Flame Acceleration Research Articles

Numerical studies on explosion hazards of vehicles using clean fuel in short vehicular tunnels

Short vehicular tunnels (SVT) with heavy traffic are commonly constructed in urban areas of densely populated cities of the Asia-Oceania Region. The introduction of vehicles using clean fuel poses new concerns on the fire safety of existing road tunnels. The explosion of a taxi using Liquefied Petroleum Gas (LPG) occurred in a garage, alarming the need for protecting existing road tunnels against hazards in using clean energy.In this paper, explosion hazards of an LPG taxi in a section of SVT were studied by using Computational Fluid Dynamics (CFD) Flame Acceleration Simulator (FLACS). The scenario of LPG leaking into the concealed space beneath the tunnel floor level of SVT through vents at the road side is simulated. A full tank of LPG would completely be released in about 231 s. The leaked LPG in the concealed space is ignited 19 s after the complete release, i.e. at t = 250 s. Predicted results on explosion pressure and temperature illustrate that appropriate protection measures should be provided to ensure the safety of SVT. Results also suggest that the existing fire safety codes for SVT should be revised by including an explosion analysis of LPG taxis.

Formation and characteristics of composite reaction – Shock clusters in narrow channels

High-speed schlieren visualizations show that a composite reaction-shock cluster structure is formed in the last flame acceleration stage prior to detonation transition for ethylene/oxygen mixture in a narrow channel. The composite structure is bounded by a normal shock at the leading edge of the structure, and series of parallel oblique shocks interweave with reaction front on the other end in the cluster. Propagating velocity of the reaction front at the inception of the cluster is ∼ 45–50% of Chapman-Jouguet detonation velocity of the mixture. Reaction front accelerates rapidly after the formation of the reaction-shock cluster, and run into detonation in tens of microseconds except for very lean mixtures. The angle between the parallel oblique shocks in the cluster and the side wall, defined as ω-angle, is found to be constant for a specific mixture as the reaction wave propagates. Dependence of ω-angle on mixture equivalence ratio and channel size are investigated in the study. Analysis shows that DDT distance is linearly proportional to ω-angle, and an empirical correlation is derived.

Effect of CO2 on self-acceleration properties of syngas/air turbulent premixed flame at different equivalence ratios

For studying the effect of carbon dioxide (CO2) on the self-accelerating properties of syngas/air mixture's turbulent premixed flame, an experimental study was carried out on 90%H2/10%CO/air mixture at the turbulence intensity of 1.31 m/s. The effects of various equivalence ratios (0.6, 0.8 and 1.0) and carbon dioxide contents (10%, 20% and 30%) on the flame evolution and propagation speed were studied. The effects of the CO2 contents and equivalence ratios on the self-acceleration properties in the flame propagation transition stage were evaluated. The effects of effective Lewis number and Zel'dovich number on the fractal excess and acceleration exponent were analyzed. The results showed that, as the CO2 contents increased, the development process of the flame slowed down, and the rate of growth of the flame's radius and the turbulent premixed flame propagation speed decreased at the same radius. The fractal excess and acceleration exponent increased with the increasing CO2 contents and decreased with the increasing equivalence ratio. Under different equivalence ratios and CO2 contents, the syngas/air premixed flame acceleration exponent was less than 1.5 in the transition stage.

Numerical analysis of hydrogen release, dispersion and combustion in a tunnel with fuel cell vehicles using all-speed CFD code GASFLOW-MPI

Hydrogen energy is expanding world-widely in recent years, while hydrogen safety issues have drawn considerable attention. It is widely accepted that accidental hydrogen release in an open-air environment will disperse quickly, hence not causing significant hydrogen hazards. A hydrogen hazard is more likely to occur when hydrogen is accidentally released in a confined place, i.e. parking garages and tunnels. Prediction the main accident process, including the hydrogen release, dispersion, and combustion, is important for hydrogen safety assessment, and ensuring the safety installations during accidents. Hence, a postulated accident scenario induced by the operation of Thermal Pressure Relief Device in a tunnel is analysed for hydrogen fuel cell vehicles with GASFLOW-MPI in this study. GASFLOW-MPI is a well validated parallel CFD code focusing on the transport, combustion, and detonation of hydrogen. It solves compressible Navier-Stokes equations with a powerful all-speed Arbitrary-Lagrangian-Eulerian (ALE) method; hence can cover both the non-compressible flow during the hydrogen release and dispersion phases, and the compressible flow during deflagration and detonation. In this study, a 3D model of real-scaled tunnel is modelled, firstly. Then the hydrogen dispersion in the tunnel is calculated to evaluate the risk of Flame acceleration and the Deflagration-Detonation Transient (DDT). The case with jet fire is analysed with assuming that the hydrogen is ignited right after being injected forming a jet fire in the tunnel, the consequence of this case is limited considering the small hydrogen inventory. The detonation in the tunnel is calculated by assuming a strong ignition at the top of the tunnel at an unfavourable time and location. The pressure loads are calculated to evaluate the consequence of the hazard. The analysis shows that the GASFLOW-MPI is applicable at a widely range for tunnel accidents, meanwhile, the safety issues related to tunnel accidents is worthy further study considering the complexity of tunnels.

