Effect of fluidic obstacles on flame acceleration and DDT process in a hydrogen-air mixture

Using solid obstacles to accelerate the deflagration to detonation transition (DDT) process induces additional thrust loss, and fluidic obstacles can alleviate this problem to a certain extent. A detailed simulation is conducted to investigate the effects of multiple groups of fluidic obstacles on the flame acceleration and DDT process under different initial velocities and gas types. The results show that, initially, the propagation of reflected shock wave formed by jet impingement is opposite to the flame acceleration direction, thus increasing the initial jet velocity will hinder the flame acceleration. Later, the vortex structure and enhanced turbulence can promote flame acceleration. As the flame accelerates, the virtual blockage ratio of the fluidic obstacles decreases, and increasing initial jet velocity or using reactive jet gases both affect the virtual blockage ratio. Further, increasing initial jet velocity or using reactive jet gases can shorten the detonation initiation time and distance. Compared with solid obstacles, it is concluded that fluidic obstacles can achieve faster detonation initiation with a smaller blockage ratio. Overall, the detonation phenomena in this study are all triggered by hot spots formed by the interaction between reflected waves and distorted flame, but the formation of reflected waves varies.     

Effect of solid obstacle distribution on flame acceleration and DDT in obstructed channels filled with hydrogen-air mixture

Ensuring hydrogen safety has become of great significance nowadays, whose leakage can possible result in the deflagration to detonation (DDT). The numerical study aims to explore the effect of solid obstacle distribution on the DDT in a homogeneous hydrogen-air mixture. Results show that the detonation initiation process can be classified into two types: i) local spherical detonation caused by the coupling of flame surface and high-pressure region; ii) detonation triggered by the interaction between the upper wall and multiple compression waves in front of the flame. Also, this study finds that the flame acceleration (FA) experiences two periodic “acceleration-deceleration” processes before the detonation initiation, and the initiation distance and time are the shortest when the obstacles are symmetrically distributed. Further, the higher the unilateral blockage ratio, the more unfavorable DDT occurs. The present results highlight the effect of different solid obstacle distribution patterns on the FA and DDT process.     

Influence of longitudinal obstacle spacing on the deflagration-to-detonation transition through a bank of obstacles in a hydrogen-air mixture

Flame acceleration and deflagration-to-detonation transition (DDT) in a channel containing an array of staggered cylindrical obstacles and a stoichiometric hydrogen-air mixture were studied by solving the fully-compressible reactive Navier-Stokes equations using a high-order numerical algorithm and adaptive mesh refinement. Four different longitudinal spacings (ls) of the neighboring obstacle rows (i.e., ls = 15.28, 19.1, 25.4, and 38.2 mm, corresponding to 1.2, 1.5, 2 and 3 times of obstacle diameter, respectively) were used to examine the effect of obstacle spacing on flame acceleration and DDT. The results show that the main mechanisms of flame acceleration and transition to detonation in all the cases studied are consistent. While the flame acceleration is caused by the growth of flame surface area in the initial stage, it is governed by shock-flame interactions in the later stage when shock waves are generated. The focusing of strong shocks at flame front is responsible for the initiation of detonation. It was found that the flame propagation speed and the DDT run-up distance and time are highly dependent on ls. Specifically, the flame acceleration declines as ls increases, since a larger ls leads to less disturbance of flow by obstacles per unit channel length. For detonation initiation, both the run-up distance and time increase with the increase of ls. It is interesting to note that the DDT distance and time increase significantly as ls increases from 19.1 mm to 25.4 mm. This is related to the slowdown of the increase rate of energy release over a period before DDT occurs under large ls condition, because every time the flame passes over an obstacle row the shock-flame interaction is delayed and numerous isolated pockets of unburned gas material are formed.     

