratio range of 0.7–1.4, laminar burning velocities are reported with an accuracy of ± 5%. The current measurements exhibit a good match with existing experimental measurements, and agree closely with the predictions of Zhao, LLNL and PoliMi mechanisms at different mixture temperatures. The present measurements show an excellent match of temperature exponent (α) variation with equivalence ratio (ϕ) with the predictions of distinct kinetic models as well as experimental measurements. This study reveals that a substantial scatter exists among the predictions of different kinetic models. A variation of 20–30 cm/s in the burning velocity is observed at 610 K mixture temperature. Reaction R16 (H + HO2 = H2 + O2), which inherently reduces the burning velocity becomes insignificant at an elevated mixture temperature of 610 K, and the reaction R15 (H + HO2 = 2OH) plays a dominant role in accelerating the flame propagation. From reaction pathway diagrams, it is clear that a higher burning velocity at 610 K is associated with the increased reaction rate. The elemental-flux value associated with the formation of C2H3 from C2H4 at 610 K mixture temperature is ≅34% higher in comparison to the 470 K mixture temperature. [ABSTRACT FROM AUTHOR]

Hydrogen and ammonia are playing an increasingly vital role in combustion technologies. Recently, some detailed reaction mechanisms (DRMs) can predict reliable laminar burning velocities (LBVs). Meanwhile, despite their broad applicability in practical combustion engineering, global reaction mechanisms (GRMs) suitable for ammonia and its mixtures with hydrogen or nitrogen have yet to be reported. This study aims to address this gap by investigating new GRMs. Five DRMs were used to calculate the LBVs, and their average values were employed as a reference. First, a single-step GRM was developed for hydrogen (H 2 + 0.5O 2 → H 2 O), and another single-step GRM was developed for ammonia (NH 3 + 0.75O 2 → 0.5N 2 + 1.5H 2 O). However, their combined GRM did not provide reasonable predictions of LBVs for the H 2 +NH 3 mixtures. Therefore, a 4-step GRM was developed consisting of the single-step hydrogen and three additional reactions, i.e., an endothermic ammonia decomposition (NH 3 + 0.5O 2 → NO + 3H), an exothermic NO reduction (NO + 2H → 0.5N 2 + H 2 O), and an exothermic H recombination (H + H → H 2). In conclusion, this 4-step GRM and its revisions could predict reasonable LBVs, flame temperatures, and primary product compositions for mixtures of hydrogen, ammonia, and cracked ammonia gas over a wide range of temperatures (300–600 K) and pressures (1–5 atm). • Global reaction mechanisms (GRMs) were investigated for H 2 +NH 3 +N 2 mixtures. • Laminar burning velocities (LBVs) from the detailed reaction mechanisms were compared. • Single-step GRMs were improved for the individual fuels H 2 and NH 3. • Available ranges were extended in temperatures (300–600 K) and pressures (1–5 atm). [ABSTRACT FROM AUTHOR]

G-equations are popular level set Hamilton–Jacobi nonlinear partial differential equations (PDEs) of first or second order arising in turbulent combustion. Characterizing the effective burning velocity (also known as the turbulent burning velocity) is a fundamental problem there. We review relevant studies of the G-equation models with a focus on both the existence of effective burning velocity (homogenization), and its dependence on physical and geometric parameters (flow intensity and curvature effect) through representative examples. The corresponding physical background is also presented to provide motivations for mathematical problems of interest. The lack of coercivity of Hamiltonian is a hallmark of G-equations. When either the curvature of the level set or the strain effect of fluid flows is accounted for, the Hamiltonian becomes highly nonconvex and nonlinear. In the absence of coercivity and convexity, the PDE (Eulerian) approach suffers from insufficient compactness to establish averaging (homogenization). We review and illustrate a suite of Lagrangian tools, most notably min-max (max-min) game representations of curvature and strain G-equations, working in tandem with analysis of streamline structures of fluid flows and PDEs. We discuss open problems for future development in this emerging area of dynamic game analysis for averaging noncoercive, nonconvex, and nonlinear PDEs such as geometric (curvature-dependent) PDEs with advection. [ABSTRACT FROM AUTHOR]

This article investigates the impact of H 2 and O 2 enrichment on the laminar burning velocity (S L) of syngas in a constant-volume combustion chamber. By analyzing changes in both chemical and thermal effects, it examines the impact of variations in H 2 and O 2 content on the S L independently. Furthermore, this article investigates the pressure variations of spherical expanding flames, flame instability, and the formation of mixed fuel NO. Results show increased H 2 and O 2 content enhance S L , with O 2 addition leading to a linear increase. H 2 affects thermal-diffusive instability differently in lean and rich fuel regions, while increasing O 2 content mitigates it. The synergistic influence of flame thickness and thermal expansion ratio amplifies fluid dynamic instability with higher H 2 and O 2 content. O 2 addition is more effective in promoting the rate of production for thermal NO and H 2 addition is more effective in promoting the rate of production for non-thermal NO. • The influence of H 2 and O 2 enrichment on combustion is analyzed. • The change of laminar combustion pressure is analyzed. • Decoupling analysis of the influence of the virtual gas method on H 2 and O 2. • The influence of H 2 and O 2 on the instability of spherical flame is analyzed. • The effects of H 2 and O 2 on different NO formation mechanisms are analyzed. [ABSTRACT FROM AUTHOR]