Numerical simulation of the influence of pipe length on explosion flame propagation in open-ended and close-ended pipes.

The pipeline length exerts great influence on flame propagation characteristics, Realizable model and Premixed combustion model were used to study the influence of pipe length on propane-air explosion flame in open-ended and close-ended pipes. Using the numerical model verified by experiments, the changes of flame structure and flame propagation speed are studied. The result showed that the Realizable model was in good agreement with the experimental results. It also proved that the reflected wave produced a strong interference on the flame front, which promoted the formation of tulip flame. Besides, some obvious vortices were usually generated in the burned gas after the tulip flame formed, which will affect the flow field around the flame front and thus exert influence on the flame structure. The formation mechanism of tulip flame as well as the flame self-acceleration is different in open-ended and close-ended pipes. In close-ended pipes, the reflection wave at the pipe end and the reflection-induced countercurrent both promote the formation of tulip flame. As the flame propagates to the pipe end, the flame propagation is inhibited by the compression wave formed by the rapid expansion of combustion products under high temperature. While, in open-ended pipes, the turbulence induced by the opening at the pipe end is the main cause of tulip flame formation. The flame acceleration depends on the combustion reaction of unburned gas, so the velocity of flame propagation continues to increase. Generally, the maximum flame propagation velocity in the open-ended pipe is larger than that in the close-ended pipe.

Effects of inert gas jet on the transition from deflagration to detonation in a stoichiometric methane-oxygen mixture

Detonation is an energetic combustion mode augmenting high flow momentum and thermodynamic efficiency, it has been applied in detonation engines, such as pulse detonation engines (PDEs) and rotating detonation engines (RDEs), they have become potential aerospace propulsion equipment. Recently, fluidic jet-in-cross flow (JICF) has been demonstrated experimentally and numerically that can accelerate the deflagration-to-detonation transition (DDT) process. Nonetheless, most of previous studies focused on the jets using combustible mixture or oxygen, which may bring additional risk for turbulence-generated system in detonation engines. In this study, a more safe and controllable inert gas (i.e., Ar) is applied for JICF, experiments are carried out to investigate effects of argon jet as an enhancement method on promoting the DDT in a stoichiometric methane-oxygen mixture. The effects of local argon concentration, turbulence intensity and injection position on the DDT process are systematically examined. Two-dimensional numerical simulations are also performed to elucidate the details of the injection evolution. The experimental results show that turbulence generated by the argon injection can promote flame acceleration and the onset of detonation only in the fast deflagration regime. The enhancing effect is more prominent at higher turbulence intensity by increasing jet injection pressure and shorter injection time. Too long injection duration increases argon local concentration that leads to an adverse effect prohibiting the DDT occurrence. During the initial laminar flame acceleration, referred to as the slow deflagration regime, no enhancement by the argon jet on DDT can be observed. By looking numerically at the flow structure of the argon jet, the vortical features enhance the transport and mixing between reactants and products. The interaction between the reactive travelling wave and the jet structure further induces turbulence and thus accelerates the chemical reaction rate. With the time elapsed, the injected argon entrains largely and dilutes the ambient combustible mixture, and restrains the DDT. Furthermore, a novel dimensionless criterion and a characteristic parameter Turc are proposed, quantitatively analyzing the dominate mechanism in flame propagation and the initial stage of DDT as inert jet is introduced.

