The quenching of propane deflagrations by crimped ribbon flame arrestors

Experimental studies of deflagration flame quenching by crimped ribbon flame arrestors were performed in a circular ducting with propane-air flammable mixture. The corresponding flameproof velocities were determined systemically. The results showed that the channel length, the expansion ratio and the aperture size must be taken into consideration to predict the performance of the crimped ribbon flame arrestors. To explore the relationship between flameproof velocity and arrestor structure, numerical simulations were carried out. The simulated results showed that with the reduction of the hydraulic diameter of the aperture size, the flameproof velocity is increased, which implies the basic angle must be considered when crimped ribbon flame arrestors are used to quench deflagration flame. In addition, the influence of the expansion ratio is of great significance on the efficiency of flame arrestors. The flameproof velocity can be reduced to be a value related to the expansion ratio. Two empirical formulas were derived to exhibit the relation between the flameproof velocity and the characteristics of the flame arrestor, which can be used to predict the performance of crimped ribbon arrestors.     

FA型高炉煤气管道火焰捕器研制

研制满足高炉煤气管道阻火的火焰捕器.内径88 mm、199 mm和305 mm组合爆炸管道模拟实验结果表明,FA型火焰捕器的阻火性能良好,满足高炉煤气管道阻火的技术要求,各项技术性能指标达到了设计要求.     

Safety devices for gas welding, cutting and allied processes

This paper describes the results of extensive research to determine the limit of safety against flame transmission for sintered metal flame arrester elements when stressed by a flashback in a fuel-gas/oxygen mixture and when stressed through a stabilized burning of a flowing mixture of fuel-gas and air and of fuel-gas and oxygen at the sintered metal element. On the basis of the results of these investigations, the limit of safety against flame transmission for sintered metal flame arrester elements can be estimated and the conditions for testing can be specified. An analysis of the protection of gas outlets on gas distribution lines for welding, cutting and allied processes has been carried out, resulting in recommendations for necessary changes to the regulations for testing of safety devices with sintered metal flame arrester elements, which should be made in the next revision of the corresponding technical regulations for acetylene installations and calcium carbide stores (TRAC), and the standards DIN 8521, EN 730 and ISO 5175.     

Limit of safety against flame transmission for sintered metal flame arrester elements in the case of flashback in fuel gas/oxygen mixtures

In autogenous welding, cutting and allied processes, so-called dry safety devices are used to protect the outlet terminals of gas distribution lines. To prevent flame transmission, these units are fitted with sintered metal flame arrester elements. At the Federal Institute for Materials Research and Testing (BAM), extensive investigations have been carried out with an experimental flame arrester to determine the limits of safety against flame transmission for sintered metal flame arrester elements in the case of flashback in fuel gas/oxygen mixtures. On the basis of the results of these investigations, it is possible to estimate the pore sizes up to which a sintered metal element can prevent any flame transmission with a given fuel gas.     

An experimental study of flame acceleration and deflagration to detonation transition in representative process piping

The paper summarizes the results of experimental tests and accompanying analyses to investigate the factors that govern flame acceleration and potential transition to detonation in a relatively long unobstructed piping system. The overall aim of the work was to obtain sufficient experimental data so as to be able to develop and evaluate methodologies for classifying and predicting potential detonation flame acceleration and deflagration to detonation transition (DDT) hazard in industrial process pipes and mixtures. The present results show that the flame acceleration process in an unobstructed pipe exhibit three distinct phases: an initial establishment phase; a second rapid acceleration phase and a final transition to detonation phase. Test results with ethylene indicate that the acceleration process is not sensitive to initial pressure (all other parameters remaining constant) but can be sensitivity to initial pipe wall temperature or possibly mixture humidity. The presence of bends increases the local rate of turbulent combustion, an effect attributed to the additional turbulence generated downstream of the bend. For straight pipes, detonation was only observed to develop for hydrogen–air and ethylene–air mixtures. Detonation was not observed with methane, propane or acetone as fuel in the present piping apparatus.     