The role of shock-flame interactions on flame acceleration in an obstacle laden channel

Flame acceleration was investigated in an obstructed, square-cross-section channel. Flame acceleration was promoted by an array of top and bottom surface mounted obstacles that were distributed along the entire channel length at an equal spacing corresponding to one channel height. This work is based on a previous investigation of the effects of blockage ratio on the early stage of flame acceleration. This study is focused on the later stage of flame acceleration when compression waves, and eventually a shock wave, form ahead of the flame. The objective of the study is to investigate the effect of obstacle blockage on the rate of flame acceleration and on the final quasi-steady flame-tip velocity. Schlieren photography was used to track the development of the shock-flame complex. It was determined that the interaction between the flame front and the reflected shock waves produced from contact of the lead shock wave with the channel top, channel bottom, and obstacle surfaces govern the late stage of flame acceleration process. The shock-flame interactions produce oscillations in the flame-tip velocity similar to that observed in the early stage of flame acceleration, but only much larger in magnitude. Eventually the flame achieves a globally quasi-steady velocity. For the lowest blockage obstacles, the velocity approaches the speed of sound of the combustion products. The final quasi-steady flame velocity was lower in tests with the higher obstacle blockage. In the quasi-steady propagation regime with the lowest blockage obstacles, burning pockets of gas extended only a few obstacles back from the flame-tip, whereas burning pockets were observed further back in tests with the higher obstacle blockage.     

Effect of multi-bend geometry on deflagration to detonation transition of a hydrocarbon-air mixture in tubes

We present a numerical investigation of gaseous deflagration-to-detonation transition (DDT) triggered by a shock in a multi-bend geometry. The ethylene-air mixture filled rigid tube with obstacles is considered for understanding the effects of complex confinement and initial flame size on DDT. Our calculations show generation of hot spots by flame and strong shock interactions, and flame propagation is either restrained or accelerated due to the wall obstacles of both straight and bent tubes. The effect of initial flame size on DDT in complex confinement geometry is analyzed as well as the hot spot formation on promoting shock–flame interaction, leading to a full detonation.     

Fast turbulent deflagration and DDT of hydrogen–air mixtures in small obstructed channel

An experimental study of flame propagation, acceleration and transition to detonation in hydrogen–air mixture in 2-m long rectangular cross-section channel filled with obstacles located at the bottom wall was performed. The initial conditions of the hydrogen–air mixture were 0.1 MPa and 293 K. Three different cases of obstacle height (blockage ratio 0.25, 0.5 and 0.75) and four cases of obstacle density were studied with the channel height equal to 0.08 m. The channel width was 0.11 m in all experiments. The propagation of flame and pressure waves was monitored by four pressure transducers and four in-house ion probes. The pairs of transducers and probes were placed at various locations along the channel in order to get information about the progress of the phenomena along the channel. To examine the influence of mixture composition on flame propagation and DDT, the experiments were performed for the compositions of 20%, 25% and 29.6% of H₂ in air by volume. As a result of the experiments the deflagration and detonation regimes and velocities of flame propagation in the obstructed channel were determined.     

Flame acceleration and transition from deflagration to detonation in hydrogen explosions

Computational Fluid Dynamics solvers are developed for explosion modelling and hazards analysis in Hydrogen air mixtures. The work is presented in two parts. These include firstly a numerical approach to simulate flame acceleration and deflagration to detonation transition (DDT) in hydrogen–air mixture and the second part presents comparisons between two approaches to detonation modelling. The detonation models are coded and the predictions in identical scenarios are compared. The DDT model which is presented here solves fully compressible, multidimensional, transient, reactive Navier–Stokes equations with a chemical reaction mechanism for different stages of flame propagation and acceleration from a laminar flame to a highly turbulent flame and subsequent transition from deflagration to detonation. The model has been used to simulate flame acceleration (FA) and DDT in a 2-D symmetric rectangular channel with 0.04 m height and 1 m length which is filled with obstacles. Comparison has been made between the predictions using a 21-step detailed chemistry as well as a single step reaction mechanism. The effect of initial temperature on the run-up distances to DDT has also been investigated.     