Effect of the plasma location on the deflagration-to-detonation transition of a hydrogen–air flame enhanced by nanosecond repetitively pulsed discharges

This work presents a method for using nanosecond repetitively pulsed (NRP) plasma discharges for accelerating a propagating flame such that the deflagration-to-detonation transition occurs. A strategy is developed for bringing the location of the plasma near the tube wall and, thus, reducing the presence of the electrodes in the combustion tube as well as presenting a configuration in which cooling of the electrodes is viable for practical applications. Time-of-flight measurements were used in combination with energy deposition measurements and high-speed OH*-chemiluminescence imagery to investigate the flame acceleration process. For stoichiometric hydrogen–air flames, successful transition to detonation was achieved by applying a burst of 110 pulses at 100 kHz, with energies as low as 10 mJ per pulse. This was also achieved when plasma discharges were applied in the vicinity of the wall. Two enhancement mechanisms for flame acceleration were identified. The essential role of shock–flame interaction was established as being the main mechanism for flame acceleration when the discharges are located near the wall. This work presents an effective alternative that allows for NRP discharges to be applied near the wall while successfully maintaining a promising success rate for detonation transition.

Flame acceleration in a narrow channel with flow compressibility and diverging or converging walls

We study the effects of non-parallel (diverging or converging) channel walls on flame propagation and acceleration in planar and cylindrical narrow channels, closed at the ignition end and open at the other, accounting for thermal expansion in both the zero Mach number and weakly compressible flow limits. For parallel channel walls, previous work has shown that thermal expansion induces an axial flow in the channel, which can significantly increase the propagation speed and acceleration of the flame. In this study, we consider examples of diverging/converging linear walls, although our asymptotic analysis is also valid for curved walls. The slope of the channel walls is chosen so that the magnitude of the thermal-expansion induced flow through the channel obtained for parallel walls is modified at leading-order, thereby influencing the leading-order flame propagation. For zero Mach number flows, the effect of the diverging/converging channel walls is moderate. However, for weakly compressible flows, the non-parallel walls directly affect the rate at which pressure diffuses through the channel, significantly inhibiting flame acceleration for diverging walls, whereas the flame acceleration process is enhanced for converging walls. We consider several values of the compressibility factor and channel wall slopes. We also show that the effect of a cylindrical channel geometry can act to significantly enhance flame acceleration relative to planar channels. The study reveals several new physical insights on how non-parallel channel walls can influence the ability of flames to accelerate by modifying the flow and pressure distribution induced by thermal expansion.

Experimental study on the effects of different fluidic jets on the acceleration of deflagration prior its transition to detonation

Fluidic jet-in-crossflow (JICF) is a reliable technique facilitating the flame acceleration and the transition from deflagration to detonation, most previous investigations on this project have focused on JICF using combustible mixtures but there are relatively few studies explored the mechanism of non-reactive gas JICF in the process of deflagration-to-detonation transition (DDT). In this study, three non-reactive gases (i.e., He, N2 and CO2) are considered as the medium of fluidic jets, the detailed mechanism of JICF enhancing deflagrations in stoichiometric methane-oxygen mixture is discussed. We experimentally demonstrate that JICF prominently enhances the propagation characteristics of deflagration, i.e., the 1st shock wave (1st SW), the 2nd shock wave (2nd SW), the combustion wave (CW) and the formation of precursor shock wave. Non-reactive gas JICF has concurrently positive turbulence-enhancement as well as negative inert gas-inhibition effects, which simultaneously influence the propagation behavior of deflagration. By adjusting the pressure difference (i.e., pJ0/p0) in JICF and test mixture, the performance of non-reactive gas jet on the propagation characteristics of deflagration is varied and mainly dependent on the attributes of the JICF. Our observation sheds light on the competing mechanism of JICF in the acceleration of a deflagration.

Effect of separation distance on gas dispersion and vapor cloud explosion in a storage tank farm determined using computational fluid dynamics

Storage tank separation distance, which considerably affects forestalling and mitigating accident consequences, is principally determined by thermal radiation modeling and meeting industry safety requirements. However, little is known about the influence of separation distance on gas dispersion or gas explosion, which are the most destructive types of accidents in industrial settings. This study evaluated the effect of separation distance on gas dispersion and vapor cloud explosion in a storage tank farm. Experiments were conducted using Flame Acceleration Simulator, an advanced computational fluid dynamics software program. Codes governing the design of separation distances in China and the United States were compared. A series of geometrical models of storage tanks with various separation distances were established. Overall, increasing separation distance led to a substantial reduction in vapor cloud volume and size in most cases. Notably, a 1.0 storage diameter separation distance appeared to be optimal. In terms of vapor cloud explosion, a greater separation distance had a marked effect on mitigating overpressure in gas explosions. Therefore, separation distance merited consideration in the design of storage tanks to prevent gas dispersion and explosion.