Experimental study on detonation flame penetrating through flame arrester

In-line detonation flame arresters are important safety apparatus to prevent group tank fires caused by the spreading of fire through vapor connection lines. In this study, a DN50 experimental apparatus aimed at the detonation flame penetration characteristics and failure mechanisms in a flame arrester was set up, and a series of experiments were carried out with 6.6% C2H4 and air mixture. Pressure, and velocity of flame penetrating through flame arrester housing and filters were analyzed. Experimental results showed that the attenuation of pressure and velocity was proportional to the thickness of the filters. Two failure modes of the fire-extinguishing process in the flame arrester were captured directly with a high-speed camera. In Mode I, the detonation flame could go straight through the flame arrester filters when the filters were too thin. In Mode II, when the filters were not sufficiently thick, the remained shock wave pressure of detonation flame was still several times of the initial pressure and could rise sharply at the downstream contraction section, resulting in that the flammable gas at the downstream transition section could be compressed and reignited even the flame had been extinguished by filters. These conclusions are helpful to reveal the nature of failure modes of fire-extinguishing process and design flame arresters with high fire-resisting performance by structure improved.     

Numerical simulation of propane detonation in medium and large scale geometries

A modelling strategy has been developed for consequence analysis of medium and large scale gaseous detonation. The model is based on the solution of Euler equations with one-step chemistry. The Van Leer flux limited method which is a total variation diminishing scheme is used for shock capturing. Preliminary calculations were firstly conducted for small domains with fine grids which resolve the wave, relatively coarse grids which have less than 10 grids across the wave and coarse grids in which the minimum grid size is larger than the wave thickness to ensure that the reaction scheme has been properly tuned to capture the correct detonation pressure, temperature and velocity in the resolutions used in the different cases. The model was firstly tested against a medium scale detonation test in a shock tube with U-bends. Reasonably good agreement is achieved on detonation pressure and mean shock wave velocities at different measuring segments of the tube. Following the validation, the detonation of a hypothetical planar propane-air cloud is simulated. The predictions uncovered some interesting features of such large scale detonation phenomena which are of significance in the safety context, especially for accidental investigations. The findings from the present analysis are in line with the forensic evidence on damages in some historic accidents and challenges previous analysis of a major accident in which forensic evidence suggested localised detonation but was considered as the consequence of fire storms by the investigation team.     

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.     

瓦斯爆炸火焰波在分叉管路中的传播规律

笔者在研究分叉管路中瓦斯爆炸火焰波传播特征的基础上,分析了管路分叉对瓦斯爆炸过程中火焰传播规律的重要影响。研究结果表明,管道分叉时,管道分叉处为一扰动源,诱导附加湍流,气流湍流度增大,使瓦斯爆炸过程中火焰的传播速度迅速提高,分叉管路支管中火焰在前端是增大的,然后迅速减小;而分叉管直管端口封闭反射对直管管段火焰传播影响很小(端口距各测点较远),火焰在分叉管路直管管段范围是加速的。因此,在矿井开拓中,应尽可能减少不必要的巷道分叉。在巷道分叉处采取保护措施,减少爆炸造成的损失。研究结果对指导现场如何防治瓦斯爆炸,减轻瓦斯爆炸的威力具有重要作用。     

Combustion modeling in large scale volumes using EUROPLEXUS code

Most of the numerical benchmarks on combustion in large scale volumes for hydrogen safety, which were performed up until today have demonstrated, that current numerical codes and physical models experience poor predictive capabilities at the industrial scale, both due to under-resolution and deficiencies in combustion modeling. This paper describes a validation of the EUROPLEXUS code against several large scale experimental data sets in order to improve its hydrogen combustion modeling capabilities in industrial settings (e.g. reactor buildings). The code is based on the Euler equations and employs an algorithm for the propagation of reactive interfaces, RDEM, which includes a combustion wave, as an integrable part of the Reactive Riemann problem, propagating with a fundamental flame speed (being a function of initial mixture properties as well as gas dynamics parameters). Validation of the first combustion model implemented in the code is based on obstacle-laden channels, interconnected reactor-type compartments, vented enclosures and covers all major premixed flame combustion regimes (slow, fast and detonation) with an aim to obtain conservative results. An improvement of this model is found in a direction of transient interaction of flame fronts with regions of elevated integral length scales presented in the velocity gradient field due to e.g. interactions with geometrical non-uniformities and pressure waves.     