This paper presents detailed investigations of the combustion characteristics of NH 3 /H 2 blends as a possible alternative fuel to address the pressing environmental and energy security challenges. The experimental measurements of laminar burning velocity (LBV) are reported for a range of mixture equivalence ratios (φ) varied from 0.8 to 1.2 and mixture temperatures up to 720 K using the externally heated diverging channel (EHDC) method. It is observed that LBV increases nearly four times for specific NH 3 /H 2 blends with an increase in mixture temperature from 300 K to 650 K. While predictions from various kinetic models align well at ambient conditions, discrepancies emerge at higher temperatures, emphasizing the need for refining these models to accurately predict flame propagation behaviour. A detailed sensitivity analysis of the NH 3 /H 2 /air fuel blend provides insights into critical reactions and species influencing burning velocity across different mixture conditions. Notably, reactions involving NNH radicals are found to strongly influence LBV for low hydrogen fractions in NH 3. Additionally, reaction pathway analysis (RPA) highlights dominant reactions under various conditions and reveals temperature-dependent NO emissions. The present study helps highlight the importance of understanding the combustion behaviour of NH 3 /H 2 blends for achieving carbon-free energy needs. The experimental data at elevated temperatures offers valuable insights into the combustion behaviour of these mixtures, enabling the advancements for cleaner energy solutions. [Display omitted] • LBV for X H2 = 0.3 increased fourfold from 300 K to 650 K. • Predicted LBVs aligned with low-temperature data but differed at higher temperatures. • Burning velocity rises with H 2 addition due to increased H, O, and OH species. • The significance of critical NNH radical reactions increased with H 2 addition. • Higher hydrogen fractions introduce NH + H N + H 2 resulting in Zeldovich mechanism. [ABSTRACT FROM AUTHOR]

In this paper, we explore the deflagration combustion of methane–air mixtures through both experimental and numerical analyses. The key parameters defining deflagration combustion dynamics include maximum explosion pressure (Pmax), maximum rate of explosion pressure rise (dP/dt)max, deflagration index (KG), and laminar burning velocity (SU). Understanding these parameters enhances the process of safety design across the energy sector, where light-emissive fuels play a crucial role in energy transformation. However, most knowledge on these parameters comes from experiments under standard conditions (P = 1 bar, T = 293.15 K), with limited data on light hydrocarbon fuels at elevated temperatures. Our study provides new insights into methane–air mixture deflagration dynamics at temperatures ranging from 293 to 348 K, addressing a gap in the current process industry knowledge, especially in gas and chemical engineering. We also conduct a comparative analysis of predictive models for the laminar burning velocity of methane mixtures in air, including the Manton, Lewis, and von Elbe, Bradley and Mitcheson, and Dahoe models, alongside various chemical kinetic mechanisms based on experimental findings. Notably, despite their simplicity, the Bradley and Dahoe models exhibit a satisfactory predictive accuracy when compared with numerical simulations from three chemical kinetic models using Cantera v. 3.0.0 code. The findings of this study enrich the fundamental combustion data for methane mixtures at elevated temperatures, vital for advancing research on natural gas as an efficient "bridge fuel" in energy transition. [ABSTRACT FROM AUTHOR]

This study focused on using swirling flow as a method to stabilize the detonation behavior and shorten the deflagration-to-detonation transition (DDT) distance of pulse detonation engines; additionaly, the study investigates the effect of swirling flow on flame propagation behavior and DDT distance. A swirling flow in which the circumferential velocity in the radial direction gradually increased from the center toward the wall and decreased near the wall was confirmed. Laser doppler velocimetry (LDV) confirmed that the swirl number is smaller in the axial velocity distribution when the number of swirling injector couplings is small. The DDT shortening ratio obtained using ten swirling injector couplings is larger than that obtained using five couplings. These results indicate that injector-induced swirling flow can increase the burning velocity and is an effective method to shorten the DDT distance. • Swirling flow used to stabilize detonation behavior and shorten DDT distance. • Circumferential velocity distribution across pipe section measured using a particle image velocimetry. • Swirl number of pipe swirling flow measured using a laser Doppler velocimeter. • The swirling flow increased the flame propagation velocity and reduced the DDT distance. [ABSTRACT FROM AUTHOR]

The next-generation energy economy relies heavily on hydrogen. Handling hydrogen during production, storage, and transportation requires risk mitigation. In this regard, the application of halide-free additives is very crucial. The present study experimentally and theoretically investigates the individual and combined inhibiting effect of propane and propene on hydrogen-air explosions. The experiments are carried out at 298 K and 1 bar, with additives ranging from 0.5 to 5% for the individual and 1–2% each for the combined effect in the 20–50% hydrogen-air mixture. Experimentally, the additive content in the stoichiometric and rich mixture decreases the explosion pressure, maximum pressure rise (MPR) rate, laminar burning velocity (LBV), and flame temperature, while increasing minimum ignition energy. For instance, the addition of 2% propane in a 50% hydrogen-air mixture, reduces the MPR rate by 98%, whereas such an effect is observed with 2.5% propene. Theoretical study shows that 3% equivalent additive augments inhibiting reactions, countering the main flame-promoting reaction R1 (H + O 2 O + OH) and thereby reducing LBV. Due to its low activation energy, propene consumes the active radicals twice the rate of propane, while propane not only scavenges the active radicals but also promotes oxygen consumption in the prior phase of combustion, weakening flame propagation. Overall, this approach exhibits a significant step forward in harnessing the potential of hydrogen while mitigating the safety risks associated with its use. [Display omitted] • The effect of C 3 -Hydrocarbons and their Mixtures on H2-air explosion is explored. • The additives decrease explosion pressure, MPR rate and LBV while increasing MIE. • Propene increases the consumption rate of active radicals in the additive mixtures. • Propane promotes oxygen consumption in the prior phase of combustion, countering R1. [ABSTRACT FROM AUTHOR]

This study provides a chemical mechanism reduction strategy based on entropy production analyses in both auto-ignition and laminar flames, which enhances the predictive accuracy for laminar burning velocities. In addition to chemical reactions, other irreversible sources causing entropy generation, such as mass diffusion and heat conduction, are considered in the modified approach. Specifically, initial skeletal mechanisms are first generated based on important reactions that contribute to entropy production in auto-ignition processes. Mechanism patches are then constructed to include important species and reactions, which contribute to entropy production from mass diffusion and heat conduction in laminar premixed flames beyond the pre-defined thresholds, respectively. Finally, the initial skeletal mechanisms and mechanism patches are combined to establish the final skeletal mechanisms. In this way, two final skeletal mechanisms for n-dodecane, consisting of 162 species and 2276 reactions, and 160 species and 1916 reactions, respectively, are developed from the detailed POLIMI mechanism with 451 species and 17,848 reactions. The two final skeletal mechanisms are proven to accurately predict laminar burning velocities and entropy production in n-dodecane flames with insignificant variations in the simulation results compared to the detailed mechanism, while their accuracy in predicting ignition delay times relies on the initial skeletal mechanisms. Specifically, the reduced mechanism with 160 species and 1916 reactions exhibits less satisfactory performance in predicting ignition delay compared to that with 162 species and 2276 reactions, indicating that a lower threshold is required to generate the initial skeletal mechanism through entropy production analysis of homogeneous auto-ignition processes. Additionally, compared with the reduced mechanisms with similar sizes obtained with other mechanism reduction strategies, the two final skeletal mechanisms accurately capture the characteristics of laminar burning velocities and ignition delay times, with similar calculation time required. [ABSTRACT FROM AUTHOR]