Propagation of Darrieus–Landau unstable laminar and turbulent expanding flames

The propagation of laminar and turbulent expanding flames subjected to Darrieus–Landau (DL), hydrodynamic instability was experimentally studied by employing stoichiometric H2/O2/N2 flames under quiescent and turbulent conditions performed in a newly developed medium-scale, fan-stirred combustion chamber. In quiescent environment, DL unstable laminar flame exhibits three-stage propagation, i.e. smooth expansion, transition acceleration, and self-similar acceleration. The self-similar acceleration is characterized by a power-law growth of acceleration exponent, α, with normalized Peclet number, which is different from the usually suggested self-similar propagation with a constant α. The imposed turbulence advances the onset of both transition acceleration and self-similar acceleration stages and promotes the strength of flame acceleration as additional wrinkles are invoked by turbulence eddies. A DL–turbulent interaction regime is confirmed to be the classical corrugated flamelets regime. Furthermore, the DL instability significantly facilitates the propagation of expanding flames in medium and even intense turbulence. The development of DL cells is not suppressed by turbulence eddies, and it needs to be considered in turbulent combustion.

Effect of diluent gases on end-gas autoignition and combustion modes in a confined space

Diluent gases are used in practical industrial applications that involve flame propagation and detonation. In this study, we investigated the effect of diluent gases on end-gas autoignition and detonation development in a constant volume combustion chamber. High-speed Schlieren photography was employed to capture the flame–shock wave interaction and detonation evolution. Two reactants (hydrogen and methane) and three diluent gases (argon, nitrogen, and carbon dioxide) were used for the process. In this study, statistical analysis confirmed that the ratio of maximum pressure to equilibrium pressure (Pmax/Pequ) can be a criterion to determine the occurrence of end-gas autoignition. In addition, it was observed that the mechanism of end-gas autoignition with detonation development was dominated by the flame propagation velocity and associated shock wave intensity in the present experimental conditions. With an increase in the flame propagation velocity, the combustion mode transitioned from normal combustion to autoignition Mode 1 (the end gas suffers two compressions by shock wave before autoignition), and to autoignition Mode 2 (the end gas suffers one compression by shock wave before autoignition), which was observed for different reactants and diluent gases. The diluent acted as an inhibitor for the chemical reaction and moderated the flame acceleration, and thus, restrained the end-gas autoignition and detonation intensity. CO2 is a better inhibitor of end-gas autoignition with high-pressure oscillation and detonation than N2 and Ar; CO2 can be employed to prevent knock or super-knock in boosted gasoline engine. Further, we discussed the basis of random location of the autoignition kernel, and the influence of the location on measuring the intensity and destructive capability of the detonation. Moreover, owing to the addition of carbon to the combustion, a blue–white light was observed after the propagation of the shock wave.

Suppression of hydrogen-air explosions by hydrofluorocarbons

This study examines the suppression effectiveness of fluorinated species CHF3 and C2HF5 on the premixed hydrogen-air explosions experimentally and numerically. The results demonstrate that as for stoichiometric and rich hydrogen-air mixtures, both CHF3 and C2HF5 can effectively reduce thermal expansion ratio and increase flame thickness, and then reduce the influence of hydrodynamic instability on the flame acceleration. Laminar burning velocity, the maximum of explosion pressure, the maximum rate of pressure rise and the absolute value of pressure impulse all decrease with increasing suppressant concentration at various equivalent ratios. The maximum of explosion pressures for stoichiometric flames is decreased by 11.54 % with 2 % CHF3 added, and decreased rapidly by 40.39 % with 2 % C2HF5 added. The hydrogen-rich mixtures cannot be ignited when C2HF5 content reaches 10 %. Numerical simulations have confirmed that both CHF3 and C2HF5 can effectively reduce the concentration of active radicals with the decrease order of H > OH > O. C2HF5 is more effective in suppressing explosions than CHF3 by converting more H atoms to HF formation through the elementary reactions CHF2 + H <=> CHF + HF, CF2:O + H <=> CF:O + HF, and CF2 + H = CF + HF.

