Modeling the initial flame acceleration in an obstructed channel using large eddy simulation

The propagation and acceleration of a flame surface past obstructions in a closed square channel was investigated using large eddy simulation. The dynamic Smagorinsky–Lilly subgrid model and the Boger flame surface density combustion model were used. The geometry is essentially two-dimensional with fence-type obstacles distributed on the top and bottom surfaces, equally spaced along the channel length at the channel height. Flame propagation, however, is three dimensional as ignition occurs at a point at the center of the channel cross-section. The effect of obstacle blockage ratio on the development of the flame structure was investigated by varying the obstacle height. Three-dimensional cases were simulated from the initiation of a combustion kernel through spark ignition to the acceleration of the flame front at speeds up to 80 m/s. The transition from laminar flame propagation to turbulent flame propagation within the “thin reaction zone” regime was observed in the simulations. By analyzing the development of the three dimensional flame surface and unburned gas flow field, the formation of several flame structures observed experimentally are explained. Global quantities such as the total flame area and centerline flame velocity were ascertained and compared to the experimental data. High amplitude oscillations in the centerline flame velocity were found to occur from a combination of the unburned gas flow field and fluctuations in the volumetric burning rate.     

楔形障碍物与火焰的作用

采用多镜头Cranz ＆ Schardin高速摄影机,对卧式燃烧方管内楔形障碍物与CH4/空气预混火焰的相互作用进行了实验研究,获得了高清淅度的障碍物诱导火焰失稳的分幅时序照片.基于RANS方法,对上述复杂物理现象进行了数值模拟.计算结果与实验结果基本相符,反映了火焰在管内传播与变形的详细过程,得到了楔形障碍物所导致的火焰加速与变形的内在机理,揭示了火焰传播过程中由层流燃烧向湍流燃烧转捩的本质.     

有障碍物半开口管道内氢气燃烧数值模拟研究

基于有障碍物氢气燃烧实验装置进行数值模拟研究,采用Fluent软件分析了半开口管道内障碍物对氢气/空气燃烧特性的影响。结果表明:障碍物会促进实验管段内氢气火焰加速,随着障碍物阻塞率和数量的增加,火焰加速更快且燃烧压力峰值更大;在相同阻塞率下,障碍物形状对氢气火焰速度和燃烧压力峰值的影响很小;燃烧压力随障碍物间距的增大先增大后减小,障碍物间距为3倍管道内径时产生的燃烧压力峰值最大。     

Detonation transition criteria from the interaction of supersonic shock-flame complexes with different shaped obstacles

The present study is an experimental investigation of the last stages of the deflagration-to-detonation transition. A fast flame following a lead shock was generated by passing a detonation wave through a perforated plate. The shock flame complex then interacts with an obstacle of different shape. We study the influence of the obstacle shape on the transition mechanism to a detonation. The obstacles studied are a single round or square obstacle, a flat plate, a C-shaped and an H-shaped obstacle. The experiments were performed in a thin transparent channel permitting high speed schlieren visualization. Stoichiometric propane-oxygen was investigated at sub-atmospheric conditions. For each obstacle configuration, the initial pressure was changed to modify the flame burning velocity and the Mach number of the leading shock. The burning velocity prior to the interaction was measured experimentally from the displacement velocity of the flame in the videos. This required estimating the speed of the gas ahead of the flame. A linear correction to the speed immediately behind the lead shock was applied using the shock change equations and the measured pressure gradient behind the lead shock in order to account for the non-steadiness of the lead shock and viscous losses to the walls. Three main findings were that the obstacle shape had a minimal influence on the critical flame strength required for transition, although obstacles with a forward facing cavity were able to suppress the transition by isolating the re-initiation event inside the cavity. The main transition mechanism for all geometries was the enhancement of the flame burning velocity through the flame interaction with the shock reflected on the obstacle leading to Richtmyer-Meshkov instability. Finally, it was found that the flame burning velocity of the initial flame required for transition was closely approximated by the Chapman-Jouguet burning velocity. Consistent with the visual observations, this supports the view that transition is favored when the flame is in phase with the acoustic waves, and strong internal pressure waves can be amplified.     

