Gas flame acceleration in long ducts

In many practical situations, a flame may propagate along a pipe, accelerate and perhaps transform into a devastating detonation. This phenomenology has been known, more or less qualitatively, for a long time and mitigation techniques were proposed to try and avoid this occurrence (flame arresters, vents,...). A number of parameters need to be known and in particular the “distance to detonation” and more generally the flame acceleration characteristic scales. Very often, the ratio between the detonation run-up distance and the pipe diameter is used without any strong justification other that using a non-dimensional parameter (L/D). In this paper, novel experimental evidence is presented on the basis of relatively large scale experiments using 10 cm and 25 cm inner diameter duct with a length between 7 and 40 m. Homogeneous C₂H₄-air, CH₄-air, C₃H₈-air and H₂-air mixtures were used and different ignition sources. The interpretation suggests that the self-acceleration mechanism of the flame may be much better represented by flame instabilities than by turbulence build-up. One consequence would be that the maximum flame velocity and, following, the maximum explosion overpressure, would be rather linked with the run-up distance than with the L/D ratio.     

Reactive shock and detonation propagation in U-bend tubes

The objective of the research outlined in this paper was to provide new experimental and computational data on initiation, propagation, and stability of gaseous stoichiometric propane–air detonations in tubes with U-bends. Extensive experimental and computational studies with the tube 51 mm in diameter with U-bends of two curvatures and two different shock-wave generators were performed. Numerical simulations of the process were used to reveal the salient features of the accompanying phenomena.     

Measurement of gas detonation blast loads in semiconfined geometry

The ignition and explosion of combustible vapor clouds represents a significant hazard across a range of industries. In this work, a new set of gas detonations experiments were performed to provide benchmark blast loading data for non-trivial geometry and explosion cases. The experiments were designed to represent two different accident scenarios: one where ignition of the vapor cloud occurs shortly after release and another where ignition is delayed and a fuel concentration gradient is allowed to develop. The experiments focused on hydrogen-air and methane-oxygen detonations in a semiconfined enclosure with TNT equivalencies ranging from 9 g to 1.81 kg. High-rate pressure transducers were used to record the blast loads imparted on the interior walls of a 1.8 m × 1.8 m × 1.8 m test fixture. Measurements included detonation wave velocity, peak overpressure, impulse, and positive phase duration. A comparison of the pressure and impulse measurements with several VCE models is provided. Results show that even for the most simplified experimental configuration, the simplified VCE models fail to provide predictions of the blast loading on the internal walls of the test fixture. It is shown that the confinement geometry of the experiment resulted in multiple blast wave reflections during the initial positive phase duration portion of the blast loading, and thus, significantly larger blast impulse values were measured than those predicted by analytical models. For the pressure sensors that experienced normally-reflect blast waves for the initial blast impulse, the Baker-Strehlow and TNT equivalency models still struggled to accurately capture the peak overpressure and reflected impulse. The TNO multi-energy model, however, performed better for the case of simple normally-reflected blast waves. The results presented here may be used as validation data for future model or simulation development.     

三氯杀螨醇生产过程中的DDT环境排放研究

三氯杀螨醇生产工艺流程主要包括缩合、碱解、氯化和水解等步骤。对工作场所中空气样品、生产过程排放的废酸及废水样品进行采集和分析。工作场所空气中DDT总质量浓度均值为6.69×10-3mg/m3。其中,碱解反应工序中质量浓度水平较低,为1.10×10-3mg/m3;包装车间质量浓度水平较高,为16.72×10-3mg/m3。所有空气样品中p,p’-DDE均是主要贡献物质,占DDT杂质总量的80.2%;p,p’-DDT的质量浓度范围为0.053×10-3~1.66×10-3mg/m3,平均为0.49×10-3mg/m3,低于国家标准限值。缩合废酸与水解废酸中DDT杂质总质量比分别为4.84μg/kg和334.83μg/kg;碱解废水与水解废水中的DDT杂质总质量比分别为456.48μg/kg和75.65μg/kg。废水及废酸样品中各种DDT杂质的质量比水平存在差异;生产工艺阶段不同,杂质组成也各具特点。水解废酸的p,p’-DDT的质量比最高,为146.82μg/kg;缩合废酸与水解废水处质量比水平较低,分别为0.33μg/kg和1.41μg/kg。该企业随废水及废酸排放的DDT杂质总量为1234.08 g/a,其中随碱解废水的排放量高达912.95 g/a。p,p’-DDT的年排放总量为163.37 g/a,随碱解废水和水解废酸的排放量分别为86.98 g/a和73.41 g/a。     

