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To further investigate the propagation characteristics of shock waves and flame waves in H‐type tunnel gas explosions, numerical simulation studies were conducted on a dual‐source gas explosion model using Fluent software. Three distinct operational conditions were designed and modeled, leading to the following outcomes. The shock wave flow field parameters from dual sources (with equal source energy) are symmetrically distributed in the H‐type tunnel, with high pressure and low flow velocity in the connecting roadway between the two shock waves. Under different conditions, the pressure is generally higher in closed tunnels (fully closed greater than semi‐closed) and lower in open tunnels. The largest overpressure in non‐explosion areas occurs at the closed ends and in the connecting roadway, while the areas bearing the greatest impulse are the shock wave reflection zones and pressure coupling regions. In closed conditions, the flame wave first moves forward and then propagates backward after the explosion, influenced by reflected waves and pressure differences between the ends and the tunnel. In open conditions, the pressure in the flame zone is lower than at both ends, inhibiting the forward propagation of the flame wave. [ABSTRACT FROM AUTHOR]

To study the flame propagating characteristic and its coupling effect with other parameters in the LPG pipeline, a typical pipeline model with two equal‐length branches perpendicular to each other is designed for experiment and simulation. Then, gas explosion scenarios are experimentally tested and numerically simulated, which is followed by the analysis of flame shape changing with time and peak temperature changing with space. Results show that when passing through the bifurcation, flame propagates to vertical branch B in a sharp knife shape affected by the strong vortex, reflected airflow, and compressed pressure wave in the pipeline with a diameter of 0.125 m. At the monitoring point that is 0.4 m away from the bifurcation point, the peak temperature of the vertical branch B is 57.87% bigger than that of the horizontal branch C, and its arrival time is 80% longer than that of the horizontal branch C, due to the existence of flame in vertical branch B. What's more, in both branches, the coupling results between peak temperature and peak velocity agree very well with the growth function, while the coupling results between peak temperature and peak pressure agree well with the decay function, providing aids to the optimal layout design of industrial pipeline branches as well as to the explosion suppression measures. [ABSTRACT FROM AUTHOR]

This paper is devoted to the numerical analysis of the spontaneous acceleration of a slow flame in a semi-closed channel. In particular, the flow development in the channel ahead of the propagating flame is analyzed. The applied detailed numerical model allows the clear observation of all features intrinsic to the reacting flow evolution in the channel, including the formation of perturbations on the scale of the boundary layer and their further development. In all considered cases, perturbations of the boundary layer emerge in the early stages of flame acceleration and decay afterward. The flow stabilizes more rapidly in a narrow channel, where the velocity profile is close to the Poiseuille profile. At the same time, the compression waves generated in the reaction zone travel along the channel. The interaction between compression waves in the area of combustion products can lead to the formation of shock waves. The effect of shock waves on the flow in the fresh mixture causes an increase in the flame area and a corresponding flame acceleration. In addition, shock waves trigger boundary-layer instability in wide channels. The perturbations of the boundary layer grow and evolve into vortexes, while further vortex–flame interaction leads to significant flame acceleration. [ABSTRACT FROM AUTHOR]

Gas explosions mostly occur in turbulent environments in practical situations. Meanwhile, the current research on the explosion characteristics of hydrogen-doped natural gas is mostly methane/hydrogen mixed fuel, ignoring the influence of other components of natural gas on the explosion characteristics of the actual situation. Therefore, the effects of different jet intensities (P jet), hydrogen-doping ratios (X H2) and equivalence ratios (φ) on the explosion characteristics of multi-component natural gas/hydrogen mixed fuel are experimentally investigated in cylindrical tanks of 30 L. Additionally, it extensively explored the dual influence of jet disturbance and gas inerting effect on the flame propagation characteristics of multi-component natural gas/hydrogen mixed fuel. With the increase of P jet , both P max and (d p/ d t) max initially increase and then decrease, while τ continuously decreases. When P jet = 0.4 MPa, the promotion effect on P max and (d p/ d t) max is most significant. The lifting of X H2 leads to a gradual increase in P max and decreasing τ. (d p/ d t) max exhibits a trend of initially increasing and then decreasing with the elevation of X H2 , reaching its maximum at X H2 = 40%. The introduction of jet flow significantly reduces the promotion or inhibition effect on explosions of high hydrogen blending ratio mixtures. Furthermore, the introduction of jet flow is more effective in promoting the explosive performance of multi-component natural gas/hydrogen mixed fuel with high equivalence ratios and low hydrogen doping ratios under rich fuel conditions. This study can provide a key reference for the explosion hazard assessment during the practical application of natural gas/hydrogen mixed fuel. • Hydrogen-doped natural gas is used in explosion experiments collected in real environments. • A gas jet disturbance experimental platform has been established. • The flame propagation characteristics under turbulent conditions have been summarized. • The promotion effect on the explosion is most significant when the jet intensity reaches 0.4 MPa. [ABSTRACT FROM AUTHOR]

This experimental study investigated the influence of different turbine guide vane parameters on the ignition limit and light-round processes in a triple-dome combustor. It was found that for the triple-dome combustor, the minimum fuel/air ratios at the ignition limit all show a trend of initial decrease followed by subsequent increase with the growth of incoming air mass flow rate. The ignition fuel/air ratio decreases with the increase in turbine blockage ratio, and the optimal scheme is achieved with a blockage ratio of 0.8. Under the condition where the incoming air mass flow rate is 0.089 kg/s, the light-round time decreases with the increase in fuel/air ratio, and the light-round time of the combustor without guide vanes is shorter than that of the other schemes. With the increase in incoming air mass flow rate, the light-round time of the schemes with guide vanes is shortened. Under the same incoming air mass flow rate and fuel/air ratio, the increase in the blockage ratio will lead to an increase in the light-round time of the triple-dome combustor. [ABSTRACT FROM AUTHOR]

Gas explosion accidents can easily lead to large-scale casualties and economic losses, significantly impeding the urban development. The purpose of this study was to comprehensively review and investigate a significant gas fuel explosion accident in Yinchuan City, China, and to conduct an in-depth discussion on process traceability, failure risk, hazard prevention, and urban development related to the accident. The research found that the accidental failure of double-valve liquefied petroleum gas cylinders and human error were identified as the direct causes of gas leakage. The numerical results indicated that the progression of the accident disaster was chaotic and highly destructive. The maximum explosion overpressure of 92 kPa resulted in severe shock-wave damage to personnel, leading to the complete destruction and collapse of the 0.2 m thick solid brick wall and obstructing the stairway for escape. The rapid change in temperature and oxygen levels caused by the explosion led to the risk of burns and asphyxiation for personnel at the scene. By utilizing the system safety theory, a gas leakage accident control structure system was developed. This system comprised four key levels: the local government, gas management department, gas company, and individual user. The tragedy of 31 deaths was ultimately caused by a serious lack of safety constraints on the behavior of the lower level by the higher level. The research conclusions are of great significance for preventing clean fuel explosion accidents and ensuring sustainable urban development, especially in the face of the negative impact of accidents. [ABSTRACT FROM AUTHOR]

• The flame propagation process of R152a under different RH conditions was recorded • The R152a reaction activation energy is reduced in O 2 and O 2 /H 2 O environments. • OH is the crucial active fragment that promotes the oxidation of R152a. • The combustion mechanism of R152a in humid air was revealed. 1,1-difluoroethane (R152a) is one of the most prospective low GWP refrigerants but may trigger fires and explosions once it leaks. Understanding the flame propagation process and combustion mechanisms at different relative humidity conditions is essential to use flammable refrigerants safely. In this study, a high-speed camera has recorded the flame propagation of R152a combustion near the flammability limit. Combining with macroscopic experimental phenomena, we revealed the effect of H 2 O on the microscopic mechanism of R152a combustion by using reactive force field molecular dynamics simulations with a reliable force field. The promotion effects of H 2 O on the oxidation of R152a were revealed from an atomistic perspective. The differences of oxidation intermediates and products in different environments were analyzed for the first time. The H 2 O/O 2 environment was the most potent promoter of R152a decomposition, followed by the O 2 and the pyrolysis environment. The O 2 /H 2 O environment reduced the apparent activation energy of R152a, significantly enhanced the consumption rate of R152a, and enriched the number of species. O 2 reacts with H 2 O or H to form OH to accelerate the reaction process. The H 2 O could provide more OH active fragments for the reaction. Moreover, it promotes the formation of HF and H 2, H 2 O + F→HF + OH, H 2 O + H→H 2 + HO. This study aims to provide a guiding theory for the safe application and disaster prevention of flammable refrigerants. [ABSTRACT FROM AUTHOR]