A recently proposed scalar forcing scheme that maintains the mixture fraction mean, root-mean-square and probability density function in the unburned gas can lead to a statistically quasi-stationary state in direct numerical simulations of turbulent stratified combustion when combined with velocity forcing. Scalar forcing alongside turbulence forcing leads to greater values of turbulent burning velocity and flame surface area in comparison to unforced simulations for globally fuel-lean mixtures. The sustained unburned gas mixture inhomogeneity changes the percentage shares of back- and front-supported flame elements in comparison to unforced simulations, and this effect is particularly apparent for high turbulence intensities. Scalar forcing does not significantly affect the heat release rates due to different modes of combustion and the micro-mixing rate within the flame characterised by scalar dissipation rate of the reaction progress variable. Thus, scalar forcing has a significant potential for enabling detailed parametric studies as well as providing well-converged time-averaged statistics for stratified-mixture combustion using Direct Numerical Simulations in canonical configurations. [ABSTRACT FROM AUTHOR]

The potential for ammonia/hydrogen (NH 3 /H 2) blends as fuel in modern, turbocharged, gas engines is evaluated by comparing the combustion properties of these blends with those of methane. To ensure proper combustion phasing in existing spark-ignited (SI) engines, the laminar burning velocity (LBV) is used, while the ignition delay times (IDT) of the mixtures are compared to assess the risk of engine knock. The chemical mechanisms used for the computation of IDT and/or LBV of NH 3 /H 2 mixtures are selected from 8 chemical mechanisms based on their ability to predict experimental data. The application of a turbocharger is shown to "boost" the temperature in the cylinder after compression to values that admit the use of NH 3 /H 2 mixtures in natural-gas SI engines without increasing the compression ratio (CR). The analysis also identifies regions of temperature and H 2 fraction that increase the risk of knock. Using the conditions in a practical turbocharged SI engine as an example, 40–50% H 2 in NH 3 yields combustion phasing equivalent to that obtained with methane, while avoiding the risk of knock. Analysis of the IDT for the conditions in a high-pressure direct-injection natural-gas engine also shows that the use of a turbocharger in compression-ignition (diesel-type) engines strongly reduces the demands on CR for achieving the conditions necessary for reliable ignition of NH 3 /H 2 mixtures. Increasing the temperature by only a few tens of degrees above the conditions studied will yield an IDT for ∼20% H 2 in NH 3 that satisfies the ignitibility requirements for use in a turbocharged diesel engine. • Turbocharging facilitates the use of NH 3 /H 2 mixtures in gas engines. • The risk of engine knock limits the "boost" temperature and maximum H 2 fraction. • 40–50% H 2 in NH 3 mimics CH 4 combustion without knocking in the SI engine considered. • Modest preheating enables the use of NH 3 with 20% H 2 in a turbocharged CI engine. • The methodology used is readily applied to different fuels and engine conditions. [ABSTRACT FROM AUTHOR]

While ammonia (NH 3) and hydrogen (H 2) are promising carbon-free fuels, there are significant challenges presented by their combustion properties. Blending them can help mitigate some of these issues; however, the fundamental behaviors of these blends need to be well understood. A constant volume combustion chamber (CVCC) is used to study the emissions of nitrogen oxides (NO, NO 2 , N 2 O), laminar burning velocity (LBV), and intrinsic instabilities for H 2 -NH 3 blends across a wide range of equivalence ratios (Φ = 0.5 to 1.5) and volume fractions of H 2 in the blends (0% to 50%). Measurements of LBV are consistent with existing data where available. The largest NO x and N 2 O emissions are found at lean conditions and decrease significantly for rich mixtures. NO x emissions tend to increase with H 2 fractions in fuel for lean mixtures. Intrinsic instabilities are more prominent at lean conditions and heavily influenced by the amount of H 2 in the blend. [Display omitted] • NO x production peaks at Φ =0.7, increases with H 2 content for lean mixtures. • N 2 O production peaks at lowest Φ , no significant variation with H 2 content. • Higher H 2 content results in earlier and more pronounced flame wrinkling. • LBV and emissions data extends available range for mechanism validation. [ABSTRACT FROM AUTHOR]

To investigate the flame propagation characteristics and chemical kinetics of premixed propane/hydrogen/air mixtures at elevated pressure, a series of experiments on propane/hydrogen/air mixtures with varying initial pressures (P 0 = 100–300 kPa) and equivalence ratios (φ = 0.6–1.4) at a hydrogen fraction of 0.6 and an ambient temperature of 298 K were carried out in a closed duct with an aspect ratio of 15. The chemical kinetics of the elementary reaction were analyzed numerically. The results revealed that the combustion-induced rapid phase transition (cRPT) occurred in the premixed C 3 H 8 /H 2 /air flames. Both the combustion pressure and flame propagation velocity increase with initial pressure except at P 0 = 300 kPa, where the flame propagation velocity decreases. Moreover, the linear dependence between the laminar burning velocity and peak mole fraction [H + O + OH] max was found by the effect of initial pressure on the peak mole fraction (H, O, OH). The explosion pressure P rea and the maximum rate of production of free radical H/O show excellent correlations. The maximum rate of pressure rise can be predicted by the maximum heat release rate (HHR). These results provide essential data on elevated-pressure duct accidents and have important implications for explosion prevention. • C 3 H 8 /H 2 /air flames at elevated pressure in a closed duct were investigated. • Combustion-induced rapid phase transition was observed in C 3 H 8 /H 2 /air flames. • An equation is established for combustion pressure with peak rate of H production. • A relationship between explosion parameters and fundamental parameters are found. [ABSTRACT FROM AUTHOR]