阻塞比对氢-空预混火焰加速影响的可视化研究

在顶置点火定容燃烧弹内布置网孔板,通过火焰传播路径上的孔板诱导实现火焰加速。利用纹影法和压力采集系统,研究了阻塞比对氢-空预混气孔板诱导火焰加速的影响规律。试验结果表明:阻塞比的增加可增大孔板对火焰的扰动,使火焰传播速度大幅增加;任意初始工况下均存在一个最佳阻塞比使燃烧持续期达到最短,在试验范围内,较低初始压力、较小当量比和较高初始温度的最佳阻塞比为0.90,其余工况的最佳阻塞比均在0.84;孔板诱导燃烧加速的效果非常显著,在试验工况范围内,各阻塞比孔板诱导下的燃烧持续期均缩短45%以上。     

Numerical simulation and validation of flame acceleration and DDT in hydrogen air mixtures

Combustion of hydrogen can take place in different modes such as laminar flames, slow and fast deflagrations and detonations. As these modes have widely varying propagation mechanisms, modeling the transition from one to the other presents a challenging task. This involves implementation of different sub-models and methods for turbulence-chemistry interaction, flame acceleration and shock propagation. In the present work, a unified numerical framework based on OpenFOAM has been evolved to simulate such phenomena with a specific emphasis on the Deflagration to Detonation Transition (DDT) in hydrogen-air mixtures. The approach is primarily based on the transport equation for the reaction progress variable. Different sub-models have been implemented to capture turbulence chemistry interaction and heat release due to autoignition. The choice of sub-models has been decided based on its applicability to lean hydrogen mixtures at high pressures and is relevant in the context of the present study. Simulations have been carried out in a two dimensional rectangular channel based on the GraVent experimental facility. Numerical results obtained from the simulations have been validated with the experimental data. Specific focus has been placed on identifying the flame propagation mechanisms in smooth and obstructed channels with stratified initial distribution. In a smooth channel with stratified distribution, it is observed that the flame surface area increases along the propagation direction, thereby enhancing the energy release rate and is identified to be the key parameter leading to strong flame acceleration. When obstacles are introduced, the increase in burning rate due to turbulence induced by the obstacles is partly negated by the hindrance to the unburned gases feeding the flame. The net effect of these competing factors leads to higher flame acceleration and propagation mechanism is identified to be in the fast deflagration regime. Further analysis shows that several pressure pulses and shock complexes are formed in the obstacle section. The ensuing decoupled shock-flame interaction augments the flame speed until the flame coalesces with a strong shock ahead of it and propagates as a single unit. At this point, a sharp increase in propagation speed is observed thus completing the DDT process. Subsequent propagation takes place at a uniform speed into the unburned mixture.     

Numerical simulation of flame acceleration and deflagration to detonation transition in hydrogen-air mixture

A numerical approach has been developed to simulate flame acceleration and deflagration to detonation transition in hydrogen-air mixture. Fully compressible, multidimensional, transient, reactive Navier–Stokes equations are solved with a chemical reaction mechanism which is tuned to simulate different stages of flame propagation and acceleration from a laminar flame to a turbulent flame and subsequent transition from deflagration to detonation. Since the numerical approach must simulate both deflagrations and detonations correctly, it is initially tested to verify the accuracy of the predicted flame temperature and velocity as well as detonation pressure, velocity and cell size. The model is then used to simulate flame acceleration (FA) and transition from deflagration to detonation (DDT) in a 2-D rectangular channel with 0.08 m height and 2 m length which is filled with obstacles to reproduce the experimental results of Teodorczyk et al.The simulations are carried out using two different initial ignition strengths to investigate the effects and the results are evaluated against the observations and measurements of Teodorczyk et al.     

管内爆燃转爆轰的热力学原理

本文首先说明了燃烧的两种传播机制，一种是燃烧自身的蔓延，国一种是由运动速度比火焰传播速度快的点火源引导而形成的传播，进而指出了可能存在的两种不稳定燃烧状态和两种极端物理过程的爆轰波，一种不稳定燃烧状态由爆燃加速到超过临界速度而致，另一种不稳定燃烧状态则由激波诱导燃烧引起，并采用简化理论计算了燃烧产物的压力和熵增随燃烧度的变化规律。由此出发，本文试图从热力学角度说明管内火中速及爆燃转爆轰的原理。爆燃     