Propagation Characteristics of Gas Explosion in Linked Vessels Based on DDT Criteria

To study the occurrence conditions and propagation characteristics of deflagration to detonation transition (DDT) in linked vessels, two typical linked vessels were investigated in this study. The DDT of the methane–air mixture under different pipe lengths and inner diameters was studied. Results showed that the CJ detonation pressure of the methane–air mixture was 1.86 MPa, and the CJ detonation velocity was 1987.4 m/s. Compared with a single pipe, the induced distance of DDT is relatively short in the linked vessels. With the increase in pipeline length, DDT is more likely to occur. Under the same pipe diameter, the DDT induction distance in the vessel–pipe–vessel structure is shorter than that in the vessel–pipe structure. With the increase in pipeline diameter, the length of the pipe required to form the DDT is reduced. For linked vessels in which detonation formed, four stages, namely, slow combustion, deflagration, deflagration to detonation, and stable detonation, occurred in the vessels. Moreover, for a pipe diameter of 60 mm and a length of 8 m, overdriven detonation occurred in the vessel–pipe–vessel structure.     

Comparison of explosion models for detonation onset estimation in large-scale unconfined vapor clouds

Although industrial denotations in semi-open and congested geometries are often neglected by many practitioners during risk assessment, recent studies have shown that industrial detonations might be more common than previously believed. Therefore, from the explosion safety perspective, it becomes imperative to better assess industrial detonation hazards to improve robustness of explosion mitigation design, emergency response procedures, and building siting evaluation. Having that in mind, this study aims to review current empirical vapor cloud explosion models, understand their limitations, and assess their capability to indicate detonation onset for elongated vapor clouds. Six models were evaluated in total: TNO Multi-Energy, Baker-Strehlow-Tang (BST), Congestion Assessment Method (CAM), Quest Model for Estimation of Flame Speed (QMEFS), Primary Explosion Site (PES), and Confinement Specific Correlation (CSC). Model estimations were compared with large-scale test data available in the open literature. The CAM model demonstrated good performance in indicating deflagration-to-detonation transition (DDT) for test conditions experiencing detonation onset without any modification in the methodology. Some suggestions are provided to improve simulation results from PES, BST and QMEFS.     

Effect of ignition position on the run-up distance to DDT for hydrogen-air explosions

The method described in this paper enabled reliable and accurate positioning of an overdriven detonation by calculation of shock wave velocities (detonation and retonation) for hydrogen explosions in a closed 18 m long horizontal DN150 pipe. This enabled an empirical correlation between the ignition position and the run-up distance to DDT to be determined. It was shown that the initial ability of the flame to expand unobstructed and the piston-like effect of burnt gas expanding against the closed end of the tube contributed to initial flame acceleration and hence were able to affect the run-up distance to overdriven detonation. Flame speeds and rates of initial pressure rise were also used to explain how these two competing effects were able to produce a minimum in the run-up distance to DDT. The shortest run-up distance to DDT, relative to the ignition position, for this pipe and gas configuration was found when the ignition position was placed 5.6 pipe diameters (or 0.9 m) from the closed pipe end. The shortest run-up distance to DDT relative to the end of the pipe was recorded when the ignition source was placed 4.4 pipe diameters or 0.7 m from the pipe end.     

