Experimental study on unconfined methane explosion: Explosion characteristics and overpressure prediction method

Explosion accidents have become the main threat for the high-efficiency use of cleaner gas energy sources, such as natural gas. During an explosion, obstacle causing flame acceleration is the main reason for the increase of the explosion overpressure, which still remains to be fully understood. In this research, field experiments were conducted in a 1 m³ cubic frame apparatus to investigate the effect of built-in obstacles on unconfined methane explosion. Cage-like obstacles were constructed using square steel rods with different cross section size. The results demonstrated that the flame could get accelerated due to the hydrodynamic instability and obstacle-induced turbulence, which enhanced the explosion overpressure. In the near field, the overpressure wave travelled slower and the maximum overpressure could almost keep constant. Reducing the cross section size, or increasing the obstacle height or the obstacle number per layer could determine the rise of the maximum overpressure, the maximum pressure rising rate and the overpressure impulse. For uniformly constructed obstacles, self-similar theory was chosen to measure the influence of the hydrodynamic instability, and a parameter β was adopted to measure the flame acceleration caused by obstacle-induced turbulence, the value of which was 2 in this research. Based on the acoustic theory, an overpressure prediction model was proposed and the predicted results agreed with the measured values better than previous models, such as TNT equivalency model and TNO multi-energy model.     

A method for simulation of vapour cloud explosions based on computational fluid dynamics (CFD)

The effectiveness of the application of CFD to vapour cloud explosion (VCE) modelling depends on the accuracy with which geometrical details of the obstacles likely to be encountered by the vapour cloud are represented and the correctness with which turbulence is predicted. This is because the severity of a VCE strongly depends on the types of obstacles encountered by the cloud undergoing combustion; the turbulence generated by the obstacles influences flame speed and feeds the process of explosion through enhanced mixing of fuel and oxidant. In this paper a CFD-based method is proposed on the basis of the author’s finding that among the various models available for assessing turbulence, the realizable k-? model yields results closer to experimental findings than the other, more frequently used, turbulence models if used in conjunction with the eddy-dissipation model. The applicability of the method has been demonstrated in simulating the dispersion and ignition of a typical vapour cloud formed as a result of a spill from a liquid petroleum gas (LPG) tank situated in a refinery. The simulation made it possible to assess the overpressures resulting from the combustion of the flammable vapour cloud. The phenomenon of flame acceleration, which is a characteristic of combustion enhanced in the presence of obstacles, was clearly observed. Comparison of the results with an oft-used commercial software reveals that the present CFD-based method achieves a more realistic simulation of the VCE phenomena.     

The acceleration of flames in tube explosions with two obstacles as a function of the obstacle separation distance

The separation distance (or pitch) between two successive obstacles or rows of obstacles is an important parameter in the acceleration of flame propagation and increase in explosion severity. Whilst this is generally recognised, it has received little specific attention by investigators. In this work a vented cylindrical vessel 162 mm in diameter 4.5 m long was used to study the effect of separation distance of two low blockage (30%) obstacles. The set up was demonstrated to produce overpressure through the fast flame speeds generated (i.e. in a similar mechanism to vapour cloud explosions). A worst case separation distance was found to be 1.75 m which produced close to 3 bar overpressure and a flame speed of about 500 m/s. These values were of the order of twice the overpressure and flame speed with a double obstacle separated 2.75 m (83 characteristic obstacle length scales) apart. The profile of effects with separation distance was 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 further downstream than the position of maximum turbulence determined in the cold flow studies. It is suggested that this may be due to the convection of the turbulence profile by the propagating flame. The present results would suggest that in many previous studies of repeated obstacles the separation distance investigated might not have included the worst case set up, and therefore existing explosion protection guidelines may not be derived from worst case scenarios.     

Prediction of distance to maximum intensity of turbulence generated by grid plate obstacles in explosion-induced flows

The interaction of unburnt gas flow induced in an explosion with an obstacle results in the production of turbulence downstream of the obstacle and the acceleration of the flame when it reaches this turbulence. Currently, there are inadequate experimental measurements of these turbulent flows in gas explosions due to transient nature of explosion flows and the connected harsh conditions. Hence, majority of measurements of turbulent properties downstream of obstacles are done using steady-state flows rather than transient flows. Consequently, an empirical based correlation to predict distance to maximum intensity of turbulence downstream of an obstacle in an explosion-induced flow using the available steady state experiments was developed in this study. The correlation would serve as a prerequisite for determining an optimum spacing between obstacles thereby determining worst case gas explosions overpressure and flame speeds. Using a limited experimental work on systematic study of obstacle spacing, the correlation was validated against 13 different test conditions. A ratio of the optimum spacing from the experiment, x_exp to the predicted optimum spacing, x_pred for all the tests was between 2-4. This shows that a factor of three higher than the x_pred would be required to produce optimum obstacle spacing that will lead to maximum explosion severity. In planning the layout of new installations, it is appropriate to identify the relevant worst case obstacle separation in order to avoid it. In assessing the risk to existing installations and taking appropriate mitigation measures it is important to evaluate such risk on the basis of a clear understanding of the effects of separation distance and congestion. It is therefore suggested that the various new correlations obtained from this work be subjected to further rigorous validation from relevant experimental data prior to been applied as design tools.     