Experimental study of flame propagation across flexible obstacles in a square cross-section channel

A comparative study was performed to investigate flame propagation in a square-cross section channel filled with either flexible- or rigid-obstacles with blockage ratio (BR) of 0.429. Experiments were conducted in premixed hydrogen-air mixtures with different equivalence ratios, at initial conditions of 100 kPa and 298 K. High-speed Schlieren photography was used to obtain the detailed flow structure, flame front evolution and the flame tip velocity. Also, pressure transducers were employed to monitor the pressure around the obstacles. Flame propagation across the obstacles was found to be strongly affected by flow contraction induced by obstacles and separated flow pattern downstream of obstacles. Flame propagation with rigid obstacles is mainly governed by the turbulent burning of the fresh gas in the pockets. For the flexible cases, the flow structure is characterized by the shear layer coming off the obstacles leading-corner and the vortex downstream from the obstacles. These special flow structures together provide a flow contraction and constrict flame propagation in the obstacles-free channel, and therefore the flame maintains acceleration. Most notable, the gas flow ahead of the flame purges the flexible obstacle to tilt, yielding an increase in BR, which is correlated with the stronger acceleration as the flame propagates through the obstacles. However, exposing the obstacles to the overpressure for a long period also induces too much deformation. Therefore, the instantaneous BR (BR_real) will also decelerate slightly. Interestingly, BR_real is closely related with the overpressure level.     

Effects of porous materials with different thickness and obstacle layout on methane/hydrogen mixture explosion with low hydrogen ratio

The effects of porous materials with different thickness and obstacle layout on the explosion of 10%H₂/90%CH₄ at stoichiometric condition were studied. Three kinds of porous materials with different thickness were selected in the experiment, which are 1 cm, 2 cm and 3 cm respectively. Three kinds of obstacle layout were designed, which are symmetrical distribution, ipsilateral distribution and staggered distribution. Results show that porous materials with different thickness can promote or inhibit the explosion flame and overpressure when the obstacles are symmetrically distributed. The quenching failure of 1 cm thick porous material is similar to the action of mesh obstacles, which accelerates the flame to break through the bondage of porous material and continue to propagate, with a maximum speed of 87.74 m/s. When the thickness of porous material is 2 cm and 3 cm, the solid structure increases, the energy absorption increases, the flame impact porous materials quench, and the overpressure peak decreases. The greater the thickness of porous material, the better the attenuation effect, and the maximum overpressure attenuation can reach 59.71%. The change of obstacle layout has an important impact on the flame propagation structure. Compared with the ipsilateral distribution and staggered distribution, when the obstacles are symmetrically distributed, the vortex dynamic induced flame turbulence area is larger, the flame combustion rate is increased, and the explosion hazard is greater.     

Flame propagation regimes and critical conditions for flame acceleration and detonation transition for hydrogen-air mixtures at cryogenic temperatures

A series of more than 100 experiments with hydrogen-air mixtures have been performed at cryogenic temperatures from 90 to 130 K and ambient pressure. A wide range of hydrogen concentrations from 8 to 60%H2 in a shock tube of 5-m long and 54 mm id was tested. Flame propagation regimes were investigated for all hydrogen compositions at three different blockage ratios 0, 30% and 60% as a function of initial temperature. Piezoelectric pressure sensors and InGaAs photo-diodes have been applied to monitor the flame and shock propagation velocity of the combustion process. More than 150 experiments at ambient pressure and temperature were conducted as the reference data for cryogenic experiments. The critical expansion ratio σ1 for an effective flame acceleration to the speed of sound was experimentally found at cryogenic temperatures. The detonability criteria for smooth and obstructed channels were used to evaluate the detonation cell sizes at cryogenic temperatures as well. The main peculiarities of cryogenic combustion with respect to the safety assessment were that the maximum combustion pressure was several times higher and the run-up-distance to detonation was two times shorter compared to ambient temperature independent of lower chemical reactivity at cryogenic conditions.     