Simple method for predicting pressure behavior during gas explosions in confined spaces considering flame instabilities

It is indispensable to predict the pressure behavior caused by gas explosions for the safety management against accidental gas explosions. In this study, a simple method for predicting the pressure behavior during gas deflagrations in confined spaces was examined. Previously the pressure behavior was calculated analytically assuming laminar flame propagation. However, the results of this method often provide underestimation compared with experimental data. It was known the underestimation intensifies as the scale of explosion spaces becomes larger. On the large scale gas deflagration, flame instability (especially hydrodynamic instability) might be more effective and wrinkles appeared on the flame front. Then, the flame surface area was increased and the propagating flame was gradually accelerated. The ordinary prediction methods led to the underestimation because the propagating flame was assumed to be laminar. In this study, we considered the effect of flame wrinkles caused by flame instabilities. By regarding the flame front as a fractal structure, the flame surface area could be modified. Because a flame surface starts to be wrinkled on a certain flame radius, proper determination of the critical flame radius provided accurate prediction of pressure behavior on a large scale deflagration. In addition, correction of the K_G value in a large vessel was discussed.     

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.     

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.     

Evaluating the overall efficiency of a flameless venting device for dust explosions

Results from cornstarch explosion tests using a flameless venting device (mounted over a burst disc) on an 8 m³ vessel are presented and used to determine the overall efficiency of the device, which is defined as the ratio between its effective vent area and the nominal vent area. Because these devices are comprised of an arrestor element mounted over an impulsively-actuated venting device (such as a burst disc), the functional form of the overall efficiency is taken as the product of the area efficiency (i.e., the ratio between the effective vent area of the entire assembly to that of the venting device without the arrestor element) and the burst efficiency (i.e., the ratio of the effective vent area of the venting device without the arrestor element to the nominal vent area). The effective vent areas are calculated from measured overpressures using three different empirical correlations (FM Global 2001, NFPA 2007, and VDI 2002). Furthermore, due to significant variations in the effective reactivity from test to test, a correction factor proportional to the initial flame speed is applied when determining the area efficiency. In general, it was found that the FM Global and NFPA methodologies yield consistent results with less scatter than VDI 3673.     

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.     

Dust explosion venting of small vessels and flameless venting

Dust explosion venting is an established method of protecting against damaging explosion over-pressures, and guidance is available for many industrial situations. However, there is a need to: (a) establish the venting requirements of small vessels and whether current guidance and predictions in BS EN 14491:2006 need revising, and (b) improve understanding of the potential and limitations of flameless venting. This paper describes initial results from an ongoing programme of research.Small vessel tests are carried out using cornflour and wood dust on: a commercial sieve unit, a commercial cyclone, and a 0.5 m³ test vessel with explosion-relief openings without vent covers. Initial 0.5 m³ vessel tests give reduced explosion pressures that are lower than those predicted. This is because the predicted pressures are based on openings with vent covers. The reduced explosion pressures measured in the sieve unit and the cyclone are also less than predicted: the reasons are discussed.Flameless vesting tests are carried out using cornflour and wheat flour on a commercial flame arrestor unit. Initial tests demonstrate benefits, particularly a high level of flame extinguishment, but a problem of reduced venting efficiency compared to conventional venting.These initial results indicate that further research is needed.     