Using the experimental and large eddy simulation (LES) methods, the impacts of equivalence ratio and adjacent obstacle blockage ratio on flame propagation, flame moving velocity, and explosion overpressure in a closed obstructed duct are investigated. The results indicate that the developed LES method is able to reproduce the tendency of flame propagation, flame moving velocity, and explosion overpressure of BR = 30%, 50%, 70% and BR = 70%, 50%, 30%. The early flame propagation and flame moving velocity are not affected by the first obstacle. As the explosion flame passes through the three obstacles, the flame acceleration and deceleration occur in sequence due to the decreasing cross-section and vortex-flame interaction, eventually forming three ascending peak flame moving velocity. The higher vorticity magnitude of BR = 70%, 50%, 30% and BR = 30%, 50%, 70% is mainly concentrated in the early stage and later stage of hydrogen explosion, which directly results in the fact that the flame acceleration and rise rate of explosion overpressure of BR = 70%, 50%, 30% is higher than that of BR = 30%, 50%, 70%. Additionally, the maximum value of flame moving velocity and explosion overpressure of BR = 30%, 50%, 70% is higher than that of BR = 70%, 50%, 30%. • Flame propagation and explosion overpressure are experimentally obtained. • LES method is established to predict flame propagation and explosion overpressure. • The velocity vectors and vorticity magnitude are analyzed. [ABSTRACT FROM AUTHOR]

The explosion characteristics of AlH 3 dust clouds with varying micron particle sizes are investigated in this paper. The explosion mechanism of AlH 3 dust clouds is initially explored. The results indicate that the activation energy for hydrogen evolution shows an increasing trend with the AlH 3 particle size, and both of these values are comparable to the 30 kJ/mol desorption energy of hydrogen from large AlH 3 clusters. Due to the rapid dehydrogenation rate and small-size residual aluminum core, fine AlH 3 particles exhibit a continuous gas-phase flame and fast propagation velocity. With increasing particle size, the lower dehydrogenation rate, reduced specific surface area, and higher volume heat capacity contribute to a higher level of incomplete reaction. Consequently, large micro-sized AlH 3 particles display a discrete flame, slower flame velocity, and lower explosion pressure. The flame delamination phenomenon is not observed in our experiments on AlH 3 dust clouds. [Display omitted] • The explosion characteristics of AlH 3 dust with different particle sizes are studied. • The phenomenon of flame delamination is not observed in the AlH 3 dust clouds. • The activation energy for H 2 release increases with particle size, similar to large AlH 3 clusters' desorption energy. • The explosion mechanism of AlH 3 dust with different particle size is revealed. [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]

Until recently, it was attempted to explain the flame propagation without considering the chain nature of combustion, but the contradictions between theory and experiments failed to be explained. The difference in the roles of reaction chains and self-heating was not considered, as well as the role of heterogeneous chain termination. These issues are addressed in this chapter. Greater uncertainty in the quantitative interpretation of gas combustion is due to the limited information on heterogeneous reactions of active particles and the nonstationary state of the surface. Under these limitations, the propagation of flame of the 4H2 + O2 mixture was simulated. To reduce the role of the nonstationary state of the surface, conditions for the limiting role of diffusion of H atoms to the walls were chosen. A method for solving differential equations and conditions for replacing the Laplace operator with an averaging rate constant for the chemisorption of atomic hydrogen are described. The flat flame approximation is used. The flame characteristics for their comparison with the results of experiments conducted with the participation of the author were obtained. Experimental results showing the strong dependence of all characteristics of flame propagation on the changing properties of the surface are presented. [ABSTRACT FROM AUTHOR]

This work primarily focuses on a three-dimensional model of flame propagation and stable combustion in a scramjet chamber. The one-equation LES turbulence model is adopted to close the sub-grid-scale turbulent viscosity terms. The finite-rate combustion model, along with the Jachimowski detailed hydrogen reaction mechanism with eight components and nineteen steps, is used to analyze the flame propagation characteristics of hydrogen combustion in the scramjet combustion chamber. Initially, based on the combustion chamber model, the effect of different injection locations and equivalence ratios on flame kernel formation and the flame propagation process is analyzed. The relationship between different fuel injection conditions and the oxygen consumption rate of the combustion chamber, as well as the total pressure recovery coefficient changes, is investigated. The research focuses on changes in equivalence ratios and injection hole distributions, with injection holes arranged upstream, downstream, and inside of the cavity. The result indicated that when the injection holes were arranged downstream of the cavity, there was a phenomenon of flame backflow into the cavity, which was related to the size of the injection pressure. For this work, the pressure causing flame backflow was approximately 2 MPa. When the injection hole was arranged inside the cavity, the relative distance difference between the injection hole and the upper wall of the cavity led to the formation of two reaction zones in the combustion chamber. When the injection hole was arranged upstream of the cavity, different injection equivalence ratios affected the final stable position and structure of the flame. Therefore, the injection position, injection pressure, and injection equivalence ratio all had a certain impact on the flame kernel formation and flame propagation process. [ABSTRACT FROM AUTHOR]

The vented deflagration of hydrogen-air mixture is studied with the influence of hydrogen concentration. The pressure oscillation and flame propagation mechanism are investigated by analyzing the internal and external pressure histories and flame propagation images. The experimental results present that the pressure oscillation phenomenon has a great effect on the internal pressure peak. All of the internal pressure curves present the remarkable pressure oscillation. The pressure oscillation curve is processed by the fast Fourier transform(FFT) method. The internal pressure peaks of origin curves are higher than the results of the FFT filter curves, especially the fuel-rich conditions. The internal pressure peak of origin curve increases with the increase of the hydrogen concentration(c (H 2)) approximatively, but the filtered internal pressure peak increases first and then decreases as c (H 2). The duration of the external pressure rise is significantly shorter than the internal results. The external pressure curves have one or two pressure peaks. The external pressure histories just have one peak as c (H 2) ranging from 15 vol% to 25 vol%. But the external pressure histories turn out to two peaks as c (H 2) is more than 30 vol%. It can be found that the duration of the internal pressure history increases monotonically with the decrease of c (H 2). The duration of external flame propagation is contrary to the duration of internal pressure history in the change rule. The duration of external flame propagation increases with the increase of c (H 2). The external explosion phenomenon can be found as c (H 2) is higher than 30 vol%. And the backdraft phenomenon in external flame propagation process can be found as c (H 2) is higher than 40 vol%. • The pressure oscillation phenomenon has a significant impact on the internal overpressure peak. • The duration of the external pressure rise is significantly shorter than the internal results. • The external explosion phenomenon can be found as c (H 2) is higher than 30 vol%. • The backdraft phenomenon can be found as c (H 2) is higher than 40 vol%. [ABSTRACT FROM AUTHOR]

Aluminum hydride (AlH 3) dust is a high-capacity hydrogen storage material but is prone to explosion. In this paper, explosion characteristics and flame propagation behavior of AlH 3 dust are investigated. The results indicate that the exposure of bare aluminum and the increase of specific surface area lead to the maximum explosion pressure of AlH 3 dust being 4 times higher than that of Al dust. The enhancement of heat transfer by hydrogen combustion causes a higher combustion rate, leading to a higher maximum explosion pressure rise rate and flame propagation velocity of AlH 3 dust. The thermo-diffusive instabilities and the evolution of hydrogen result in a pulsation flame propagation velocity of AlH 3 dust. Combining the explosion temperature calculated by NASA CEA with the explosion residues analysis, the explosion mechanism of AlH 3 dust is further revealed. The break of the oxide film and the combustion of hydrogen result in a different explosion mechanism. • The explosion characteristics and flame propagation behavior of AlH 3 dust are investigated. • The hydrogen evolution characteristic is studied by the comparison of Al dust. • The explosion mechanism of AlH 3 dust is revealed. [ABSTRACT FROM AUTHOR]