High speed turbulent deflagrations and transition to detonation in H2air mixtures

An experimental investigation on flame acceleration and transition to detonation in H₂air mixtures has been carried out in a tube which had a 5 cm cross-sectional diameter and was 11 m long. Obstacles in the form of a spiral coil (6 mm diameter tubing, pitch 5 cm, blockage ratio BR = 0.44) and repeated orifice plates spaced 5 cm apart with blockage ratios of BR = 0.44 and 0.6 were used. The obstacle section was 3 m long. The compositional range of H₂ in air extended from 10 to 45%, the initial pressure of the experiment was 1 atm, and the mixture was at room temperature. The results indicate that steady-state flame (or detonation) speeds are attained over a flame travel of 10–40 tube diameters. For H₂ ? 13% maximum flame speeds are subsonic, typically below 200 m/s. A sharp transition occurs at about 13% H₂ when the flame speed reaches supersonic values. A second transition to the so-called quasi-detonation regime occurs near the stoichiometric composition when the flame speed reaches a critical value of the order of 800 m/s. The maximum value of the averaged pressure is found to be between the normal C-J detonation pressure and the constant volume explosion value. Of particular interest is the observation that at a critical composition of about 17% H₂ transition to normal C-J detonation occurs when the flame exits into the smooth obstacle-free portion of the tube. For compositions below 17% H₂, the high speed turbulent deflagration is observed to decay in this portion of the tube. The detonation cell size for 17% H₂ is about 150 mm and corresponds closely to the value of πD that has been proposed to designate the onset of single-head spinning detonation, in this case for the 5 cm diameter tube used. This supports the limit criterion, namely, that for confined detonations in tubes, the onset of single-head spin gives the limiting composition for stable propagation of a detonation wave.     

The flame propagation characteristics and detonation parameters of ammonia/oxygen in a large-scale horizontal tube: As a carbon-free fuel and hydrogen-energy carrier

As a carbon-free fuel and a hydrogen-energy carrier, ammonia is a potential candidate for future energy utilization. Therefore, in order to promote the application of ammonia in detonation engines and to evaluate the safety of ammonia related industrial process, DDT experiments for ammonia/oxygen mixtures with different ERs were carried out in a large-scale horizontal tube. Moreover, pressure transducers and self-developed temperature sensors were applied to record the overpressure and the instantaneous flame temperature during DDT process. The results show that the DDT process in ammonia/oxygen mixtures contains four stages: Slow propagation stage, Flame and pressure wave acceleration stage, Fast propagation and detonation wave formation stage, Detonation wave self-sustained propagation stage. For stoichiometric ammonia/oxygen mixtures, flame front and the leading shock wave propagate one after another with different velocity, until they closely coupled and propagated together with one steady velocity. At the same time, it is found that an interesting retonation wave propagates backward. The peak overpressure, detonation velocity, and flame temperature of the self-sustained detonation are 2 MPa, 2000 m/s and 3500 K, respectively. With the ER increased from 0.6 to 1.6, the detonation velocities and peak overpressures ranged from 2310 m/s to 2480 m/s and 25.6 bar–28.7 bar, respectively. In addition, the detonation parameters of ammonia were compared with those of methane and hydrogen to evaluate the detonation performance and destructiveness of ammonia.     

Visualization of the unburned gas flow field ahead of an accelerating flame in an obstructed square channel

The effect of blockage ratio on the early phase of the flame acceleration process was investigated in an obstructed square cross-section channel. Flame acceleration was promoted by an array of top-and bottom-surface mounted obstacles that were distributed along the entire channel length at an equal spacing corresponding to one channel height. It was determined that flame acceleration is more pronounced for higher blockage obstacles during the initial stage of flame acceleration up to a flame velocity below the speed of sound of the reactants. The progression of the flame shape and flame area was determined by constructing a series of three-dimensional flame surface models using synchronized orthogonal schlieren images. A novel schlieren based photographic technique was used to visualize the unburned gas flow field ahead of the flame front. A small amount of helium gas is injected into the channel before ignition, and the evolution of the helium diluted unburned gas pocket is tracked simultaneously with the flame front. Using this technique the formation of a vortex downstream of each obstacle was observed. The size of the vortex increases with time until it reaches the channel wall and completely spans the distance between adjacent obstacles. A shear layer develops separating the core flow from the recirculation zone between the obstacles. The evolution of oscillations in centerline flame velocity is discussed in the context of the development of these flow structures in the unburned gas.     