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.     

对称障碍物条件下瓦斯爆炸火焰与压力波耦合作用研究

为了研究对称障碍物条件下瓦斯爆炸压力波与火焰传播的耦合作用,在150 mm×150 mm×1 700 mm的有机玻璃瓦斯爆炸管道中,距离点火端不同距离安装0.5阻塞率的对称障碍物,进行8.5%甲烷体积分数的爆炸试验,采集瓦斯爆炸的超压信号并同步拍摄火焰传播图像。结果表明:火焰穿越板式对称障碍物的过程经历了火焰加速、火焰降速到火焰再加速的过程,火焰降速的时间仅为5 ms。距离点火焰源不同长度的对称障碍物在火焰加速过程中的作用存在明显差异,近点火源的障碍物作用主要为诱导湍流,远离点火源的障碍物作用主要为湍流增强。     

可燃气体爆炸破坏效应的试验研究

借助高速摄像机及ProAnalyst软件,研究可燃气体体积分数和障碍物对可燃气体爆炸破坏力的影响。测定不同体积分数下的甲烷-空气预混气体爆炸冲击波超压,和爆炸火焰波在有无乒乓球方向传播的平均速度。试验结果表明:超压和平均速度均随着甲烷体积分数的增加呈现先增大后减小的变化趋势,其最大值均出现在甲烷体积分数为10%～11%之间;同一体积分数下的甲烷-空气预混气体爆炸火焰波在有乒乓球方向传播的平均速度比没有乒乓球方向传播的平均速度大。根据试验结果,推导出可燃气体爆炸冲击波超压和爆炸火焰波传播平均速度与可燃气体体积分数之间的函数关系,并得出障碍物对爆炸火焰波传播的加速作用随着体积分数的增加呈现先加强后减弱的变化趋势。     

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.     

大尺度障碍物与泄爆面对天然气内爆炸的协同作用规律研究

为阐释民用建筑内部大尺度物品与门窗等泄爆面对天然气爆炸灾害的协同作用机制,基于典型厨房空间布局及内部物品特征,借助计算流体动力学技术研究了不同泄爆面开启压力和不同大尺度障碍物体积阻塞率条件下天然气内爆炸火焰速度、爆炸超压的分布规律。研究结果表明:大尺度障碍物与泄爆面对室内天然气爆炸过程具有显著的协同作用,共同促进火焰速度与爆炸超压的显著增长,并缩短峰值超压到达时间;大尺度障碍物的存在虽然显著降低了室内天然气的体积,但从增加房间内湍流源和相对长径比的角度进一步促进了泄爆效应;大尺度障碍物与泄爆面协同作用下,室内火焰速度呈现明显的阶段性特征,并在泄爆面附近发生波动。研究结论可为民用建筑物内气体爆炸事故调查分析和灾害评估提供科学依据。     

The effect of orifice plates with different shapes on explosion propagation of premixed methane–air in a semi-confined pipeline

The effect of internal shape of obstacles on the deflagration of premixed methane–air (concentration of 10%) was experimentally investigated in a semi-confined steel pipeline (with a square cross section size of 80 mm × 80 mm and 4 m long). The obstacles used in this study were circular, square, triangular and gear-shaped (4-teeth, 6-teeth and 8-teeth) orifice plates with a blockage ratio of 75%, and the perimeter of the orifice was regarded as a criterion for determining the sharpness of the orifice plate. The overpressure history, flame intensity histories, flame front propagation speed, maximum flame intensity and peak explosion overpressure were analyzed. The explosion in the pipeline can be divided into two stages: initial explosion and secondary explosion. The secondary explosion is caused by recoiled flame. The perimeter is positively related to the intensity of the recoiled flame and the ability of orifice plate to suppress the explosion propagation. In addition, the increase in the perimeter will cause the acceleration of the flame passing through the orifice plate, while after the perimeter of the orifice reaches a certain value, the effect of the increase in perimeter on explosion excitation becomes no obvious. The overpressure (static pressure) downstream of the orifice plate is the result of the combined effect of explosion intensity and turbulence. The increase in perimeter leads to the increase in turbulence downstream of the orifice plate which in turn causes more explosion pressure to be converted into dynamic pressure.     