A theoretical method was optimized based on a wrinkled flame model for predicting the explosion pressure in confined chambers for typical low-carbon fuels, e.g., hydrogen (H 2), methane (CH 4), and ammonia/dimethyl ether/hydrogen (NH 3 /DME/H 2) blends, etc. It was found that the previous model with a wrinkling factor of Ξ Δ = e t (t is time) is only applicable to the explosions of H 2 fuel, while considerably underestimating the explosion pressures of other fuels such as CH 4 and NH 3 /DME/H 2 blends. For H 2 fuel where the pressure power exponent β > 0, the effect of flame wrinkling on the laminar burning velocity S L is less significant compared to that of adiabatic compression, so that an underestimated wrinkling factor does not affect the overall prediction of pressure by the model. However, for most fuels with β < 0, a more accurate representation of the wrinkling effect is necessary for pressure prediction. The results show that the model can be optimized by increasing the wrinkling factor to Ξ Δ = e 20 t that satisfactorily represents the effect of flame instabilities. The wrinkling factor remains constant (Ξ Δ = 2.46) after it reaches its maximum value of 2.46. Comparison of the model predictions to experiments shows that the optimized model can well predict the explosion pressures of various low-carbon fuels that experienced flame instability. • The wrinkled flame model for predicting the explosion pressure in confined chambers was optimized. • For H 2 , the effect of flame wrinkling on flame speed is less significant compared to that of adiabatic compression. • For fuels with β < 0, the wrinkling factor was increased to Ξ Δ = e 20 t with the maximum value of 2.46. • The optimized model can well predict the explosion pressures of different fuels at various initial conditions. [ABSTRACT FROM AUTHOR]

The laminar burning velocity and NO formation process of ammonia-hydrogen combustion within a two-stage combustion chamber were investigated numerically in the present study. A chemical reactor network method involving perfectly stirred reactor, plug flow reactor, and partially stirred reactor configurations with the 24-species Xiao mechanism was implemented to simulate the premixed ammonia-hydrogen-air combustion process. The effects of inlet temperature and pressure conditions on the laminar burning velocity were investigated. Results proved that elevated pressure condition decreased primary flame thickness leading to lower laminar burning velocity while inlet temperature increased flame temperature which in turn increased the laminar burning velocity. Investigation of the effect of humidification on the laminar burning velocity showed that humidification can counteract the effect of high inlet temperature. The NO emission studies indicated a twofold impact of pressure on NO formation processes: preventing NO formation in the primary combustion zone, and promoting thermal NO formation in the lean combustion zone. The minimum amounts of NO emission were obtained at total equivalence ratios of 0.4. Humidification prevented the NO formation in the lean combustion through the competitive effect of H2O on O, whilst temperature effect was comparatively small. Humidity and pressure were optimized in the two-stage configuration achieve both low emission and high efficiency. [ABSTRACT FROM AUTHOR]

The hydrodynamics of the dense confining fuel shell is of great importance in defining the behavior of the burning plasma and burn propagation regimes of inertial confinement fusion experiments. However, it is difficult to probe due to its low emissivity in comparison with the central fusion core. In this work, we utilize the backscattered neutron spectroscopy technique to directly measure the hydrodynamic conditions of the dense fuel during fusion burn. Experimental data are fit to obtain dense fuel velocities and apparent ion temperatures. Trends of these inferred parameters with yield and velocity of the burning plasma are used to investigate their dependence on alpha heating and low mode drive asymmetry. It is shown that the dense fuel layer has an increased outward radial velocity as yield increases, showing that burn has continued into re-expansion, a key signature of hotspot ignition. A comparison with analytic and simulation models shows that the observed dense fuel parameters are displaying signatures of burn propagation into the dense fuel layer, including a rapid increase in dense fuel apparent ion temperature with neutron yield. [ABSTRACT FROM AUTHOR]

The combustion characteristics of ammonia/hydrogen/air mixtures flame under different equivalent ratios (0.6–1.0), and hydrogen ratio (0.3–0.5) in a wide range of turbulence intensity were investigated. The result shows both differential diffusion and flame chemistry affect the flame morphology and burning velocity of lean and stoichiometric mixtures. With the increase of hydrogen ratio, not only can chemical reaction be enhanced, but also heat and mass diffusion of the flame front can be promoted, thus accelerating flame propagation. In addition, the strong turbulent stretch of the turbulent field gradually plays a dominant role in the development of flame and causes the flame to wrinkle even more. Based on the turbulent flame Reynolds number (Re T,flame), the accelerated propagation processes are analyzed, and further, the influences of differential diffusion and turbulent stretching are taken into account by Le and Ka numbers. The general correlation of turbulent burning velocity (S T) is obtained, and the applicability of the correlation based on Da and Ka for ammonia/hydrogen/air mixtures turbulent combustion is verified. In addition, a unified scaling of turbulent burning velocity (S T) suitable for different equivalent ratios and hydrogen content conditions is obtained by rearranging with Le. • Effect of differential diffusion and turbulent stretch on S T are studied. • The self-accelerating propagation is verified and propose the correlation of S T. • A unified scaling of S T is obtained based on Da , Ka , and Le. [ABSTRACT FROM AUTHOR]

This study experimentally and numerically determined the effect of CO 2 on H 2 /air explosion. The results indicated that CO 2 can effectively reduce the maximum explosion pressure, the maximum explosion pressure rise rate and flame propagation speed of hydrogen explosion. Numerical simulations have confirmed that adiabatic flame temperature, laminar burning velocity, free radical concentration and the rate of product gradually decrease of hydrogen explosion with the increase of CO 2 concentration. Among them, the changing trend of H radicals and laminar burning velocity is most closely related. Further analysis of the formation of H radicals found that as the CO 2 concentration increases, reaction R31:CO + OH CO 2 +H(ΔH = -203.5 kJ/mol) gradually proceeds in the reverse direction. While removing H radicals, the system temperature is absorbed, but a certain amount of CO is produced. In addition, analysis found that the physical inhibition effect of CO 2 is significantly greater than the chemical inhibition effect. When φ = 0.6, physical inhibition effect accounts for 77 % of the total inhibition effect, and chemical inhibition effect accounts for 23 % of the total inhibition effect. When φ > 0.8, as the equivalence ratio and CO 2 concentration increase, the proportion of physical inhibition effects gradually increases, and the proportion of chemical inhibition effects gradually decreases. The reason for the increase in the proportion of physical inhibition is that the dilution effect gradually shifts from promotion to inhibition. • The physical endothermic effect is the most important inhibitory effect of CO 2. • The dilution effect has a dual effect of promoting and inhibiting the hydrogen laminar flame speed. • CO 2 has inhibitory effect on different equivalence ratios of hydrogen. • CO 2 suppresses hydrogen explosions to produce higher concentrations of CO. [ABSTRACT FROM AUTHOR]