Deflagration to detonation transition in a vapour cloud explosion in open but congested space: Large scale test

The paper reviews large scale experiments with various fuels in air where successful deflagration to detonation transition (DDT) took place. This includes a recent experiment disclosed in the Buncefield R&D program, where DDT developed in the propane/air mixture. The DDT occurred in branches of deciduous trees in a premixed stagnant mixture. An internal R&D investigation programme was initiated to better understand the phenomena. A large scale experiment in an open space with ethane air mixture is presented in the paper. The premixed mixture was ignited at the edge of the congested three-dimensional rigs which consisted of vertical and horizontal pipes. After ignition, the flame accelerated in the congestion and transitioned to detonation at the end of congestion. Stable detonation propagated through the remaining open and uncongested space.The flame acceleration process leading to DDT is scale dependent. It also depends on many parameters leading to a large investigation array and, significant cost. However, such R&D efforts aimed toward a safer plant design, i.e. the prevention of occurrence of a major accident, are a small fraction of a real accident cost.     

障碍物对瓦斯煤尘爆炸火焰传播规律的影响

为了进一步探究瓦斯煤尘耦合爆炸火焰的传播规律,用自行搭建的半封闭垂直管道爆炸试验系统,研究障碍物对瓦斯煤尘耦合爆炸火焰传播规律的影响。研究结果表明:障碍物能显著提高瓦斯煤尘爆炸火焰的传播速度,其加速机理主要是障碍物诱导的湍流区会促进火焰的传播;火焰在传播过程中的加速度不是一直增加,随着火焰速度的增加会出现上下波动;煤尘的加入会使瓦斯爆炸产生的火焰传播速度显著增大及速度的最大值距离点火端较远;通过障碍物时爆炸产生的火焰形状发生较大的改变,出现拉伸和褶皱现象。     

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。     

油池火作用下钢管混凝土柱温度分布研究

在开敞空间条件下利用油池火模拟不同的火灾场景,对火源燃烧阶段和燃烧结束后钢管混凝土柱的温度变化进行研究。测量了燃料质量变化和钢管混凝土柱的温度分布等参数,分析不同火灾场景下钢管混凝土柱轴向、横截面的温度变化,并以实验工况HS3为例进行深入分析,同时分析了火源的燃烧时间、热释放速率对钢管混凝土柱温度场分布的影响。研究结果表明,钢管混凝土柱纵向温度分布与横向温度分布一样都存在明显的温度梯度,其表面钢管温度变化受火场热释放速率影响较大,而内部混凝土的温度变化则主要受火场持续时间的影响。     

Regimes and critical conditions of detonation propagation in expanding channels in gas suspensions of ultrafine aluminum particles

Under study are the regimes of detonation propagation in channels with linear expansion filled with monodisperse mixtures of oxygen and ultrafine aluminum particles of various loading; the methods of numerical simulations are used. The detonation combustion of submicron aluminum particles is described within the semi-empirical model of reduced kinetics with due regard to the transition from the diffusion-limited regime of combustion to the kinetic one. Waves of both planar and developed cellular detonation are considered as initial conditions. The characteristics of the main flow regimes are obtained and described: the subcritical (detonation failure), critical (detonation failure in some part of the channel) and supercritical (continuous detonation propagation). The maps of flow regimes in suspensions of 200-nm – 400-nm particles are presented in the plane of parameters: the channel width, expansion angle. The obtained critical conditions are similar to those observed in the gas detonation. The critical channel width linearly depends on the expansion angle up to a first critical value (35°–38°). Behind the second critical value (50°), the channel width is independent on the expansion angle. Between these values, there is an interval of nonmonotonicity similar to the detonation of micro-sized suspensions of aluminum particles. The effect of particle loading on the critical conditions in poor mixtures appears in the form of a sharp increase in the critical channel width, if the mass concentration falls below 0.25.     