Fast and tiny: A model for the flame propagation of nanopowders

To avoid the influence of external parameters, such as the vessel volume or the initial turbulence, the explosion severity should be determined from intrinsic properties of the fuel-air mixture. Therefore, the flame propagation of gaseous mixtures is often studied in order to estimate their laminar burning velocity, which is both independent of external factors and a useful input for CFD simulation. Experimentally, this parameter is difficult to evaluate when it comes to dust explosion, due to the inherent turbulence during the dispersion of the cloud. However, the low inertia of nanoparticles allows performing tests at very low turbulence without sedimentation. Knowledge on flame propagation concerning nanoparticles may then be modelled and, under certain conditions, extrapolated to microparticles, for which an experimental measurement is a delicate task. This work focuses on a nanocellulose with primary fiber dimensions of 3 nm width and 70 nm length. A one-dimensional model was developed to estimate the flame velocity of a nanocellulose explosion, based on an existing model already validated for hybrid mixtures of gas and carbonaceous nanopowders similar to soot. Assuming the fast devolatilization of organic nanopowders, the chemical reactions considered are limited to the combustion of the pyrolysis gases. The finite volume method was used to solve the mass and energy balances equations and mass reactions rates constituting the numerical system. Finally, the radiative heat transfer was also considered, highlighting the influence of the total surface area of the particles on the thermal radiation. Flame velocities of nanocellulose from 17.5 to 20.8 cm/s were obtained numerically depending on the radiative heat transfer, which proves a good agreement with the values around 21 cm/s measured experimentally by flame visualization and allows the validation of the model for nanoparticles.     

Evaluation of a CFD porous model for calculating ventilation in explosion hazard assessments

In the past, gas explosion assessment relied on worst case scenarios. A more realistic approach is to look at the probability of explosions and their likely severity. The most flexible way of investigating many different scenarios is to estimate a ventilation flow, feed this into a flammable volume calculation and then calculate the explosion severity. The procedure allows many parameters to be varied efficiently. A Computational Fluid Dynamics porous model is evaluated for modelling the ventilation flow through congested regions, including a new method that has been developed to derive the resistance. Comparison with velocity measurements from a large scale model of an offshore module showed that overall the CFD model performs very well, especially considering that the homogenous porosity block does not model any of the internal obstructions and therefore would not predict any local flow effects. This gives confidence that the overall flow pattern is sufficiently close to the local flow patterns, to be used in explosion assessments. The porous approximation in CFX is found to underpredict the turbulence intensity in the obstacle array compared to the explosion model EXSIM. Improving the turbulence prediction in the porous model would be valuable, so a relatively simple method of increasing the turbulence in porous regions is proposed. The CFD model will provide the non-uniform natural ventilation flowfields of complex regions for future explosion assessments at a hierarchy of levels.     

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.     

Initial phase modelling in numerical explosion applied to process safety

The utilisation of computational fluid dynamics (CFD) in process safety has increased significantly in recent years. The modelling of accidental explosion via CFD has in many cases replaced the classical Multi Energy and Brake Strehlow methods. The benefits obtained with CFD modelling can be diminished if proper modelling of the initial phase of explosion is neglected. In the early stages of an explosion, the flame propagates in a quasi-laminar regime. Proper modelling of the initial laminar phase is a key aspect in order to predict the peak pressure and the time to peak pressure. The present work suggests a modelling approach for the initial laminar phase in explosion scenarios. Findings are compared with experimental data for two classical explosion test cases which resemble the common features in chemical process areas (confinement and congestion). A detailed analysis of the threshold for the transition from laminar to turbulent regime is also carried out. The modelling is implemented in a fully 3D Navier–Stokes compressible formulation. Combustion is treated using a laminar flamelet approach based on the Bray, Moss and Libby (BML) formulation. A novel modified porosity approach developed for the unstructured solver is also considered. Results agree satisfactorily with experiments and the modelling is found to be robust.     

湍流对粉尘云电火花点火过程的影响

随着现代工业的发展,粉尘爆炸事故发生的频率也逐年增加,因此,对粉尘云点火敏感程度进行测量和计算就变得十分重要。粉尘云最小点火能是粉尘爆炸重要的特性参数之一,是采取粉尘爆炸防护的基础。最小点火能在测量的过程中受到多个敏感条件的影响,其中湍流则是最复杂的影响因素之一。文中对实验过程中粉尘云的湍流进行了定义,并分析了湍流对粉尘云最小点火能影响的内在原因;同时对通过数值模拟计算粉尘云最小点火能过程中的湍流计算给出了数学模型。从实验和数学模型两个方向对湍流进行了全面描述,对粉尘云电火花点火过程中湍流影响的分析结论,可有效的指导实验。     

A numerical study of the evolution of the blast wave shape in rectangular tunnels