Numerical investigation of the mechanism behind the deflagration to detonation transition in homogeneous and inhomogeneous mixtures of H2-air in an obstructed channel

The accidental release of hydrogen into enclosures can result in a flammable mixture with concentration gradients and possible deflagration-to-detonation transition (DDT). This numerical study aims to investigate the effect of obstacle spacing and mixture concentration on the DDT in a homogeneous and inhomogeneous hydrogen-air mixture. The paper focuses on the mechanisms behind the DDT in two mixtures with an average hydrogen concentration of 15% and 30%. Unlike the near-stoichiometric mixture, in the lean mixture, DDT only occurs in the inhomogeneous mixture. Depending on obstacle spacing, three different regimes of DDT were observed in the near-stoichiometric inhomogeneous mixture: i) Detonation was ignited when a strong Mach stem formed and propagated between the obstacles; ii) two explosion centers appeared when incident shock and Mach stem reflected from upper and lower obstacles, respectively; iii) Mach stem did not form but DDT occurred behind the flame front at the top of the obstacle.     

Characteristics of gasoline–air mixture explosions with different obstacle configurations

The effects of obstacle distance from ignition point, the blockage ratio of obstacle (BR) and the separation distance of obstacles on the characteristics of gasoline–air mixture explosions have been examined by a series of contrast experiments in a semi-confined organic glass pipe (with a square cross section size of 100 mm*100 mm and 1000 mm long, L/D = 10, V = 0.01 m³). It was shown that before the flame fronts propagated to the obstacle, the flame fronts remained regular shape and spread in a low speed, while passed across the obstacle, the flame fronts could be sharply accelerated and became distorted. And it was obvious that the shorter the distance between obstacle and ignition point, the earlier the flame was accelerated, and eventually led to a higher maximum flame speed. Meanwhile, the maximum overpressures and maximum rates of overpressure rise were obtained at L_i = 400 mm, and the shorter the distance between the obstacle and ignition point, the shorter the time taken to reach the maximum overpressure. Three kinds of blockage ratios (BR = 36.4%, 49.8%, 71.7%) were tested, and it was found that the maximum flame speeds, maximum overpressures, average rates of overpressure rise and maximum rates of overpressure rise increased with the growth of blockage ratio. It was also discovered that the maximum effect of the combined obstacles on flame acceleration behavior could be obtained at an obstacle separation distance of 1 time to 4 times the length of pipe diameter. And the time taken to obtain the maximum overpressures became shorter with the growth of the obstacle separation distance, while the maximum overpressures and maximum rates of overpressure rise were obtained at an obstacle separation range from D_i/D = 3 to 5 (or 300 mm–500 mm).     

Effect of obstacle arrangement on premixed hydrogen flame: Eddy-dissipation concept model based numerical simulation

In this paper, computational fluid dynamics (CFD) numerical simulation is used to analyze and discuss the horizontal propagation process of premixed hydrogen flame with obstacles. A total of three different obstacle channel arrangements at the blocking ratio of 0.5, which will affect the explosion flame and pressure development. The results show that the premixed flame is affected by flow instabilities and vortices when propagating through the obstacle channel, thereby distorting the flame. The vortices outside the flame boundary are more conducive to the acceleration of the flame. The continuous acceleration and synergistic promotion of the flame is more prominent due to the existence of the channel in the central axis of flame propagation, and the maximum velocity even achieved 307.91  m/s. The degree of the wrinkle of flame increases with the number of obstacle channels. The flame propagation process is always accompanied by pressure variations, and the dynamic pressure builds up at the flame front and intensifies periodically. But the downstream pressure gradually increases as the number of obstacle channels increases. CFD simulation of the explosion process clearly reveals the changing trends and interactions of explosion characteristic factors.