In this paper, we present a mathematical approach for studying the changes in pressure and temperature variables in flames. This conception extends beyond the traditional second-order Laplacian diffusion model by considering the p-Laplacian operator and a bi-stable reaction term, thereby providing a more generalized framework for flame diffusion analysis. Given the structure of our equations, we provide the boundedness and uniqueness of the solutions in a weak sense from both analytical and numerical approaches. We further reformulate the governing equations in the context of traveling wave solutions, applying singular geometric perturbation theory to derive the analytical expressions of these profiles. This theoretical development is complemented by numerical assessments, which not only validate our theoretical predictions, but also optimize the traveling wave speed to minimize the error between numerical and analytical solutions. Additionally, we explore self-similar structured solutions. The paper then concludes with a perspective on future research, with emphasis being placed on the need for experimental validation in laboratory settings. Such empirical studies could test the robustness of our model and allow for refinement based on actual measurements, thereby broadening the applicability and accuracy of our findings in practical scenarios. [ABSTRACT FROM AUTHOR]

Aluminum hydride (AlH 3) is an excellent hydrogen storage material, whereas hydrogen is easily released because of the low-temperature dehydrogenation characteristic. Therefore, the explosion mechanism of AlH 3 dust is explored by comparison with the Al/H 2 mixture. The findings indicate that since the residual aluminum after hydrogen evolution combusts more completely, AlH 3 has a higher explosion pressure value than Al/H 2 mixture. However, the explosion pressure rise rate and average flame propagation velocity of AlH 3 dust are lower than those of Al/H 2 mixture. It is attributed to the initial free hydrogen state and initial higher hydrogen concentration of Al/H 2 mixture. The key radicals that generate main explosion residues Al 2 O 3 are AlO and O. AlH 3 (+M) ≤> AlH + H 2 (+M) has the greatest sensitivity on the formation of O and AlO. Hydrogen combustion expedites the phase transition process of aluminum dust, while their competition for oxygen is not conducive to the formation of Al 2 O 3. • The explosion characteristics of AlH 3 dust are analyzed by comparison with the Al/H 2 mixture. • The initial state and content of hydrogen lead to different explosion processes. • The explosion mechanism of AlH 3 dust is revealed by reaction kinetics simulation. [ABSTRACT FROM AUTHOR]

In this paper, we studied the effects of transverse concentration gradients on methane explosion characteristics using a fully-transparent pipeline and finally came to the following conclusions. The peak explosion overpressure changed in a waveform of "M" along different measuring points, but a set of non-equivalent concentration gradients maximized this peak value. Higher methane concentrations in the Q2 region could cause the secondary peak overpressure near the both ends of the pipe. In this process, the flame outside the pipe tends to form a near-spherical shape. However, at lower methane concentrations in the Q2 region, the secondary peak overpressure was only detected near the outlet. Its occurrence at different sites may relate to the turbulence effect heightened by concentration gradients. Besides, under the same gradient, the smallest propagation speed was measured on the right of the ignition source when the flame propagated fastest on the left, which was attributed to different inhibiting effects of reflected waves on flame propagation. • There exists a set of non-equivalent concentrations to maximize the explosion overpressure peak. • The spatial evolution law of gas explosion overpressure in pipeline with concentration gradient was revealed. • The formation mechanism of slope flame and secondary overpressure peak were described. [ABSTRACT FROM AUTHOR]

A 20L spherical device and a schlieren system were used to study the suppression effect of CF 3 I on the hydrogen explosion pressure and flame characteristics. The results show that CF 3 I can promote and inhibit the pressure parameters of hydrogen under lean fuel, but only inhibit the pressure parameters of hydrogen under equivalence ratios and rich fuel. Such as, φ = 0.6, adding 10 % CF 3 I, the maximum explosion pressure increased by 8.8 %. φ = 1, adding 5 % CF 3 I, the maximum explosion pressure decreased by 25 %. Different from the explosion pressure, the laminar burning velocity of hydrogen with different equivalence ratio decreases gradually with the increase of CF 3 I concentration. Numerical simulation was used to reveal the mechanism of CF 3 I inhibiting hydrogen. The results show that the concentration of active free radicals(H, O, OH) decreases with the increase of trifluoroiodomethane concentration. Further analysis on the rate of production and sensitivity of H free radicals found that R822 and R839 are the main generation reactions of H free radicals in the fluorine- and iodine-containing elemental reactions, and R1397, R1396, R1391 and R859 are the main consumption of H free radicals reaction. The elementary reaction with the greatest inhibitory effect on H free radicals is R1391, and the elementary reaction with the greatest promoting effect is R850. • CF 3 I has two effects of promoting and suppressing hydrogen explosion pressure. • Analysis of the effect of suppressant on hydrodynamic instability. • CF 3 I has inhibitory effect on laminar burning velocity of hydrogen. • The kinetic mechanism of CF 3 I suppressing hydrogen explosion was analyzed. [ABSTRACT FROM AUTHOR]

To research the influence of inert gases on the explosion behaviors of LPG/air mixtures, LPG explosion tests suppressed by CO2 and N2 were conducted in a spherical chamber and the chemical reaction kinetic properties were analyzed. The results show that CO2 has a better inhibitory effect than N2, especially when the added amount is small. With the increasing volume fraction of inert gases, the LPG explosion flame propagation speed continuously decreases, and the phenomenon of flame uplift and wrinkling deformation occurs. The explosion pressure of LPG/CO2(N2)/air mixture and the average flame propagation speed in the initial stage follow an exponential function with an offset. The laminar flame structure of 4% LPG is similar to its mixtures with N2 and CO2. After adding inert gases, the maximum temperature of the LPG laminar flame decreases significantly, and the flame thickness increases. The key elementary reactions affecting the N2(CO2)/LPG/air explosions are given. The research results have important implications for suppression of LPG explosions. [ABSTRACT FROM AUTHOR]

Unsymmetrical dimethylhydrazine (UDMH) is a common liquid propellant widely used in rocket engines and other applications. The safety of UDMH in service is affected by its slow oxidation during long-term storage to form impurities such as dimethylamine (DMA) and formaldehyde dimethylhydrazone (FDH). How these impurities affect combustion performance is not known, and in order to assess these effects, the present experiments investigated the combustion characteristics of self-igniting fuels and carried out ignition delay time measurements and flame propagation velocity measurements of pure UDMH and its denatured mixtures in a nitrogen tetroxide (NTO) atmosphere. This experiment was carried out to measure the delay time of hedge ignition of pure UDMH and qualitative analysis of its flame propagation properties under vacuum conditions at room temperature (T = 293 K). Ignition delay time measurements and flame propagation characterization were performed under the same experimental conditions for UDMH mixed with 1%, 5% and 10% FDH, UDMH mixed with 1%, 5% and 10% H2O, UDMH mixed with 1%, 5% and 10% DMA, as well as for UDMH mixed with the same proportions of the three substances (1%, 5% and 10%). The flame propagation characteristics were analyzed. The results showed that the incorporation of DMA, H2O and FDH in different proportions could inhibit the combustion of UDMH to varying degrees and prolong its ignition delay time. It is worth noting that the introduction of FDH had the least effect on it, and the least effect was observed at a concentration of 1%. In contrast, the effect of DMA on UDMH is more obvious, and the addition of H2O has the largest increase in the ignition delay time of UDMH. In the flame propagation experiment, the flame of the experimental group adding H2O can no longer fill the whole experimental window, while the other experimental groups can still make the window full of flame. Combined with the measurements of the ignition delay time, it can be seen that the moisture content has the greatest effect on the combustion characteristics. [ABSTRACT FROM AUTHOR]