When the explosion of condensed materials occurs in square or circular cross-section tunnel, the subsequent blast wave reveals two patterns: three-dimensional close to the explosive charge and one-dimensional far from the explosion. Pressure decays for these two patterns have been thoroughly studied. However, when the explosion occurs in rectangular cross-section tunnel, which is the most regular geometry for underground networks, the blast wave exhibits a third, two-dimensional, patterns. In order to assess the range of these three patterns, several numerical simulation of blast waves were carried out varying the width and the height of the rectangular cross-section as well as the mass of the charge. Laws are presented to localize the transition zones between the 3D and the 2D patterns, and between the 2D and the 1D patterns, as functions of non-dimensional width and height. The numerical results of the overpressure are compared to existing 3D and 1D laws. An overpressure decay law is proposed to represent the 2D pattern. Knowing the two transition zones and the overpressure decays within these zones, an algorithm is presented to efficiently predict an overpressure map. This algorithm is validated by comparison with experimental data.     

Experimental study on the mitigation via an ultra-fine water mist of methane/coal dust mixture explosions in the presence of obstacles

In this paper, experimental investigations were performed for the mitigation via an ultra-fine water mist of methane/coal dust mixture explosions in the presence of obstacles to reveal the effects of the obstacles in this scenario. Two PCB piezo-electronic pressure transducers were used to acquire the pressure history, a Fastcam Ultima APX high-speed video camera was used to visualize both the process of the mixture explosion and its mitigation. The diameters of the coal dust, the types of obstacles and the volumes of ultra-fine water mist were varied in the tests. The parameters of the explosion overpressure and the range of critical volume flux of the ultra-fine water mist for explosion mitigation were determined. The results show that the mixture explosion and its mitigation are primarily influenced by the number, shape and set locations of the obstacles. When the volume flux of the water mist is larger than a certain amount, the mixture explosions and the effects of obstacles can be completely mitigated with the ultra-fine water mist.     

铝粉分散过程中粒径效应的三维数值研究

为了研究不同粒径的铝粉在20 L爆炸测试装置中的分散规律,基于计算模型的非结构网格划分,耦合欧拉和拉格朗日方法,实现了描述可压缩气体演化的时间平均Navier-Stokes方程组和粒子运动的DPM动量平衡方程的求解,获得了不同粒径（25,50和100 μm）的铝粉在20 L爆炸仓内分散的三维时空演化规律。研究结果表明:铝粉粒径的差异对爆炸仓点火中心的湍动能和速度的演化过程影响不显著,但对粉尘浓度的变化率和峰值均具有重要影响;随着粒径的增大,峰值浓度越小,但均高于形式浓度0.25 kg/m3,达到峰值浓度的时间越滞后。     

A quantitative risk assessment tool for the external safety of industrial plants with a dust explosion hazard

A quantitative risk assessment (QRA) tool has been developed by TNO for the external safety of industrial plants with a dust explosion hazard. As a first step an industrial plant is divided into groups of modules, defined by their size, shape, and constructional properties. Then the relevant explosion scenarios are determined, together with their frequency of occurrence. These include scenarios in which one module participates, as well as domino scenarios. The frequency is partly based on casuistry.

A typical burning velocity is determined depending on the ignition type, the dust properties and the local conditions for flame acceleration. The resulting pressure development is predicted with the ‘thin flame model’. Module failure occurs when the explosion load exceeds thresholds, which are derived from single degree of freedom (SDOF) calculations for various types of modules. A model has been developed to predict the process of pressure venting after module failure and the related motion of launched module parts.

The blast effects of the primary explosion are based on results from calculations with BLAST3D. The blast and flame effects of the secondary external explosion due to venting are calculated using existing models. The throw of fragments and debris is quantified with a recently developed model. This model is based on trajectory calculations and gives the impact densities, velocities, and angles as output. Furthermore the outflow of bulk material is taken into account. The consequences for external objects and human beings are calculated using existing models. Finally the risk contours and the Societal risk (FN curve) are calculated, which can be compared to regulations.     

Large-scale dust explosions in vessel-pipe systems

This study investigates dust explosions in vessel-pipe systems to develop a better understanding of dust flame propagation between interconnected vessels and implications for the proper application of explosion isolation systems. Cornstarch dust explosions were conducted in a large-scale setup consisting of a vented 8-m³ vessel and an attached pipe with a diameter of 0.4 m and a length of 9.8 m. The ignition location and effective dust reactivity were varied between experiments. The experimental results are compared against previous experiments with initially quiescent propane-air mixtures, demonstrating a significantly higher reactivity of the dust explosions due to elevated initial turbulence, leading to higher peak pressures and faster flame propagation. In addition, a physics-based model developed previously to predict gas explosion dynamics in vessel-pipe systems was extended for dust combustion. The model successfully predicts the pressure transients and flame progress recorded in the experiments and captures the effects of ignition location and effective dust reactivity.