Experimental and numerical study on the effect of low vent burst pressure on vented methane-air deflagrations

This paper experimentally investigated the effect of low vent burst pressure on the overpressure buildup and flame evolution during the vented methane-air deflagrations in a 1m3 rectangular vessel. CFD software FLACS was assessed in simulation of vented methane-air explosions against the experiment. Experimental results demonstrate that four overpressure peaks of Popen, Pout, Pext and Pacc from the vent failure, flame venting, external explosion and acoustically enhanced combustion, respectively, were observed except for the free venting case, in which only two overpressure peaks of Pout and Pext were monitored. The flame behaviors corresponding to different overpressure peaks were monitored especially at Pacc, where an abrupt increase in flame luminosity was observed on the bottom of the vessel due to the combustion of pockets of flammable mixture. Popen and Pacc increase with the vent burst pressure while Pout and Pext are nearly independent of the vent burst pressure. Double flame accelerations were observed owing to the vent rupture and flame venting. The phenomena of Helmholtz oscillation and the bulk motion of flame bubble always appeared after the vent rupture. The predicted overpressure agrees reasonably well with the experimental data. The discrepancy of the predicted and measured maximum peak overpressures reduces with increasing vent burst pressure. The predicted maximum peak overpressure and external flame length are slightly larger than those in experiment, which would facilitate the safe design in explosion venting and offer a better guidance in risk assessment. The simulation results also show that two overpressure peaks were recorded resulting from the pressure wave propagating above the vessel and the external explosion, respectively.

Flame propagation characteristics and overpressure prediction of unconfined gas deflagration

Shock wave and thermal radiation were two main hazards of gas cloud deflagration in the open space. Actually, the overpressure can’t be forecast easily, because the deflagration process was often affected by the gas cloud size, ignition energy, combustion mode of the cloud, flame speed. So, the prediction of the gas cloud deflagration overpressure in the open space was relatively difficult. In this study, a series of small-scale gas cloud explosions were made to investigate the deflagration characteristics, flame acceleration process and overpressure distribution. The gas cloud was restricted by different sizes of latex balloon and ignited by different energy sources. High-speed video camera was used to record the flame propagation process. Experimental results showed that the rupture of the balloon can strongly accelerate the flame speed for about 10 times which was about 18 m/s, but the flame speeds at different directions came into different values and the flame speed was higher when the acceleration distance was longer within certain limits. What’s more, when the gas cloud was ignited by a weak ignition the distribution of overpressures in the repeated experiments was not stable. Overpressure and maximum flame speed increase with the size of the gas cloud, but not linear dependence and can’t be used to predict large-scale gas explosion. A new overpressure prediction model was proposed, which was based on the maximum flame speed. This new model was feasible, but the maximum flame speed prediction model should be corrected basing more experimental data.

Multiscale modeling of transients in the shock-induced detonation of heterogeneous energetic solid fuels

We pursued a quantitative comparison of the continuum and mesoscale simulations of metal particle reactions in a condensed explosive. The continuum or homogeneous approximation assumed a single combustible mixture; contrastingly, in the present heterogeneous treatment of a mixture, aluminum and 1,3,5,7-tetranitro-1,3,5,7-tetrazocane were considered independent reactive granules of micron size with binder being inert material with strength. Both simulations similarly estimated the steady detonation velocity and peak pressure, while the early flame acceleration transients, marked by a complex flow structure behind the flame front, were observed only in the mesoscale simulation. The precise multi-interface processing techniques, combined with granule-level reactive flow models developed in this study, are deemed applicable to general heterogeneous solid fuel combustion studies related to granules of various size, shape, and reactivity.

Visual experimental investigation on initiation process of H2/air rotating detonation wave in plane-radial structure

Using visual observation methods, assisted by synchronous measurement with high-frequency pressure sensors and ion probes, the initiation process of the plane-radial rotating detonation combustor under different operating modes are revealed. Results show that, the H2/air rotating detonation wave (RDW) cannot be obtained directly by the low-energy spark plug owning to its low ignition energy. The RDW initiation processes under different modes are similar, which undergo the following processes: ignition, deflagration, chaotic turbulent flame acceleration (deflagration to detonation transition), unstable detonation, and stable rotating detonation. For multi-wave mode, the collision and mode switching mostly appear to occur in the initiation. The chaotic turbulent flame development process is further analyzed. The reactants’ continuous injection and accumulation to a certain height is an important prerequisite for RDW initiation and stable propagation. The radial propagation of turbulent flames and collision promote the formation of hot spots, and the development of hot spots in reactants layer is an essential condition for the formation of circumferential flame.