The combustion process of traditional diesel engines is mainly determined by the injection timing of diesel. There is a trade-off relationship between the soot and NOx (nitrogen oxides) during this combustion process, making it difficult to reduce these two emissions simultaneously. The use of methanol can not only solve the above problem, but also replace some fossil fuels. However, the effects of methanol injection into the intake duct on the flame propagation in diesel/methanol dual-fuel engines is not yet clear, and there is relatively little research on it. The effects of methanol addition on the combustion process of diesel/methanol dual fuel (DMDF) were achieved based on a modified optical engine in this paper. One injector is installed on the intake inlet to inject methanol, and the other injector is installed in the cylinder to inject diesel in two stages before the top dead center of compression. There are three tests conducted separately in this paper. Firstly, the effects of the methanol ratio (40%, 50%, 60%, and 70%) on the combustion process are investigated, with the total heat remaining unchanged. Secondly, the effects of the pre-injection mass of diesel (20%, 30%, 40%, and 50%) on the combustion process are investigated, which keeps the total diesel mass unchanged. Finally, the effects of the total mass of diesel on the combustion process are investigated while maintaining the mass of methanol unchanged. The dual-fuel combustion process is recorded by a high-speed camera. A combustion analyzer and other equipment were used to analyze the combustion. The results showed that CA10 is delayed, the pressure and the heat release rate (HRR) are reduced, and the number of pixels of the KL factor (KL) decreases significantly with the increasing methanol ratio. CA10 and CA50 are advanced, the pressure and HRR decrease, and the KL increases when the mass of pre-injected diesel increases. CA10 and CA50 are advanced, respectively, and CA90 is postponed due to the increase in diesel mass. The pressure and HRR increase, and the KL increases when the total mass of diesel increases. [ABSTRACT FROM AUTHOR]

The present study focuses on the examination of the combustion behavior of Iraqi LPG-air mixtures, flame propagation, and the tulip flame phenomenon in tubes. A new experimental facility was designed and constructed, featuring a complete setup comprising a combustion chamber unit, ignition unit, fuel injection and control unit, mixture preparation unit, and flame imaging unit. The combustion process was initiated through ignition by an electric spark, and both experimental measurements and numerical simulations were conducted using various turbulence models, including the realizable k-e model, the k-e model, and the Launder Sharma k-e model. The study aimed to compare and validate the numerical results with empirical observations, specifically focusing on flame propagation velocity, peak pressures, and tulip flame formation. The investigation revealed that the k-e model exhibited the closest agreement with experimental findings, accurately capturing flame propagation dynamics and tulip flame formation. However, discrepancies were noted in the numerical simulations, attributed to the absence of a wrinkling factor in the laminar combustion model, leading to smoother flame boundaries compared to experimental flames. The research highlighted the influence of turbulence on flame propagation beyond the tulip flame zone, necessitating further inquiries and model refinements to improve predictive capabilities. The findings affirm the appropriateness of the k-e model in simulating the combustion process and provide valuable information for optimizing combustion efficiency andmitigating pollutant emissions. Future investigations will be essential to enhance turbulence models and advance the understanding of flame dynamics in confined environments. The comprehensive analysis of flame behavior presented in this study contributes to the advancement of combustion science and engineering, offering potential applications in various industrial and environmental settings. [ABSTRACT FROM AUTHOR]

Rice is relatively rich in minerals, including iron, magnesium, and selenium. In the presence of a spark, rice dust suspended in air in a range of concentrations can ignite and explode. The explosion characteristics of brown, black, white, glutinous, and Bario rice were studied in a 20‐L spherical vessel. Brown rice recorded the highest explosion pressure and rate of pressure rise in both dried and undried forms, while Bario rice recorded the lowest. Brown rice was more combustible and tended to develop a fast flame mechanism once ignited owing to its low moisture content and high volatility, which made its explosion more severe compared to that of black, white, glutinous, and Bario rice. [ABSTRACT FROM AUTHOR]

Hydrogen energy is considered as one of the most recognized future energy systems by most of countries. Much hydrogen related technologies have been developed such as H 2 engine. However, safety issue could be specially taken care since leaked hydrogen could disperse quickly into the enclosed environment and lead to the possible explosion. The spherical flame propagation is a basic process for explosion. Initial spherical flame characteristics is important due to its effect on the ignition in the hydrogen explosion hazard. Thus, numerical research on the initial flame propagation characteristics of the hydrogen spherical premixed flame at various initial temperature and radius are investigated. Results show that the initial flame temperature and radius will affect the self-sustained hydrogen flame characteristics. For smaller initial radius and temperature, H, OH radicals and HRR increase greatly to a maximum value and then decrease to almost zero in a short time. However, HRR is still very low and the initial flame kernel could not propagate without external energy. When the initial flame temperature or radius increase, the reaction rate becomes to be stronger and H, OH radicals and HRR all increase with the time. Both of the flame kernel could be self-sustained. With the increase of the initial temperature, the heat release increases. There is a critical initial temperature for various initial radius. For smaller initial radius of 0.2 mm, the critical initial temperature is around 3075 K for equivalence ratio of 1.0. With the increase of the initial radius, the critical initial temperature will decrease but change little after the initial radius of 0.8 mm. The critical initial temperature is around 1500 K. Theoretical analysis shows that initial temperature, laminar flame speed, reaction heat release and thermal diffusivity are the most important parameters which affect the initial flame kernel development. Larger laminar flame speed, reaction heat release and smaller thermal conductivity could be favor of the flame kernel development. • Flame characteristics of self- and no-self-sustained flames are discussed. • Critical initial temperature decreases with initial radius. • Influencing parameters of initial flame propagation are analyzed. [ABSTRACT FROM AUTHOR]

The paper presents a fully three-dimensional coupled simulation of flame propagation through epoxy resins reinforced with glass fiber (glass–epoxy) under the action of a heat flux from two sides, using the Fire Dynamic Simulator package. The model includes three-dimensional heat and mass transfer, diffusion of basic substances in the gas phase, and diffusion transport of pyrolysis products in the solid phase. The pyrolysis and oxidation processes are represented by macroreactions and take into account the effect of the phosphorus-containing flame retardant DDM-DOPO. The model satisfactorily predicts the experimentally observed dynamics of flame propagation. [ABSTRACT FROM AUTHOR]

Natural gas (NG) is a low-carbon fuel that can achieve low carbon emissions and high efficiencies. However, the low mean molecular mass and low energy density of methane result in low engine performance, and blending methane with gasoline is an effective way of improving the performance of NG engines. In this study, flame characteristics (including abnormal combustion) and engine performance are optically studied under methane port injection and isooctane direct injection conditions. The results indicate that the addition of isooctane can significantly increase the power output of NG engines. Under partial-load conditions, the minimum ignition advance for best IMEP is delayed with the addition of isooctane because of the higher flame speed of isooctane, and isooctane addition primarily improves the turbulent flame propagation rather than the initial flame formation. As for the isooctane injection timing, the indicated mean effective pressure (IMEP) first increases and then decreases with the delay in isooctane injection. The hot spots in the flame images confirm that late injection results in isooctane inhomogeneity, which reduces the engine's thermal efficiency. At high loads, the low energy density of methane can lower the knocking intensity, even in the presence of auto-ignition at a speed of 120.7 m/s. The knocking pressure can lead to the ejection of oil droplets, which can increase the flame speed in the subsequent cycle. [ABSTRACT FROM AUTHOR]