To enhance ammonia's flammability, blending methane with ammonia is a viable strategy to improve the laminar burning velocity of ammonia combustion. We conducted three-dimensional numerical simulations to investigate the thermal performance, the 2nd thermodynamic law efficiency, and NO emissions of a micro-combustor fuelled by ammonia/methane. Three key parameters are identified and examined. They include: 1) the inlet volume flow rate, 2) the CH4 mole blended ratio, 3) the N 2 dilution rate. It is found that increasing the inlet volume rate give rise to an increase of the combustor outer wall temperature, and so the thermodynamic second-law efficiency, but a reduction of NO emissions. For example, when the inlet volume flow rate is set to 14.4 mL/s, the wall temperature and 2nd law efficiency are increased by 36 % and 21 % respectively, but the NO emission is reduced by 15 %, in comparison with those in the presence of 7.2 mL/s inlet flow. While an increase of the CH 4 blended ratio (molar fraction) is shown to have limited impact on the thermal performances. This variation of such blended ratio is found to be associated with a notable 22 % reduction of NO emissions. Additionally, injecting N 2 as a dilution gas is shown to be not beneficial to the thermal performance. However, NO emissions are reduced. When the N 2 dilution rate is set to 0.6, the wall temperature is found to be reduced by 173 K. However, 47 % more NO reduction is observed in comparison with those in the presence of the N_2 dilution rate being set to 0.3. • 3D numerical investigation of a micro-combustor fuelled by methane/ammonia is conducted. • Entropy generation analyses of the ammonia/methane-fuelled micro-combustor are performed. • Examining the effect of blending methane with ammonia on thermodynamic performances is conducted. • NO emissions from the ammonia/methane-fuelled micro-combustor are evaluated. • A reduced chemical reaction mechanism of ammonia-methane combustion is proposed. [ABSTRACT FROM AUTHOR]

Hydrogen can be easily produced by methanol steam reforming. This work aims to investigate the effects of methanol addition and its reformed gas of H 2 /CO 2 on ammonia combustion under lean-burn operation. A newly-developed reaction mechanism for NH 3 , H 2 , CH 3 OH, CO and their blends was established based on the highly-precise validations of laminar burning velocity (LBV), nitrogen monoxide (NO) and ignition delay time (IDT). The premixed flame analysis showed that methanol blends and reforming with hydrogen addition can greatly enhance the combustion rate, resulted from the chemical effect. The equivalence ratio (ER) can be extended to 0.45 for a methanol blending ratio of 60 % and reforming ratio of 90 % (M60R90) to achieve the similar level of LBV at pure ammonia at an ER of 1.0 under ambient condition. The methanol blends and reforming lead to the increase of NO, while the lean-burn operation can help to reduce the NO emission. • A chemical reaction mechanism for H 2 /NH 3 /CH 3 OH/CO was developed. • Chemical kinetics of methanol and reformed gas (H 2 /CO 2) blending with ammonia were investigated. • Methanol reforming leads to a higher LBV due to the enhancement of free radicals. • Ammonia blending with reformed gas at a lower ER can achieve lower NO generation. [ABSTRACT FROM AUTHOR]

This study measures the accurate Laminar Burning Velocities (LBVs) of (H 2 /NH 3 /CH 4) ternary fuels-air flames under elevated pressure and oxygen enrichment using a constant volume chamber, providing reliable experimental data to verify the detailed chemical kinetic mechanism. Results show that the LBV values for blending ratios of H 2 /NH 3 of 10%–50% demonstrate a suitable alternative energy-generated fuel for conventional fuels in industrial applications, and the maximum value of LBV is achieved at an equivalence ratio close to 1.05. Increasing H 2 blends will increase LBV non-linearly under any conditions and decrease with an increase in ammonia blends. Increasing the initial pressure decreases LBV; at 10% H 2 , LBV decreases by 33% at 0.3 MPa and 45% at 0.5 MPa compared to atmospheric pressure. Furthermore, increasing oxygen content improves the reactivity of the mixture and accelerates LBV due to increasing the OH, NH 2 , and CH 3 radicals. Maintaining a constant value of LBV requires controlling the blending ratio of the ternary fuels, where increasing LBV values shift the blending of the mixture towards higher hydrogen and methane concentrations and fewer ammonia concentrations. Finally, the effect of maintaining a constant value of adiabatic flame temperature and density ratio on flame characteristics is explained, making the selection of an alternative ternary blending fuel more precise. [Display omitted] • LBV of H 2 /NH 3 /CH 4 -air flames at elevated pressures and oxygen enrichment were measured. • H 2 /NH 3 /CH 4 mixture can be considered a suitable alternative for conventional fuels in industrial applications. • Different blending ratios maintain a consistent laminar burning velocity at different fuel blends and initial pressures. • The effect of constant adiabatic flame temperature and density ratio on LBV was evaluated. [ABSTRACT FROM AUTHOR]

Ammonia (NH3) is a carbon-free fuel. Therefore, many researchers have proposed detailed mechanisms for NH3, yet the existing mechanisms have widely varied combinations of reactions and were validated using different experimental datasets. Thus, this study suggests developing a NH3 mechanism by exploring the vast reaction pool of the five existing mechanisms. Several reaction combinations from the reaction pool were tested to validate the experimental datasets. The developed mechanism showed the lowest mean squared error (MSE) among the referenced mechanisms for predicting the ignition delay times (IDT). Furthermore, the MSE of the laminar burning velocity (LBV) prediction was less than those of the three referenced mechanisms. Although the mechanism was developed by employing only NH3 fuel mixtures, the estimated IDT and LBV of NH3/H2 fuel mixtures showed low MSE. Analyses of many tested sample mechanisms revealed that the rate coefficients of NH3 combustion reactions should be further elucidated for enhanced prediction. [ABSTRACT FROM AUTHOR]