Experimental and LES investigation of premixed methane/air flame propagating in a chamber for three obstacle BR configurations

The paper aims at revealing the effect of blockage ratio (BR) on the flame acceleration process and the flame-vortex mechanism in an obstructed chamber based essentially on the experimental and numerical methods. In the experiments, high-speed video photography and pressure transducer are used to study the flame shape changes and pressure dynamics. In the numerical simulations, large eddy simulation (LES) with the flame surface density (FSD) model is applied to investigate the interaction between the moving flame and vortices induced by obstacle. The results demonstrate that the flame propagation process can be divided into four stages, namely spherical flame, finger-shaped flame, jet flame and volute flame for three obstacle BR configurations, and a small recirculation zone is observed above the obstacle only for BR = 0.5. The peak of flame tip speed and pressure growth rate increases with the blockage ratio. The generation and evolution of the vortex behind the obstacle can be attributed to the initial flame acceleration, while the subsequent flame deceleration is due to the flame-vortex interaction. In addition, the transition from a “thin reaction zones” to a “broken reaction zones” is also observed in the simulation.     

A study on the obstacle-induced variation of the gas explosion characteristics

A study on the variation of the gas explosion characteristics caused by the built-in obstacles was conducted in enclosed/vented gas explosion vessels. It has been well known that the obstacles in pipes and long ducts would accelerate the flame propagation, and cause the transition from deflagration to detonation. In this study, the explosion characteristics and the flame behavior of vented explosions and constant-volume explosions were investigated. Experiments were carried out in a 270-liter and 36-liter hexahedron vessels filled with LPG–air mixture. The explosion characteristics of the gas mixture were determined by using a strain-responding pressure transducer. The flame behavior was recorded by using a high-speed video camera. The shape and the size of the obstacle, and the gas concentration, were adjusted in the experiments.

It can be seen from the experimental results that, instead of being accelerated, the flame propagation inside the explosion vessel is decelerated by the plate obstacles fixed at the bottom of the vessel. Also, the characteristics of the enclosed explosion are not so affected by the built-in obstacles as those of the vented explosion are. It is believed that the eddy-induced turbulence behind the obstacle decelerates the flame propagation.     

Experimental study of the effect of nitrogen addition on gas explosion

Flame propagation and combustion characteristics of methane/air mixed gas in gas explosion were studied in a constant volume combustion bomb. Stretched flame propagation velocity, unstretched laminar flame propagation velocity, unstretched laminar combustion velocity and Markstein length were obtained at various ratios of nitrogen to gas mixture. Combustion stability at various ratios of nitrogen to gas mixture was analyzed by analyzing the pictures of flame propagation. Furthermore, the effect of initial pressure on the flame propagation and combustion characteristics of methane/air mixed gas in gas explosion was analyzed. The results show that the unstretched laminar flame propagation velocity, the unstretched laminar combustion velocity, Markstein length, flame stability, and the maximum combustion pressure decrease distinctly with the increase of nitrogen fraction in the gas mixture. At the same ratios of nitrogen to gas mixture, Markstein length, unstretched laminar flame propagation velocity and unstretched laminar combustion velocity decrease and the maximum combustion pressure increase with the increase of initial pressure of the gas mixture. When nitrogen fraction in the gas mixture is over 20%, the flame will be unstable and is easy to exterminate.     