封闭管道油气爆炸超压及火焰传播特性

对油气在封闭管道内的爆炸特性进行研究，发现爆炸超压发展过程可以分为3个阶段:第1次超压上升阶段、第2次超压上升阶段和超压下降阶段。初始油气浓度对爆炸初始阶段的发展有很大影响，油气浓度为1.73%时发展最激烈；当初始油气浓度较高时，在最大超压峰值附近，会产生压力振荡现象；初始油气浓度对Tulip火焰的形成及发展有较大影响，各种浓度油气的爆炸，都有形成Tulip火焰的趋势；当油气浓度适中时，Tulip火焰会一直传播到管道末端，当油气浓度较高或较低时，火焰锋面会经由鲨鱼嘴形状火焰转变为刀尖形火焰，当初始油气浓度为1.73%时，最容易发展形成Tuilp火焰。     

Prevention and suppression of explosions in gas-air and dust-air mixtures using powder aerosol-inhibitor

The prevention and suppression of explosions is a very topical field of research because annually hundreds of coal mine workers became their victims. In this research a very effective powder “powder for suppression of explosions” (“PSE”) for the suppression of explosions has been developed and tested. The experiments on suppression of explosions of a methane–air mixture (MAM) at a laboratory conditions using “PSE”-powder have been carried out. The possibility of lowering the power of coal-dust explosion with the help of a “PSE”-powder has been investigated. The feasibility of almost instantaneous disperse of powders using intentionally created mini-explosions (ammonal) was investigated. The barrel-suppressor of explosion in the experimental adit (tunnel) was studied and the large-scale tests for suppression of MAM-explosions in experimental adit were also subjects of study.     

Flame acceleration in tube explosions with up to three flat-bar obstacles with variable obstacle separation distance

The effect of obstacle separation distance on the severity of gas explosions has received little methodical study. It was the aim of this work to investigate the influence of obstacle spacing of up to three flat-bar obstacles. The tests were performed using methane-air (10% by vol.), in an elongated vented cylindrical vessel 162 mm internal diameter with an overall length-to-diameter, L/D, of 27.7. The obstacles had either 2 or 4 flat-bars and presenting 20% blockage ratio to the flow path. The different number of flat-bars for the same blockage achieved a change of the obstacle scale which was also part of this investigation. The first two obstacles were kept at the established optimum spacing and only the spacing between the second and third obstacles was varied. The profiles of maximum flame speed and overpressure with separation distance were shown to agree with the cold flow turbulence profile determined in cold flows by other researchers. However, the present results showed that the maximum effect in explosions is experienced at 80 to 100 obstacle scales about 4 times further downstream than the position of maximum turbulence determined in the cold flow studies. Similar trends were observed for the flames speeds. In both cases the optimum spacing between the second and third obstacles corresponded to the same optimum spacing found for the first two obstacles demonstrating that the optimum separation distance does not change with number of obstacles. In planning the layout of new installations, the worst case separation distance needs to be avoided but incorporated when assessing the risk to existing set-ups. The results clearly demonstrate that high congestion in a given layout does not necessarily imply higher explosion severity as traditionally assumed. Less congested but optimally separated obstructions can lead to higher overpressures.     

Flame acceleration and transition to detonation in an array of square obstacles

We study flame acceleration and DDT in a two-dimensional staggered array of square obstacles by solving the compressible multidimensional reactive Navier–Stokes equations. The energy release rate for a stoichiometric H₂-air mixture is modeled by a one-step Arrhenius kinetics. The space between obstacles is filled with a stoichiometric H₂-air mixture at 1 atm and 298 K. Initially, the flow is at rest, and a flame is ignited at the center of the array. Computations show effects of the obstacles as a series of events leading to DDT. During the initial flame acceleration, the speed of the flame depends on the direction of flame propagation since some directions are more obstructed than others. This affects the macroscopic shape of the expanding burned region, which forms concave boundaries in more obstructed directions. As the flame accelerates, shocks form ahead of the flame, reflect from obstacles, and interact with the flame. There are more shock–flame interactions in more obstructed directions, and this leads to a greater flame acceleration and stronger leading shocks. When the shocks become strong enough, their collisions with obstacles ignite the gas mixture, and detonations form. The simulation shows four independent DDT events within a 90-degree sector, all in more obstructed directions. Resulting detonations spread in all directions. Some parts of detonation fronts are quenched by diffractions around obstacles, but they are reignited by collisions of decoupled shocks, or overtaken by other detonations. Thus detonations continue to spread and quickly burn all the material between the obstacles.