Adding methane to methanol can solve the problem of difficult cold starts of methanol engines. Therefore, it is important to understand the combustion of methane and methanol fuel blends. Because of this, this study explores the effect of methane addition on the laminar burning velocity and Markstein length of methanol/methane premixed flames under stoichiometric conditions at the initial temperatures of 353 K, 373 K, and 393 K, initial pressures of 1 bar, 2 bar, and 4 bar, using methane addition ratios of 0%, 25%, 50%, 75%, and 100% in a constant volume combustion chamber. The results show that the laminar burning velocity decreases linearly with the increase of methane addition ratio due to the linear decrease of the Arrhenius factor. The sensitivity analysis revealed that the kinetic effect is the main reason for the inhibition of laminar burning velocity, which is insensitive to initial temperature but enhanced at a high initial pressure. The Markstein length decreases with the addition of methane and the increase of initial pressure, which is mainly caused by the high mass diffusivity of methane and the decrease of flame thickness due to the increase of initial pressure. [ABSTRACT FROM AUTHOR]

Explosion hazards associated with hydrogen-oxygen mixtures remain a significant challenge for the energy industry, despite hydrogen garnering attention as a carbon emissions-free alternative energy source. While several studies have been conducted on hydrogen-air explosions, research on hydrogen-oxygen explosions has been limited. In the current study, we employed a spherical soap bubble method to experimentally investigate the flame propagation behavior of an unconfined hydrogen-oxygen explosion. The experimental values of the laminar burning velocity were in agreement with its calculated values and were notably higher than those observed hydrogen-air mixtures. In the case of hydrogen-oxygen mixtures, a spherical expanding flame is accelerated due to the presence of Darrieus-Landau and diffusive-thermal instabilities. Furthermore, we compared the correlation between the critical Pѐclet number and the critical Karlovitz number with the Markstein number for hydrogen-oxygen mixtures to results obtained with propane-oxygen and methane-oxygen mixtures. [ABSTRACT FROM AUTHOR]

Hydrogen stands out as a compelling alternative to fossil fuels for combustion engines. Predictive combustion models are instrumental in developing hydrogen-fuelled engines. A fundamental metric of these models is the laminar burning velocity (LBV), which can be precisely determined through laminar flame propagation simulations. In this context, the selection of an appropriate combustion mechanism is critical. This paper aims to propose the appropriate combustion mechanism for calculating LBV in predictive combustion models of hydrogen-fuelled engines. 15 state-of-the-art combustion mechanisms were applied to reproduce the LBV measurements in engine-like conditions, especially considering the application of lean combustion and water injection. The FFCM 1.0 mechanism was identified from them and further optimised to improve its prediction accuracy at elevated pressures for the lean mixture. The maximum deviation of LBV was reduced from 17.6 % to 8.7 % by this optimisation, in comparison to 10.5 % for its closest competitor mechanism, ELTE (Varga et al., 2015). • 15 combustion mechanisms were selected as the candidates for optimisation. • Test cases consider applications of lean combustion and water injection. • Many mechanisms predict laminar burning velocities at elevated pressure inadequately. • FFCM 1.0 mechanism was optimised to a new mechanism for modelling H2-fuelled engines. • Prediction errors of laminar burning velocity by the new mechanism are within 8.7 %. [ABSTRACT FROM AUTHOR]

The aviation industry uses heavy hydrocarbon fuels, which release carbon-based emissions upon combustion and could suffer from flame stabilization issues due to their high Lewis numbers at lean conditions. Hydrogen is emerging as an alternative fuel which can help mitigate these issues. A viable strategy is to blend liquid hydrocarbons with hydrogen, which could strengthen the flame in positively stretched regions and help reduce carbon-based emissions. In order to investigate this, we carry out a numerical study for different blends of n-dodecane, which is a surrogate fuel for kerosene and jet fuel, with hydrogen in air. Equivalence ratios and unburnt temperatures of different blends are selected such that the laminar burning velocity remains the same. This condition allows for a more precise characterization of Lewis number effects manifested at the flame base and tip. It is found that up to 60 % addition of H 2 into n-dodecane by volume, no major difference is observable with respect to the flame stabilization process. The positively curved flame base burns weakly while the negatively curved flame tip is strengthened due to thermal diffusivity being greater than mass diffusivity. With a further increase in the percentage of hydrogen, the flame base starts to burn stronger with the flame moving closer to the flame holder. However, the flame tip does not become weak as n-dodecane continues to burn there. As positive stretch and negative curvature increase the consumption rate for H 2 and n-dodecane flames, respectively, for the blend with 95 % hydrogen and 5 % n-dodecane, a flame is observed with the flame base and the flame tip both burning strong. Overall, this demonstrates that introducing hydrogen at concentrations exceeding 60 % by volume significantly enhances flame stabilization by bringing the flame closer to the flame holder, potentially expanding flame stabilization boundaries. [Display omitted] • Addition of hydrogen into n-dodecane numerically analyzed for multi-slit flames. • Mixtures with the same burning velocity were selected for a fair comparison. • n-dodecane flames remain unaffected till 60 percent of hydrogen addition. • For higher hydrogen content, the flame base starts to burn strong due to Le < 1 effects. • The addition of hydrogen can help in the stabilization of n-dodecane flames. [ABSTRACT FROM AUTHOR]

In this work, the overall activation energy of the combustion of lean hydrogen–methane–air mixtures (equivalence ratio φ = 0.7−1.0 and hydrogen fraction in methane α = 0 , 2, 4) is experimentally determined using thin-filament pyrometry of flames stabilised on a flat porous burner under normal conditions ( p = 1 bar, T = 20 °C). The experimental data are compared with numerical calculations within the detailed reaction mechanism GRI3.0 and both approaches confirm the linear correlation between mass flow rate and inverse flame temperature predicted in the theory. An analysis of the numerical and experimental data shows that, in the limit of lean hydrogen–methane–air mixtures, the activation energy approaches a constant value, which is not sensitive to the addition of hydrogen to methane. The mass flow rate for a freely propagating flame and, thus, the laminar burning velocity, are measured for mixtures with different hydrogen contents. This mass flow rate, scaled over the characteristic temperature dependence of the laminar burning velocity for a one-step reaction mechanism, is found and it can also be used in order to estimate the parameters of the overall reaction mechanisms. Such reaction mechanisms will find implementation in the numerical simulation of practical combustion devices with complex flows and geometries. [ABSTRACT FROM AUTHOR]