Experimental investigation of hydrogen–air deflagrations and detonations in semi-confined flat layers

This paper presents results of an experimental investigation on the deflagration and deflagration-to-detonation transition (DDT) in an obstructed (blockage ratio BR = 50%), semi-confined flat layer filled with uniform hydrogen–air mixtures. The effect of mixture reactivity depending on flat layer thickness and its width is studied to evaluate the critical conditions for sonic flame propagation and the possibility for detonation onset. The experiments were performed in a transparent, rectangular channel with a length of 2.5 m. The flat layer thickness was varied from 0.06 to 0.24 m and the experiments were performed for different channel widths of 0.3, 0.6 and 0.9 m. The experimental results show flame velocity vs. hydrogen concentration for different thicknesses and widths of the semi-confined flat layer. Three different flame propagation regimes were observed: slow subsonic flame (M << 1), sonic deflagration (M ～ 1) and detonation (M >> 1). It is shown that flame acceleration (FA) to sonic speed is independent of the width of the flat layer. The critical expansion ratio for effective flame acceleration to sonic speed was found to be linearly dependent on the reciprocal layer thickness.     

Experimental investigation of fast flame propagation in stratified hydrogen–air mixtures in semi-confined flat layers

This paper presents results of an experimental investigation on fast flame propagation and the deflagration-to-detonation transition (DDT) and following detonation propagation in a semi-confined flat layer filled with stratified hydrogen–air mixtures. The experiments were performed in a transparent, rectangular channel open from below. The combustion channel has a width of 0.3 m and a length of 2.5 m. The effective layer thickness in the channel was varied by using different linear hydrogen concentration gradients. The method to create quasi-linear hydrogen concentration gradients that differ in the range and slope is also presented. The ignited mixtures were accelerated quickly to sonic flame speed in the first obstructed part of the channel. The interaction of the fast flame propagation with different obstacle set-ups was studied in the second part of the channel. The experimental results show an initiation of DDT by one additional metal grid in the obstructed semi-confined flat layer. Detonation propagation and failed detonation propagation were observed in obstructed and unobstructed parts of the channel.     

安全帽阻燃性能测试方法研究与探讨

本文采用GB/T2812-2006《安全帽测试方法》提供的方法对安全帽的阻燃性能进行了测试,测试结果表明,采用该方法,同一顶安全帽得到了两种不同的实验结果:一种实验结果为"合格",另一种实验结果为"不合格"。进而研究发现,标准仅对火焰的长度和颜色作了简单的要求,并未对燃气压力、火焰温度等作出明确的要求,从而导致同一顶安全帽有两种不同的实验结果。建议在以后的标准修订中,对试验用气体的纯度和实验时气体的压力有所要求,从而确保测试结果的唯一性和可重复性。     

水平管内不同煤质煤尘爆炸火焰传播特性实验研究

为研究水平管道空间不同煤质煤尘爆炸火焰传播特性，选取褐煤、长焰煤、不粘煤、气煤4种煤尘，对爆炸火焰焰峰特性、火焰加速传播特性、火焰传播距离与持续时间展开研究。研究结果表明:褐煤在500 ms内焰峰的形状由尖锐向平滑再向钝化不断演变，长焰煤与不粘煤在375 ms时焰峰前端出现明显焰体分离现象，分析认为这与管体冷壁效应、空间尺度效应及空间氧气消耗直接相关；气煤在375 ms时焰峰出现大面积火焰碎纹，说明气煤爆炸火焰猛烈传播的持续时间相对较短，整体爆炸强度相对较弱；褐煤与长焰煤爆炸火焰存在2次间断性加速，分析认为这与管体空间受限、常温管壁散热、局部助燃氧气瞬间不足等因素有关；褐煤在爆炸后400～600 ms内火焰2次加速完全，火焰传播距离达740 mm，明显大于长焰煤、不粘煤与气煤，说明低变质褐煤爆炸火焰持续时间更长，火焰传播距离更远且传播更剧烈；虽然气煤火焰最远传播距离比长焰煤大30 mm，但由于气煤火焰在375 ms左右出现大片火焰碎纹，因此气煤整体的爆炸强度小于长焰煤。