Experiments of two nozzle diameters at three ignition positions under three initial pressure conditions were carried out. The dynamic leakage characteristics and the stagnation parameters of flame propagation under normal temperature and high pressure conditions were studied. Based on van der Waal's equation, a model for predicting stagnation parameters, jet velocity and flow rate of hydrogen leakage was proposed. Compared with the experimental results, it was found that the maximum error occurred when the initial pressure was 200 bar. Theoretical leakage time was 1.66 s, experiment leakage time was 1.84 s, the error was 9.8%. Background-Oriented Schlieren image technology was used to record the flame development and propagation process after ignition. For the same nozzle diameter and ignition location, the higher pressure caused the flame to propagate faster upstream and downstream. For the same initial pressure and ignition position, a flame with a large nozzle diameter propagated faster upstream and downstream. For the same initial pressure and nozzle diameter, the farther the ignition point was, the greater the slope of flame attenuation when propagating upstream. Due to the attenuation of hydrogen concentration and jet velocity, the flame propagation velocity to the downstream decreased linearly with the increase of distance from the ignition location. • Dynamic leakage and combustion of hydrogen were studied experimentally. • The changing law of stagnation pressure and temperature was measured. • A mathematical model that could predict stagnation parameters was proposed. • The calculation error of theoretical model was in the range of ±9.8%. • Pressure, nozzle diameter and ignition position affect flame propagation. [ABSTRACT FROM AUTHOR]

In several works, scientific researchers targeted the development of polymeric materials with good mechanical and thermal properties and resist heat and fire propagation. In a new approach, this article goals to the usage of waste polyvinyl chloride (WPVC) based polymer composites derived from PVC pipes scrap filled with fixed percent of zinc borate particles (ZnB), which were prepared via two different techniques named melt mixing WPVC/ZnB (M) and casting WPVC/ZnB (C). The fabricated composites by the two methods were irradiated in an electron beam accelerator (EB) at 30 kGy. A comparative study of the thermal and mechanical properties based on stress-strain curves, tensile strength (TS), elongation at break (E %), and tear strength were evaluated. Furthermore, the rate of heat burning (%) for the two methods has been performed. Moreover, differential scanning calorimetry (DSC), Thermogravimetric analysis (TGA), Infrared spectroscopy analysis (FTIR), X-ray diffraction (XRD), and the activation energy (Ea) calculation of the prepared samples have been considered. The observed improvements in the mechanical, thermal, and flammability behavior of WPVC were succeeded by incorporating ZnB particles. The irradiated WPVC loaded with zinc borate presented a superior flame propagation fraction when matched with their corresponding unirradiated composites. Also, WPVC composite films prepared via the melt mixing method reveal more excellent properties than those equipped by the casting process for the most investigated factors. Graphical Abstract [ABSTRACT FROM AUTHOR]

Purpose: This article aims to study a model of flame propagation in a nonhomogeneous medium with a p-Laplacian operator. The intention with such operator is to model the effects of slow and fast diffusion, that can appear in a nonhomogeneous media, depending on the pressure driven conditions. In addition, the authors introduce a general form in the reaction term, that introduces the flame chemical kinetics. Design/methodology/approach: To introduce the governing equations, the authors depart from previously reported models in flame propagation, but the authors consider a new modeling approach based on a p-Laplacian operator. Findings: The authors provide evidences of regularity and uniqueness of solutions. Afterward, the authors introduce profiles of stationary solutions based on the definition of a Hamiltonian for the newly discussed model. Eventually, the authors obtain exponential profiles solutions with the help of a scaling, that transforms the model into a nonlinear Hamilton–Jacobi equation. Originality/value: The new model has not been previously reported in the literature. The authors consider that the mathematical properties of a p-laplacian (in particular the property known as finite propagation) is of inherent interest to model pressure drive flames with slow or fast diffusion. Indeed, the authors' approach has the value of providing an operator that can fit better to model flame propagation. In addition, the authors introduce a general form of chemical kinetics, to make the authors' model further general. [ABSTRACT FROM AUTHOR]

The processes of flame penetration and propagation in a narrow annular gap between the piston and cylinder of a hydrogen piston engine are investigated by the method of 3D mathematical modeling. The model is verified by comparing the changes in pressure and heat release rate in the cylinder obtained from an experimental hydrogen engine, considering the data collected during numerical experiments. The movement of the flame front into the gap is analyzed by changes in the instantaneous local values of the hydrogen fractions in the mixture and the local temperatures of the cold gas (unburned mixture) and combustion products. A comparative analysis of the spread of gasoline and hydrogen flames is carried out. The phenomenon of increasing heat losses in the combustion chamber of a hydrogen engine compared to a gasoline engine, previously confirmed experimentally by different authors and not yet having an acceptable theoretical interpretation, is explained by neglecting the role of heat transfer in the indicated gap. [ABSTRACT FROM AUTHOR]

It has been established that, contrary to previous ideas, chemical processes in flame propagation, explosion, and detonation of gases are chain reactions proceeding according to previously unknown laws of nonisothermal chain processes. The characteristic reaction times in deflagration and detonation in the combustion zone are less than a ten-thousandth and millionth of a second, respectively. The mechanisms and laws that determine these high rates and accelerations of reactions and their extremely strong temperature dependence were revealed. The critical role of atoms and radicals, formed in concentrations reaching tens of percent of the concentrations of the initial reagents, was shown using kinetic and spectroscopic methods. Efficient chemical methods for controlling all combustion modes were developed. [ABSTRACT FROM AUTHOR]

The pipe structures and opening conditions have an important influence on gas explosion, but little research has been done on the coupling analysis of the two. In order to reveal the effect of pipe structure on the flammability characteristics of hydrogen-air premixed gas under end-opening conditions, the flame structure, flame propagation velocity, explosion pressure and flow field distribution in the explosion process of hydrogen-air premixed gas in different pipe structures were analyzed by numerical simulation. The results show that the flame propagation velocity and pressure are less influenced by the end-opening structure in the initial ignition stage, however, when the flame propagates to the pipe end, the flame propagation velocity in each pipe structure is significantly enhanced. The 90° elbow has a certain inhibitory effect on the flame development, while the T-shaped bifurcation structure can effectively increase the degree of gas detonation. In pipe with large aspect ratio, due to the wall effect, the effect of acoustic oscillation disturbance on the flame front and the air-flow release effect of the end-opening, there are two peaks and two troughs in the pressure rise rate curve of each pipe-line structure. [ABSTRACT FROM AUTHOR]

For explosion-proof and effective suppression measures in treating magnesium–aluminum alloy, it is of great importance to study the suppression of Mg–Al alloy dust explosions to prevent explosion disasters. The addition of inert solid substances to combustible dust is a measure aimed at preventing and reducing dust explosions. The explosion characteristics and flame propagation characteristics of Mg–Al alloy powder were studied using the Hartmann tube 20 L spherical explosion experimental system. The mechanism for removing sodium bicarbonate (NaHCO3) during the Mg–Al alloy powder explosion was further studied. The results show that the explosion pressure, the height of the deflagration flame, and the speed at which the flame propagates can be effectively reduced by increasing the percentage of NaHCO3. After the addition of 80% NaHCO3, the flame was suppressed, and the maximum explosion pressure decreased to less than 0.1 MPa, causing a 93% decrease in the maximum flame propagation speed. The process of suppressing NaHCO3 powder on magnesium aluminum alloy dust explosion is relatively complex, starting primarily with physical and chemical suppression. NaHCO3 realizes physical inhibition by reducing ambient temperature and oxygen concentration through the H2O and CO2 generated by decomposition. At the same time, through the cycle of NaO ↔ Na, the transformation from highly active oxygen to low active oxygen is realized. This reduces the activity of the explosive combustion response of Mg–Al alloy powder. At the same time, sodium ions can capture free radicals in explosive responses, reducing the number of free radicals in the reaction system and terminating the combustion reaction chain in advance. The research findings are of great importance for the safety of Mg–Al alloy production. [ABSTRACT FROM AUTHOR]