The backdraft dynamics are numerically investigated using a proposed combustion model that attempts to accurately simulate the multi-combustion regime with a fast flame propagation in the inhomogeneous thermal condition. This scenario is often found in the limited-ventilation fires. The modeling strategy includes the flame-front tracking by the level-set function and the solution of non-adiabatic flamelet and a stratification parameter. In a comparison to the previous works, the present approach shows a better agreement with experiments. The results also underline the important effect of gravity current that promotes the rapid flame evolution in the case with less-dense initial mixture. Four different phases are identified based on the transient characteristics of backdraft, and they are (a) initial ignition, (b) free propagation, (c) limited-front propagation and (d) accumulation & flame-ejection. The dominant driving forces of buoyancy convection and turbulence effect on burning velocity are revealed. The backdraft process is found to encompass the initial premixed flame propagation and the diffusion burning outside the chamber following the formation of fireball. This work gains the valuable insights on the backdraft dynamics and provides useful information to design effective protection measures.

The determination of the characteristics and main combustion properties of fuels is necessary for post-implementation in different applications. Among the most important combustion properties of a fuel are the combustion products, flame temperature and laminar burning velocity. Therefore, this paper describes the step-by-step development and coding of a MATLAB application that can determine 12 combustion products, flame temperature and laminar burning velocity in order to understand the logic of calculus procedure, so any user would be able to make improvements of new functionalities (add more fuels, add more combustion products, etc.). The numerical procedure and methods (Gaussian elimination, Taylor Series and Newton–Raphson) parallel with their implementation as code lines for the development of the application are carried out using flow charts. In addition, simulations in Ansys Chemkin were performed and included in the application as part of the results comparison. It was found that: (1) The MATLAB Application codification and development were successfully explained in detail, (2) the functions and execution sequence are described by using flow charts and code extract, (3) the application is available to everyone for modifications, (4) the application can only be used for hydrocarbons fuels, (5) the application execution time registered was less than 8 s.

In this work, the laminar premixed flames for methane and propane at atmospheric pressure were numerically investigated with respect to different hydrogen concentrations, ranging from 5 % to 25 % (by vol.). Based on the two-dimensional (2D) flame predictions, hydrogen decreased the maximum temperatures and species concentrations such as formaldehyde (CH 2 O), acetylene (C 2 H 2), carbon monoxide (CO) and nitric oxide (NO) for methane and propane. However, the CH 2 O mole fractions were raised by a maximum deviation of 10 % when hydrogen was added into the fuel lean and stoichiometric propane mixtures. The decrease in C 2 H 2 eventually lowered the soot volume fractions and number densities. Besides, the axial locations of the maximum species concentrations, soot volume fractions and number densities were also affected by hydrogen. Meanwhile, the addition of hydrogen increased the flame temperatures and laminar burning velocities for the one-dimensional (1D) premixed flames of methane and propane. This trend was identically predicted for the formation of intermediate species such as CH 2 O, hydroperoxyl (HO 2) and hydroxyl (OH) whereas C 2 H 2 , CO and NO were lowered, as justified by the sensitivity analyses. For propane at an equivalence ratio (φ) = 1.3, the rates of production (ROPs) of C 2 H 2 were increased linearly with hydrogen, which resulted in higher C 2 H 2 concentrations. • Methane and propane coflow flames with hydrogen addition are modelled in 1D and 2D. • Hydrogen lowers flame temperatures, CH 2 O, C 2 H 2 , CO and NO for 2D flames. • For 1D flames, hydrogen raises flame temperatures and intermediate species. • Effects of hydrogen on emissions depend on fuel types and mixture conditions. [ABSTRACT FROM AUTHOR]

The laminar burning velocity of n-decane (an aviation kerosene surrogate) blended with hydrogen is studied using experimental and numerical analysis at the initial conditions of 470 K and 1, 2, and 4 bar over the equivalence ratios of 0.7–1.3 and hydrogen addition ratios of 0 %, 10 %, 20 %, 30 %, 40 %, and 50 %. The burning velocity is investigated using the constant volume method of the spherically expanding flame method. Here, burning velocity at elevated pressure and temperature conditions close to engine-relevant conditions have been established. Thus, using the experimental data at 470 K and 4 bar, burning velocities at the unburnt temperatures of up to 612 K and pressures of up to 11.02 bar are obtained. Also, Polimi and JetSurF2.0 mechanisms were used to simulate the burning velocities and they agreed well with the experimental data. However, as the hydrogen addition increased, especially when the hydrogen addition was 40 % there was a significant deviation between the experiment and the simulation results. The chemical reaction analysis of n-decane-hydrogen shows the flame speed is significantly driven by H + O 2 ⇔ O + OH , CO + OH ⇔ CO 2 + H , CH 3 + H (+ M) ⇔ C H 4 (+ M) , and H + OH + M ⇔ H 2 O + M at the various initial conditions. However, H + O 2 ⇔ O + OH produces more OH, H, and O radicals that accelerate the combustion chemical reaction rate of n-decane-hydrogen. The reaction pathways show that the decomposition of n-decane is mainly controlled by the OH and H radicals. This study's results provide an understanding of the combustion characteristics of aviation fuel-hydrogen blends in internal combustion engines at elevated pressures and temperatures to achieve zero emissions in aviation. • Laminar combustion characteristics of n-decane-hydrogen were studied. • Burning velocity at 11.02 bar and 612 K were obtained for n-decane-hydrogen. • H + O 2 ⇔ O + OH , CH 3 + H (+ M) ⇔ C H 4 (+ M) , and H + OH + M ⇔ H 2 O + M drove the flame speed. • OH, H and O radicals enhanced the combustion reactivity of n-decane-hydrogen. [ABSTRACT FROM AUTHOR]