Large eddy simulation and experimental study of the effect of wire mesh on flame behaviours of methane/air explosions in a semi-confined pipe

Preventing the propagation of flames in a pipeline is an effective measure for avoiding gas explosion accidents and reducing losses. To evaluate the effect of wire mesh, acting as a porous media, experimental and simulation studies are conducted to determine the influence of the wire mesh on the dynamics of premixed methane/air flame propagation in a semi-closed pipe. Four different kinds of wire mesh with different numbers of layers are chosen in the experiments and simulation, and the mechanism of wire mesh quenching of the flame is investigated. The experimental and simulation results are consistent. Flames are quenched when 4 layers of 40-mesh or 3 layers of 60-mesh wire mesh are used; however, once the flame propagates through the wire mesh, the risk of methane combustion may increase. The wire mesh becomes the key factor causing flame folds and acceleration, and the greater the number of layers or the larger the mesh size is, the more obvious the folds after the flame passes through the wire mesh. Moreover, the combination of heat absorption and disruption of the continuous flame surface by the mesh causes flame quenching. Wire mesh can effectively attenuate the flame temperature during premixed flame propagation in a pipe, and the attenuated maximum rate reaches approximately 79% in the case of adding 3 layers of 60-mesh wire mesh.

Evaluation of different models for turbulent combustion of hydrogen-air mixtures. Large Eddy Simulation of a LOVA sequence with hydrogen deflagration in ITER Vacuum Vessel

• LES with TFM model was validated vs H2 experiments. • Validated model was used to simulate ITER VV LOVA sequence with H2 combustion. • In LOVA sequence burning speed increased 1–3.5 times due to flame acceleration. • Maximum average VV pressure was 4–5.5 times to the initial one. The modelling of premixed combustion in the presence of a turbulent field in three-dimensional (3-D) confined scenarios was studied in this work, and applied to hydrogen combustion within ITER vacuum vessel (VV). Two different combustion approaches were tested with Large Eddy Simulation: a Flamelet Progress Variable (LES-FPV) and a Thickened Flame Model (LES-TFM). For the case of LES-TFM modelling, Dynamic Adaptive Chemistry (DAC) with a detailed kinetic mechanism for hydrogen combustion and in-situ adaptive tabulation (ISAT) methods were employed. Moreover, an adaptive meshing technique was used with the aim of tracking the flame front to ensure an adequate spatial resolution in this region. Experimental validation was performed to assess the ability of the different studied approaches to predict the flame burning speed, flame acceleration, and pressure evolution for lean H 2 -Air volume percent mixtures from 16 to 28 % propagating within a turbulent field. Results revealed that both approaches led to accurate predictions in terms of flame burning speed. When considering DAC and ISAT methods with detailed chemistry, LES-TFM model was found to be a cost-efficient solution. This model was used to analyse two loss of vacuum accident (LOVA) sequences within ITER VV. Results showed that turbulence might increase the flame burning speed by a factor of up to 3.5 for the case of big breaches (0.15 m 2 ) but it would not affect in case of breaches of 0.02 m 2 . Besides, results showed that autoignition with 2 kg of H 2 within the VV at 13.35 kPa might degenerate in detonation with average wall pressure levels ∼70 kPa.

Effects of obstacle position and hydrogen volume fraction on premixed syngas-air flame acceleration

There have been many studies concerning on the influence of obstacles on gas explosion, but the understanding of some phenomena is still not clear, such as explosion overpressure or the change of flame velocity with the position of obstacles. This work aims to figure out the coupling effects of obstacle position (L1, L2, L3 from near to far) and hydrogen volume fraction (φ) on syngas-air flame acceleration and overpressure variation in a closed duct. Detailed flame structure evolution in the acceleration process was visualized by a high-speed camera, and overpressure was recorded by pressure transducers. Results show that the effect of obstacles moving backward is not always monotonous on the flame acceleration, which affects the maximum flame speed and maximum overpressure. Such as when φ = 0.5, the flame acceleration that is reflected as the flame propagation speed normalized by the laminar flame speed, ranks as L2 > L3 > L1. Because the obstacles strengthened turbulence on the tulip- or distorted tulip-shaped flame in L2. While when φ = 0.1, the flame acceleration ranks as L1 > L2 > L3 since the effect of turbulence is reduced. It is found that maximum overpressure is related to both flame acceleration and local overpressure. Local overpressure is the transient overpressure before the flame front touches obstacles. Obstacles moving backward is beneficial for local overpressure accumulation. Hence, the maximum overpressure increases with obstacle position moving backward when φ ≥ 0.1. This study clarifies the role of obstacle position and hydrogen volume fraction on flame acceleration, which may help to guide the application of syngas in a complex environment.