Rapid suppression is a must in the mitigation of ventilation air methane (VAM) explosions. Flame suppression proves to be much more challenging than prevention of flame initiation due to the small physics timescale (~1 s). This study numerically investigates the effect of spherical obstructions on flame propagation dynamics in a tube closed at one end. Obstructions with an inflating geometry, installed at different locations, were examined. Noticeably, in the presence of a single or multiple obstructions that partially block the tube, flame and pressure waves propagate faster upstream than in an empty tube; this phenomenon is more pronounced when the obstruction is located further away from the ignition point. In scenarios of a full blockage of the tube, the high pressure builds up inside the blocked region, e.g., surging up to 7.5 bar in less than 0.1 s at a location 10 m away from the ignition point (tube diameter: 0.456 m). Obstructions located closer to the ignition point experience more tearing in terms of duration and strength. [ABSTRACT FROM AUTHOR]

Flammable gas explosion frequently occurs in the actual oil and gas pipeline transportation or mining operations. Given this problem, this paper studies the effect of ignition positions on the explosion characteristics of the methane-air mixture using a semi-closed transparent gas explosion pipe. According to the results, regardless of ignition position, the overpressure curve of each measuring point presents a process of rising-falling to negative-rising-stabilizing, and the flame propagation to the open end (left flame) evolves from spherical to finger shape. As the ignition position is farther away from the closed end, the overpressure at the closed end and the average flame speed on the right side of the ignition source climb gradually. In contrast, the average flame speed on the left side declines gradually. Besides, When the ignition position（IP3 and IP4）are near the open end, P1（Overpressure at the closed end）overpressure curves under both working conditions oscillate obviously before reaching the peak. The flame front undergoes a repeated compression-stretch-recompression, and the extreme point of the oscillation overpressure waveform always corresponds to the extreme point of the flame position. During this period, the amplitude of the P1 curve grows linearly and parabolically with time. According to the research, the overpressure oscillation is triggered by the reverse pressure wave after the left-side flame rushes out of the pipeline under such working conditions. The growing amplitude during the oscillation can be caused by various factors, including the thermoacoustic instability and the interaction between the flame front of the closed end and the unburned gas column. The research results deepen the understanding of the combustion and explosion characteristics of flammable gas, better the explosion safety protection in its storage, transportation, and processing, and provide a theoretical basis for the safety protection of detonation devices. [ABSTRACT FROM AUTHOR]

It is generally assumed, sometimes implicitly, that the exploding gas cloud is uniform and homogeneous. However, in practice, the cloud often results from an accidental release of a chemical substance into the air, leading to a non-uniform gas mixture and, therefore, gradients of concentration of pressure or temperature. These gradients correspond to distributions of the degree of reactivity and, consequently, to a greater or lesser ability to react. In addition, they influence the consequences of the explosion. This study aims to investigate the behaviour of these gradients in the case of explosions in a reactive, nonuniform and confined environment. The phenomena of diffusion, combustion, and flame propagation of heterogeneous mixtures within closed chambers are carried out through numerical simulations. Moreover, a comparative study with previously published works is developed and presented to validate our results. [ABSTRACT FROM AUTHOR]

Recently, multiple spark ignition has received great attention since it helps to increase thermal efficiency and to reduce misfire in engines. Multiple spark ignition also affects the combustion duration and thereby it can be used for knock control. However, previous studies reported opposite trends in terms of how multiple spark ignition affects engine knock. This work aims to assess and interpret the influence of flame propagation direction induced by different ignition configurations on combustion duration and end-gas autoignition/engine knock. Two simplified and idealised ignition configurations are studied theoretically and numerically. One is with infinite number of sparks at side circular wall, which induces an inwardly propagating flame (IPF); and the other is with a single central spark, which induces an outwardly propagating flame (OPF). In the asymptotic theoretical analysis, the canonical 1D formulations for IPF and OPF are reduced to 0D model. Based on the 0D model, OPF and IPF at different initial temperatures are studied and compared. Counterintuitively, it is found that the combustion duration of OPF is shorter than that of IPF when there is no end-gas autoignition. On the other hand, the combustion duration of IPF is shorter than that of OPF when end-gas autoignition occurs. Furthermore, end-gas autoignition is found to be more prone to occur in IPF than OPF. These interesting observations are interpreted through assessing the ignition delay time and different components of the absolute flame propagation speed. The theoretical results are validated by transient simulations considering detailed chemistry and transport which are conducted for IPF and OPF in an iso-octane/air mixture at different initial temperatures and pressures. Both theoretical and numerical results suggest that compared to infinite number of ignition sparks at side wall, the single central ignition has the advantages in shortening the combustion duration and reducing the tendency of end-gas autoignition. [ABSTRACT FROM AUTHOR]

The accidental methane explosion accident will not only pose a serious threat to the safety of industrial processes, but also cause serious environmental pollution with excessive carbon emissions and greenhouse effect. Therefore, this paper has studied the inhibition effect of modified ABC powder driven by heptafluoropropane on methane explosion. Multiple methane explosion suppression experiments were carried out using a 3.2 L horizontal experimental pipeline under various working condition. The experimental results demonstrate that the modified ABC powder has a certain inhibitory effect on methane explosions, but the explosive suppression effect of the powder improves when heptafluoropropane is used as the driving gas. When 3∼4 % heptafluoropropane is used as the driving gas, the flame is obviously inhibited. The deformation degree of flame shape is the largest at 4 % concentration. The flame totally folds inward at this point, forming an uneven finger flame. When 4 % concentration of heptafluoropropane drives the powder, there is a critical value of the concentration of the powder, which is 0.06 g/L. According to the characterization results, the average particle size of the modified ABC powder is less than 5 µm. It has good dispersibility and pyrolysis effect and is not easy to agglomerate. Under the driving action of heptafluoropropane, the powder will form aerosol, which can fully contact and react with the flame. The inert gas around the powder will reduce the forward speed of the flame and ensure that the flame and powder can fully contact and react. The combination of the two has a good synergistic effect and can effectively inhibit methane explosion. [Display omitted] [ABSTRACT FROM AUTHOR]

Because of their adverse impact on social infrastructure and economic property, gas explosions are among the most dangerous disasters that can befall utility tunnels. Flame propagation is the most intuitive manifestation during the explosion process. It can be significantly influenced by spatial obstacles. To analyze the propagation law of gas explosions in utility tunnels more realistically, an explosion model of the utility tunnel was established by considering actual spatial obstacles, including the pipeline support and reserved frame. The flame propagation characteristics, including the flame shape, temperature distribution, and propagation velocity, were analyzed systematically. Results demonstrate that spatial obstacles have a prominent influence on flame evolution laws. Flame propagation is disturbed but not completely blocked by spatial obstacles. Rather, irregular flame shapes and wrinkles of the flame front are easily developed. The explosion process is insufficient because of the existence of residual methane in the obstacle area. Flame propagation velocity is improved remarkably, whereas temperature distribution is affected slightly. In particular, the spread velocity increases with the increase of propagation distance. The maximum velocity of monitoring surfaces can be improved by 60% under conditions with obstacles. [ABSTRACT FROM AUTHOR]

• Visualization of in-cylinder flow and flame using an optically accessible engine. • Characteristics of in-cylinder flow and flame in the catalyst-heating mode. • Effects of intake valve timing and duration on tumble and flame propagation speed. • Investigation of the amount of residual gas depending on exhaust valve timings. The effects of intake and exhaust valve timing and duration were investigated using an optically accessible multi-point injection engine under catalytic heating conditions. The target engine features a quartz cylinder and an extended piston with a quartz piston crown. To understand the effects of valve timing and duration on the in-cylinder flow, particle image velocimetry (PIV) was applied to visualize the in-cylinder flow. The acquired images were used to quantitatively evaluate the in-cylinder flow characteristics, and the effects of these flow characteristics on the flame propagation were analyzed through flame visualization experiments. The results indicate that when the intake valve timing was advanced by 20°, the average flow velocity was 59.5% higher, the tumble ratio was 72.2% greater, and the flame propagation velocity was also the fastest. The longest duration of 218° was advantageous for the intake valve duration, and the formation of a tumble flow pattern was important for fuel and air mixing. Finally, in the exhaust valve timing and duration experiment, the exhaust valve closing timing affected the amount of residual gas inside the cylinder. Therefore, the ratio of diffusion flames increased when the exhaust valve timing was advanced or retarded compared to the reference exhaust valve timing. [ABSTRACT FROM AUTHOR]