障碍物对油气-空气混合气体泄压爆炸火焰传播特性影响

为了研究障碍物对油气泄压爆炸火焰传播特性的影响规律,进行了不同数量障碍物工况下的对比实验,并利用纹影仪和高速摄影仪记录了火焰传播过程,针对障碍物对火焰形态、火焰锋面位置及火焰传播速度的影响规律进行了研究,结果表明:圆柱体障碍物会导致油气泄压爆炸火焰形态产生褶皱和弯曲变形,诱导层流火焰向湍流火焰转变,加速火焰的传播,对油气泄压爆炸火焰的初始传播形态有显著影响;随着障碍物数量的增多,火焰锋面传播距离点火端的最大距离增大,但到达最远距离的时间减少;障碍物能够增强火焰的传播速度,尤其对障碍物下游火焰影响最为显著,随着障碍物数量的增多,火焰传播的最大速度也随之增大,但达到最大火焰传播速度的时间却随之减少;障碍物的存在增大了油气泄压爆炸过程外部爆炸压力,并且随着障碍物数量的增多,外部爆炸压力峰值增长幅度增大。     

管道内甲烷/空气预混火焰加速传播特性研究

搭建了基于可视化燃烧管道的预混火焰精细结构实验台,通过理论分析和实验等手段对预混火焰的瞬态传播过程及加速传播特性进行了研究,分析得出了不同甲烷含量的预混火焰加速传播规律及预混火焰由层流向湍流的转变规律.     

甲烷爆炸传播过程膜状障碍物的激励效应研究

针对管状空间内膜状障碍物对甲烷爆炸传播的激励效应现象,基于机理分析进行了数值模拟和实验研究,计算分析薄膜附近爆炸冲击波压力峰值大小与火焰速度变化,同时运用激波管道进行相同工况条件下的实验,并对两者结果对比分析,发现有无膜状障碍物的压力峰值相差6倍以上。研究表明,膜状障碍物的激励效应是破膜以后形成的带压燃烧,提高了燃烧速率,导致甲烷爆炸的火焰传播速度剧增。实验结果一定意义诠释了同样数量的甲烷气体爆炸在不同环境内后果上的巨大差异,研究结果对矿井瓦斯爆炸事故调查及防治具有指导意义。     

细水雾熄灭障碍物火的实验研究和数值模拟

为了研究细水雾灭火系统在网状和条状障碍物阻隔条件下的灭火效果,采用全尺寸试验研究和数值模拟的方法研究了障碍物的形状,位置,遮挡面积以及细水雾工作压力对灭火效果的影响.实验结果表明:提高压力有助于快速有效地熄灭障碍物火;障碍物的位置是影响灭火效果的重要因素,近距离的覆盖使细水雾难以进入火区.障碍物下出现局部高温区,余火难以扑灭,灭火时间至少需要56 s;而障碍物距离火源较远时,卷吸作用下细水雾仍能均匀的进入火区,发挥冷却作用,灭火时间在18 s以内;网状障碍物的遮挡面积,对灭火效果影响不大;相同遮挡面积条件下,条状障碍物火比较难熄灭.     

LES – DFSD modelling of vented hydrogen explosions in a small-scale combustion chamber

Accidental explosions are a plausible danger to the chemical process industries. In the event of a gas explosion, any obstacles placed within the path of the flame generate turbulence, which accelerates the transient flame and raises explosion overpressure, posing a safety hazard. This paper presents numerical studies using an in-house computational fluid dynamics (CFD) model for lean premixed hydrogen/air flame propagations with an equivalence ratio of 0.7. A laboratory-scale combustion chamber is used with repeated solid obstacles. The transient compressible large eddy simulation (LES) modelling technique combined with a dynamic flame surface density (DFSD) combustion model is used to carry out the numerical simulations in three-dimensional space. The study presented uses eight different baffle configurations with two solid obstructions, which have area blockage ratios of 0.24 and 0.5. The flame speed, maximum rate of pressure-rise as well as peak overpressure magnitude and timing are presented and discussed. Numerical results are validated against available published experimental data. It is concluded that, increasing the solid obstacle area blockage ratio and the number of consecutive baffles results in a raised maximum rate of pressure rise, higher peak explosion overpressure and faster flame propagation. Future model development would require more experimental data, probably in a more congested configuration.     