In the present paper, it was aimed to consume pure NH 3 by using an existing non-premixed burner with H 2 blending strategy, and to examine the temperature and NO emission profiles emerged in a model combustor for NH 3 /H 2 and pure NH 3 fuel compositions. The Mixture Fraction/PDF combustion model with a reduced reaction mechanism was used for the numerical modelling. During the experimental studies, a stable NH 3 /air flame was able to achieve by introducing H 2 into the burner since NH 3 /air mixture has a low reactivity and needs a high ignition energy. However, any flashback or lift-off tendency was not observed for NH 3 /H 2 /air or NH 3 /air flames. The numerical and experimental results showed that H 2 addition into NH 3 /air mixture provided higher temperature values both in the flame region and in the entire combustor. As the concentration of H 2 in the mixture was raised, the flame approached towards to the burner outlet with increment effect in burning velocity. However, H 2 addition into NH 3 /air mixture led to more NO emissions formation in the combustor. This can be attributed with that H 2 addition promoted both thermal and fuel-NO mechanisms because H 2 addition induced to increase the decomposition rate of NH 3 and flame temperature of NH 3 /air mixture. This finding was validated with the prediction of the concentrations of O and HNO radicals for all fuel compositions. Consequently, H 2 introduction contributed to the flame stability of NH 3 /air mixture even if more NO formation was observed. • H 2 addition on the performance of NH 3 /air combustion have been investigated. • A stable flame for NH 3 /air combustion has been achieved. • H 2 addition provided higher flame temperature distributions in the combustor. • H 2 addition promoted NO emissions in the combustor. [ABSTRACT FROM AUTHOR]

Combustion is the cornerstone for generating energy in various sectors and applications. The predominance of hydrocarbons combustion causes several environmental and human health issues. The ever-increasing demand for decarbonization of combustion technologies in order to reduce or eliminate undesirable emissions has created a major challenge, putting tremendous pressure on the international research community to develop alternative fuels. Ammonia has recently been explored as a promising carbon-free fuel and a great alternative to hydrocarbon fuels to achieve zero-carbon emissions. However, despite its advantages over hydrogen, ammonia combustion has some drawbacks including low burning velocity, flame stability, and high NOx emissions. This review begins with energy and environmental issues before moving on to hydrogen and ammonia as promising potential solutions, their advantages and drawbacks, and ammonia combustion kinetics. It summarises the principles of chemical kinetics of both ammonia combustion and NOx reduction coupled with the chemistry, kinetic mechanisms and impact of plasma on ammonia combustion. The effects of plasma on ammonia combustion and NOx reduction are also discussed, with a comparison of NOx reduction mechanisms and plasma approach. Finally, the review discusses the perspectives and challenges of using plasma in ammonia combustion. [Display omitted] • Plasma assisted ammonia combustion. • Advantages and drawbacks of ammonia combustion with kinetics. • NOx reduction coupled kinetics and impacts of plasma on ammonia combustion. • Perspectives and challenges of plasma ammonia combustion technologies. [ABSTRACT FROM AUTHOR]

Recently, ammonia is getting more attractive as a potential alternative fuel for future power generation and transportation purposes due to its ability to reduce consumption of fossil fuels, soot, CO 2 emissions, and hydrocarbon pollutants. In the recent past, several studies aimed to the development of ammonia combustion systems have been conducted. However, ammonia combustion systems face several challenges, such as poor ignition timings, lower burning velocity, low energy density, shorter flammability limits than hydrocarbons. These issues are compounded by the significant increase in NOx emissions, which makes practical application of ammonia challenging. Various conventional methods, such as blending ammonia with other hydrocarbons and hydrogen, have been employed to improve combustion performance and reduce NO x emission. Meanwhile, it increased the engine complexities and at the same time, it raised CO 2 and NO x emissions. Recently, plasma-assisted combustion (PAC) has emerged as a promising technology to boost the ammonia combustion process by improving ignition delay timings, increasing flame speed, extending flammability limits, and reducing NO x emissions. In recent years, plasma-assisted combustion (PAC) has emerged as a promising technology to enhance the ammonia combustion process by improving ignition delay timings, increasing flame speed, extending flammability limits, and reducing NOx emissions. Nevertheless, only a few studies on plasma-assisted ammonia combustion (PAAC) are available in the literature. This review paper aims to summarize the latest advancements in the field of PAAC during the last years, including progress in the development of numerical models and experimental studies. The paper highlights the detailed procedure of PAAC numerical modeling and the limitations of numerical models. Finally, the latest numerical and experimental developments of PAAC have been discussed. • Ammonia is getting more attraction as an alternative fuel for future power and transportation sectors. • A comprehensive outlook of non-thermal plasma for ammonia combustion is provided. • The in-depth mechanisms of plasma assisted combustion are highlighted. • The latest numerical and experimental studies of Plasma assisted combustion were analyzed. [ABSTRACT FROM AUTHOR]

Biogas is a gas resulting from the digestion of biomass, which means transforming organic waste into energy. It is composed essentially of methane (CH4) and carbon dioxide (CO2) and can also contain ammonia (NH3) as an impurity. Biogas is generally used to generate electricity or produce heat in a cogeneration system. With the renewed interest in ammonia and the increasing development of biogas caused by the urge for an energetic transition, those two carbon-neutral fuels are being investigated as a mixture in this study through the laminar burning velocity (LBV). In this paper, the LBV of biogas ammonia air mixtures are investigated experimentally for the first time over a wide range of equivalence ratios and ammonia concentrations. The biogas studied was 60% CH4 and 40% CO2 in volume. The NH3 concentration in the fuel varied from 0 to 50% vol. while the equivalence ratio varied from 0.8 to 1.2. The experiments were conducted at constant pressure in a constant volume vessel at 300 K and 1 bar. Adding ammonia to biogas decreases the LBV while the Markstein length is not very sensitive to ammonia addition. The CEU-NH3-Mech-1.1 and Okafor mechanisms show good agreement with the experimental laminar burning velocity. The effect of radiative heat losses on the measurement is also investigated. [ABSTRACT FROM AUTHOR]