• Ignition and flame propagation of NH 3 /diesel dual fuel mode with high NH 3 energy ratio were studied. • The two combustion modes, multiple ignition and flame propagation, depend on the ammonia energy ratio. • Weakening the diesel mixing is beneficial for promoting the ignition of NH 3 /diesel dual fuel. Ammonia (NH 3) is considered as one of the most promising zero-carbon fuels for internal combustion engines. Based on diesel engine modification, The ammonia/diesel dual-fuel mode with port injection of ammonia and in-cylinder direct injection of diesel has been proven to be the most feasible strategy. In this paper, the effects of ignition and flame propagation of premixed ammonia ignited by direct-injected diesel were studied based on an optical rapid compression machine (RCM). The results show that the spray and distribution of diesel play the key role in the ignition and flame propagation of NH 3 /diesel dual fuels combustion. For total excess air ratio (λ T) of 1.0, the ignition delay of NH 3 /diesel with high energy ratio of ammonia (ER of 95 %) was much shorter than ER of 66 % which was mainly due to the low injection pressure of diesel. Under the same injection pressure, with the large nozzle diameter of diesel injector of 0.28 mm, the ignition delay of NH 3 /diesel will be shortened significantly. When the ER is low, the combustion process presents the characteristics similar to the multi-point compression combustion. When the ER rises to 83 %, the combustion process is mainly dominated by NH 3 premixed combustion, and diesel mainly plays the role of ignition. Reducing the background pressure and temperature to 4 MPa and 887 K, respectively, NH 3 is not able to be ignited by diesel. [ABSTRACT FROM AUTHOR]

To investigate the comprehensive effects of the blocking ratio and the relative position of obstacles on the continuous explosion characteristics of the methane-air mixture, a series of explosion experiments were conducted in a 1.2 m long experimental tube. Methane concentration in the two connecting tubes was maintained at 10–12 vol.%, with obstacle plates featuring varying blocking ratios (Br) installed in both sections. Experimental results indicate that variations in the position of the obstacles significantly influence the shape of flame propagation. When the obstacle is positioned in the forepart of the experimental tube, the flame shape evolves through three distinct stages: spherical flame, finger-shaped flame, and vortex flame. Both the maximum flame front speed and the maximum explosion overpressure (Pmax) increase with the increasing blocking ratio. Comparatively, when the obstacle is located in the latter part of the tube, the increased turbulence intensity of the flame leads to the formation of a 'cavity' downstream as the flame interacts with the obstacle. In this scenario, the obstacles have minimal impact on both the flame front speed (Vf) and the maximum explosion overpressure. The formation of a vortex flame is a direct consequence of the interaction between the flame and the vortex, with flame acceleration occurring as the flame transitions into turbulent combustion due to the influence of the vortex. [ABSTRACT FROM AUTHOR]

To solve the problem of spontaneous combustion and dust explosion during storage and transportation of lignite, this work investigates the efficacy of melamine cyanurate (MCA) and ammonium dihydrogen phosphate (ADP) dust inhibitors in suppressing flame propagation and reducing the minimum ignition temperature of lignite dust explosion. The experimental findings reveal that ADP demonstrated a superiority of 18.33 % and 26.14 % over MCA in terms of flame propagation height and velocity at a concentration of 15 %, respectively. By comparing the inhibition effect of the two inhibitors on the minimum ignition temperature of lignite dust, ADP reached the abrupt concentration at 5 % and MCA was 10 %. Combined with thermogravimetric analysis, MCA primarily targets gas-phase reactions and exerts limited influence on the surface structure of lignite dust, resulting in suboptimal inhibition, ADP operates by generating minute quantities of inert gas and solid products, thereby exhibiting superior inhibitory effects than MCA. [Display omitted] • Flame propagation of lignite dust was investigated with MCA and ADP. • Minimum ignition temperature of lignite dust was explored with MCA and ADP. • Thermogravimetric analysis of the MCA and ADP was given. • Suppression mechanisms of MCA and ADP for lignite were elucidated. [ABSTRACT FROM AUTHOR]

To obtain the ignition performance of an integrated inclined combustor, we perform an experimental study on the ignition performance of an inclined combustor under various ignition positions, inlet flow rates, and fuel air ratios (FARs). The experimental results reveal the following. 1) During ignition at I1, the inclined combustor presents the best ignition performance. 2) The forward propagation process of flame along the swirler's inclined direction easily realizes flame propagation, whereas the backward flame propagation process in the swirler's inclined direction is difficult to achieve; forward and backward flame propagations exhibit evident differences. 3) The diffusion propagation of swirl flames at the ignition head is the main means swirl flames are generated at the nonignition head. 4) During the ignition process, the combustion intensity increases with the increase in FAR and decreases with the increase in inlet flow rate. 5) The successful ignition time decreases with the increase in inlet flow rate and FAR. • Proposed an integrated inclined combustor technology. • Explored the difference between forward and reverse flame propagation in an integrated inclined combustor. • Revealed the effects of different ignition positions on ignition circumferential propagation performance. • Quantitative analysis of the effects of inlet flow rate and fuel gas ratio on flame dynamics. [ABSTRACT FROM AUTHOR]

This numerical study investigates the effect of in-cylinder flow, regulated by combustion chamber (CC) geometry and swirl ratio (SR), on the combustion and flame propagation characteristics of an ammonia/diesel dual-fuel engine. First, a 3-D computational model considering flame propagation was constructed by fitting the parameters of Gulder model for ammonia. Then, four different CCs, namely shallow depth CC (SCC), petal type CC (PTCC), toroidal CC (TCC), and toroidal re-entrant CC (TRCC), were designed based on the original CC (OCC). Simulations of the designed CCs were performed under different SRs. The results show that OCC, PTCC, and SCC exhibit weak swirl and strong reverse squish, while TCC and TRCC feature strong swirl and weak reverse squish. The strong swirl of TCC can significantly enhance the diffusion combustion stage, resulting in the highest indicated thermal efficiency of 44.2 % with an SR of 0.978. Increasing SR can further enhance the swirl and displace it towards the bowl, which enhances combustion in the bowl region and subsequently leads to a significant increase in the heat loss through the piston bowl. However, increasing SR also leads to a further weakening of the reverse squish, resulting in a shorter flame propagation distance in the squish region. • The Gulder model parameters for ammonia were obtained. • In-cylinder flow was manipulated by changing combustion chamber and swirl ratio. • TCC and TRCC shows stronger swirl and weaker reverse squish. • Increasing the swirl ratio enhances the swirl, but weakens the reverse squish. • TCC enhances premixed combustion as a result of strong swirl. [ABSTRACT FROM AUTHOR]

[Display omitted] • The in-cylinder flow of ORP engine shows consistency at different altitudes. • General decrease in the speed of flame propagation with altitude increasing is observed. • The fuel economy is less sensitive to altitudes than power output. • NO x emissions are reduced with increasing altitude. Opposed rotary piston (ORP) engines boast a theoretical power density that is double that of traditional four-stroke reciprocating engines. Additionally, the ORP engines are free of connecting-rod mechanisms, which contributes to the compactness and efficiency of ORP engines. This study explores the performance of ORP engine under variable altitudes using three-dimensional simulation. The findings revealed that the charging efficiency diminishes as the altitude increases. During the compression stroke, the vortex formed in the intake stroke progressively dissipates, forming a flow field suitable for combustion. The combustion rate decreases as the altitude rises. However, the intense vortex formation within the cylinder at an altitude of 1000 m obstructs the flame propagation process, resulting in slower flame propagation. When the ORP engine operates at 4000 m, the combustion is enhanced due to the rich mixture around the spark plug. Additionally, the post-combustion phenomenon emerged at an altitude of 5000 m. The reduced cycle charge is a contributing factor to the lower in-cylinder temperature and pressure at high altitudes. As the altitude increases by 1000 m, the power output decreases by approximately 11.8 %. In contrast, the fuel economy is less sensitive to altitudes than power output. Nitrogen oxides (NO x) emission decreases with increasing altitudes as a result of the drop of in-cylinder temperature. NO x and hydrocarbon (HC) factors approach zero when the ORP engine operates at an altitude of 5000 m. However, the emission of carbon monoxide (CO) increases due to the significant percentage of post-combustion. [ABSTRACT FROM AUTHOR]