Effects of multiple annular obstacles on flame propagation of local corn starch dust in a vertical pipe

With high-speed camera technology, the propagation behavior of explosion flame for the local dust cloud of corn starch in a semi-open vertical pipe under the action of the annular obstacle was studied experimentally, and the blockage rate and the annular obstacle numbers as well as impact of dust cloud concentration on the flame propagation were investigated. The researches showed that both the blockage rate and the annular obstacle numbers have significant effects on the flame speed and propagation process for the dust cloud explosion of corn starch. The increase of the blockage rate of such annular obstacles will cause that the combustion of dust cloud with high concentration is mainly concentrated in the lower part of the pipe. The increase of the annular obstacle numbers will lead to the acceleration of combustion of the dust cloud. With the increase of the blockage rate and the annular obstacle numbers, the maximum flame speed shows a trend of the first increasing and then decreasing, and the phenomenon of accelerated propagation of the flame becomes more and more obvious, however, the distance of continuous acceleration for the flame is gradually decreased and the maximum flame speed is farther from the outlet of the pipe. Under the action of such annular obstacles, the concentration of dust cloud has a significant effect on the flame speed and shape of the dust cloud of the corn starch. The increase of the concentration of the dust cloud will decrease the acceleration effect of such annular obstacles to result in maximum flame speed showing a trend of the first increasing and then decreasing. However, the acceleration distance of the flame is longer, and the maximum flame speed is closer to the outlet of the pipe. The increasing concentration will make the flame speed develop more slowly, the flame color will be darker, and the flame segmentation phenomenon will be more obvious.     

障碍物场中预混燃烧火焰的数值模拟

利用k-ε湍流模型和拉切滑(SCASM)预混燃烧模型,对障碍物场中预混燃烧火焰进行了三维空间数值模拟。通过对控制方程添加不同的源项以反映障碍物对流场的影响,采用交错网格控制容积法将计算区域进行离散,用SIMPLE算法求解离散控制方程。模拟结果表明,障碍物的存在改变了燃烧流场的结构,成为加速燃烧甚至诱导爆炸过程的不稳定因素。该研究结果对有效预测障碍物场中火焰走势及其流场的分布情况,加强人们对火焰传播规律的认识,对预防工业灾害有重要参考价值。     

基于浓度敏感性分析和遗传算法的甲烷燃烧机理简化与优化

采用良搅拌反应器模型和层流预混火焰模型计算甲烷/空气燃烧过程,通过元素流通法和浓度敏感性分析法,对甲烷燃烧详细化学动力学机理GRIMECH 3.0进行简化。利用遗传算法,以甲烷/空气详细机理获得的组分浓度和一维层流火焰速度为目标,对简化机理进行优化。结果表明,相比于优化之前的简化机理,优化后的简化机理在描述甲烷/空气燃烧反应的组分浓度、层流火焰速度以及反应物和产物的时空分布方面,具有更高的精度。     

Large eddy simulation of methane–air deflagration in an obstructed chamber using different combustion models

In this paper, simulations of methane–air deflagration inside a semi-confined chamber with three solid obstacles have been carried out with large eddy simulation (LES) technique. Three sub-grid scale (SGS) combustion models, including power-law flame wrinkling model by Charlette et al., turbulent flame speed closure (TFC) model, and eddy dissipation model (EDM), are applied. All numerical results have been compared to literature experimental data. It is found that the power-law flame wrinkling model by Charlette et al. is able to better predict the generated pressure and other flame features, such as flame structure, position, speed and acceleration against measured data. Based on the power-law flame wrinkling model, the flame–vortex interaction during the deflagration progress is also investigated. The results obtained have demonstrated that higher turbulence levels, induced by obstacles, wrinkle the flame and then increase its surface area, the burning rates and the flame speed.     

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.