Direct numerical simulations (DNS) with detailed chemical kinetics and chemical explosive mode analysis (CEMA) are performed to investigate the effect of operating pressure and hydrogen–methane blending on laminar wrinkled flames at reheat combustion conditions. Building upon previous three-dimensional DNS datasets of reheat hydrogen flames, the present work considers two-dimensional configurations that render computationally-feasible simulations of high-pressure combustion and significantly more complex hydrocarbon chemistry. A geometrically-simplified representation of the reheat combustor is adopted and the thermodynamic states considered in the simulations are carefully chosen to mimic gas turbine operation conditions, which are constrained to achieve a target flame position and flame temperature. The two-dimensional DNS results are first assessed against the available three-dimensional data and then analyzed to extract the main trends exhibited by the reheat combustion process with respect to the fraction of the fuel consumed by spontaneous ignition and flame propagation, for increasing pressures and hydrogen fractions. Due to significant differences in the characteristic features of the velocity and thermal boundary layers between the three-dimensional and the two-dimensional configurations, a different global flame shape is observed (V-shaped flame vs W-shape flame) together with a ∼ 10 % bias in the fraction of fuel consumed by spontaneous ignition. Crucially, results from the scaling studies reveal very clear trends with a significant decrease in the fuel consumption by spontaneous ignition for increasing pressures and hydrogen fractions, at the conditions investigated. Finally, this study highlights the important role that first-principle direct numerical simulations, even when conducted in computationally affordable two-dimensional simplified geometrical configurations, can play in obtaining detailed insights and quantitative trends useful for the characterization of complex reactive flows. Novelty and significance The reheat burners of the two-stage sequential gas turbine combustors are designed to stabilize flames primarily due to spontaneous ignition. However, prior numerical simulations revealed the presence of an additional assisted-ignition mode aided by recirculation zones. In this study, we used practically feasible two-dimensional direct numerical simulations of premixed hydrogen–air mixtures under lean conditions to quantify the roles of the two flame stabilization modes at different operating pressures. Achieving fundamental understanding of the effect of pressure and hydrogen fraction on flame stabilization is crucial to the development of two-stage sequential combustion and the present two-dimensional calculations provide important new insights. We show that at high pressures, both modes consume fuel to a similar extent. We also investigated the effect of methane blending on flame stabilization. This study also discusses how two-dimensional direct numerical simulations can be leveraged in the design optimization of reheat burners at low technology readiness levels. [ABSTRACT FROM AUTHOR]

• Methanol spray and combustion within the narrow-confined pre-chambers were fully transparent investigated for the first time. • The effects of injection strategy was studied in an actual-geometry optical pre-chamber to fill the lack of test data in turbulence jet ignition mechanism. • Multiple spray-wall impingements and spray tumble were experimentally conformed in pre-chamber. • Flame propagation through complex walls in confined space and hot jets formation along with Mach rings were discussed. Methanol spray impingement, mixing, flame propagation and jet evolution within the narrow-confined active pre-chamber (APC) with complex walls were experimentally observed by optical method for the first time. The effects of injection pressure and duration was investigated on an actual-geometry optical APC assembled in constant volume combustion chamber (CVC). Exploration on APC internal processes within static environments forms the basis for optimization of APC and analysis of jet formation. Mie scattering coupled with flame natural luminosity imaging were used to record spray mixing and combustion. The results show that in the APC with narrow-confined space, methanol spray undergoes three wall impingements, inducing a tumble that promotes mixing. High injection pressure enhances rapid spray tip penetration, while long injection duration slows methanol diffusion. In longer injection duration cases, the spray tip after the third impingement interferes with subsequent sprays. For higher injection pressure, this interference occurs in the APC cone section due to a larger spray cone angle, whereas for lower pressure, it happens in the APC duct section. Higher injection pressure also causes more spray to impact the APC bottom after the first wall impingement, hindering flame acceleration in the duct section. Irregular flame propagation is observed in the APC cone section due to wider space and uneven mixture conditions, while the convergent geometry from the APC throat to the duct section significantly accelerates flame propagation. The APC jet appears due to the pressure difference between the APC and the main chamber, with the brightness and length of the visible flame jet increasing with APC flame luminosity and pressure difference. A Mach ring structure is observed initially, but the jet velocity decreases as the Mach ring disappears during the APC inner flame extinguishing process. Through this study, the actual spraying and combustion process in APC is experimentally captured and revealed, which are an important contribution for APC optimization and development. [ABSTRACT FROM AUTHOR]

To investigate the effect of venting diameter and static activation pressure on the explosion venting behavior of hydrogen-air mixtures, tests were performed utilizing the explosion venting test platform. The pressure and flame propagation characteristics under different venting diameters (30–70 mm) and pressures (0.1–0.4 MPa) were experimentally examined, and numerical calculation was employed to analyze the explosion venting characteristics of hydrogen at a stoichiometric ratio. Compared to fuel-lean and fuel-rich hydrogen conditions, the first peak pressure within the container is maximal under the hydrogen-air mixtures at a stoichiometric ratio. Under stoichiometric and fuel-rich hydrogen-air mixtures, pressure peak structures inside the container are both bimodal. When D = 30 mm, as P stat is increased from 0.1 to 0.2 MPa, P max11 increases. However, when P stat exceeds 0.2 MPa, with the increase of P stat , P max11 remains at the level of 0.69 MPa, reaching the "venting bottleneck". The discovery of this phenomenon is crucial for the research and development of the venting device for hydrogen storage equipment. Numerical simulations indicate the formation of the second peak pressure inside the container is primarily attributed to the intense obstruction of venting by secondary explosions within the duct. • The pressure in the container appears "venting bottleneck" phenomenon. • The venting diameter affects the "venting bottleneck" phenomenon. • The pressure waves reflected trigger the peak pressures in the duct. [ABSTRACT FROM AUTHOR]

• The relationship between the explosion overpressure and flame evolution of ethanol aerosols has been elucidated. • The intensity of the ethanol aerosol explosion and the speed of flame propagation increase with the initial pressure. • When P0 ≤ 70 kPa, an orange-yellow transitional phase occurs during the development of the flame. Ethanol is one of the most common renewable fuels and chemical raw materials, and there is a risk of aerosol explosion disasters at all stages of its production, transport, storage, and use. This paper investigates the explosion overpressure and flame propagation characteristics of ethanol aerosols at different equivalence ratio under sub-atmospheric pressure conditions in a square closed vessel. The results show that the maximum explosion pressure (P max), the maximum rate of pressure rise ((dP/dt) max), and the explosion index (K a) of ethanol aerosol explosions increase with the equivalence ratio, peaking at an equivalence ratio of 1.45, reaching values of 0.92 MPa, 20.25 MPa/s, and 4.73 MPa·m/s, respectively. The variation in Pmax with equivalence ratio can be fitted by a cubic polynomial function. P max , (dP/dt) max , and K a all increase linearly with the initial pressure. The evolution speed and luminosity of the ethanol aerosol explosion flames are stronger in rich fuel conditions than in lean fuel conditions, and the explosion combustion time is also shorter in rich fuel conditions. Moreover, when the initial pressure is less than or equal to 70 kPa, the blue flame of the ethanol aerosol explosion is initially dim in color, displaying an orange-yellow transitional phase before turning into a dazzling white. The flame propagation speed undergoes four stages as displacement increases: rapid increase, high-speed maintenance, rapid decline, and oscillatory weakening. The findings provide experimental data and theoretical support for preventing and mitigating explosion disasters of alcohol-based fuel aerosols under sub-atmospheric pressure conditions. [ABSTRACT FROM AUTHOR]