Flame behaviors and pressure characteristics of vented dust explosions at elevated static activation overpressures

Dust explosion venting experiments were performed using a 20-L spherical chamber at elevated static activation overpressures larger than 1 bar. Lycopodium dust samples with mean diameter of 70 μm and electric igniters with 0.5 KJ ignition energy were used in the experiments. Explosion overpressures in the chamber and flame appearances near the vent were recorded simultaneously. The results indicated that the flame appeared as the under-expanded free jet with shock diamonds, when the overpressure in the chamber was larger than the critical pressure during the venting process. The flame appeared as the normal constant-pressure combustion when the pressure venting process finished. Three types of venting processes were concluded in the experiments: no secondary flame and no secondary explosion, secondary flame, secondary explosion. The occurrence of the secondary explosions near the vent was related to the vent diameter and the static activation overpressure. Larger diameters and lower static activation overpressures were beneficial to the occurrence of the secondary explosions. In current experiments, the secondary explosions only occurred at the following combinations of the vent diameter and the static activation overpressure: 40 mm and 1.2 bar, 60 mm and 1.2 bar, 60 mm and 1.8 bar.     

Effect of flame propagation regime on pressure evolution of nano and micron PMMA dust explosions

Experiments using an open space dust explosion apparatus and a standard 20 L explosion apparatus on nano and micron polymethyl methacrylate dust explosions were conducted to reveal the differences in flame and pressure evolutions. Then the effect of combustion and flame propagation regimes on the explosion overpressure characteristics was discussed. The results showed that the flame propagation behavior, flame temperature distribution and ion current distribution all demonstrated the different flame structures for nano and micron dust explosions. The combustion and flame propagation of 100 nm and 30 μm PMMA dust clouds were mainly controlled by the heat transfer efficiency between the particles and external heat sources. Compared with the cluster diffusion dominant combustion of 30 μm dust flame, the premixed-gas dominant combustion of 100 nm dust flame determined a quicker pyrolysis and combustion reaction rate, a faster flame propagation velocity, a stronger combustion reaction intensity, a quicker heat release rate and a higher amount of released reaction heat, which resulted in an earlier pressure rise, a larger maximum overpressure and a higher explosion hazard class. The complex combustion and propagation regime of agglomerated particles strongly influenced the nano flame propagation and explosion pressure evolution characteristics, and limited the maximum overpressure.     

超细CaCO3惰化剂对铝合金抛光伴生粉尘爆炸的防控效果*

为探究超细粉体惰化剂对铝合金抛光伴生粉尘爆炸特性的影响规律,利用标准化实验装置及自行搭建的实验平台,在对爆炸基本参数进行测试的基础上,分别研究超细CaCO3粉体对抛光废弃物粉尘点燃敏感度的钝化作用以及对爆炸火焰传播进程的惰化效果,并在相同条件下与同等粒径高纯度铝粉的实验效果进行比对。研究结果表明:铝合金抛光废弃物粉尘最小点火能量为280 mJ,而同等粒径高纯度铝粉最小点火能量为35 mJ;在铝合金抛光废弃物粉尘质量浓度为300 g/m3条件下,发生爆炸的火焰传播速度峰值为7.4 m/s,约为高纯度铝粉的57%,铝合金抛光废弃物粉尘的爆炸敏感度及猛烈度均低于高纯度铝粉;当超细CaCO3粉体的惰化比为30%时,可将铝合金抛光废弃物粉尘的最小点火能量钝化至约1 J,爆炸火焰失去持续传播能力,惰化作用效果充分显现。     

Buncefield: A violent, episodic vapour cloud explosion

Damage caused by the 2005 Buncefield explosion indicates pressures in excess of 2000 mbar over all of the area covered by the vapour cloud. Such high overpressures are normally associated with high (super-sonic) rates of flame spread. On the other hand, evidence from witnesses, building damage analysis and CCTV cameras all suggest the average rate of progress of the explosion flame front was only around 150 m/s.The high overpressures in the cloud and low average rate of flame advance can be reconciled if the rate of flame advance was episodic, with periods of very rapid combustion being punctuated by pauses when the flame advanced very slowly. The widespread high overpressures were caused by the rapid phases of combustion; the low average speed of advance was caused by the pauses.Mechanisms of flame spread through radiative ignition of particulates ahead of the flame front provide possible explanations for such unusual episodic behaviour.The first part of this paper reviews a wide range of empirical evidence on average flame speed and rate of blast pressure increase.The second part explores the theoretical consequences of forward radiation and how the new theory might be developed into a practical means of assessment.     

CO2-超细水雾对瓦斯/煤尘爆炸抑制特性研究

为了解CO2-超细水雾对瓦斯/煤尘爆炸抑制特性,用自行搭建的实验系统,从超压、火焰传播速度和火焰结构3个方面研究了CO2-超细水雾形成的气液两相介质对9.5%瓦斯/煤尘复合体系爆炸的抑爆效果、影响因素与原因。研究结果表明:随着CO2体积分数和超细水雾质量浓度的增加,爆炸火焰最大传播速度、爆炸超压峰值均出现明显下降,火焰到达泄爆口时间显著延迟;尤其当CO2体积分数达到14%与超细水雾的共同抑爆效果凸显,瓦斯/煤尘复合体系爆炸超压的“震荡平台”消失,同时火焰结构呈现“整体孔隙化”。所得结论为煤矿井下高效防爆抑爆技术进行了完善和增强。     

Numerical study on premixing characteristics and explosion process of starch in a vertical pipe under turbulent flow

Fire and explosion accidents are frequently caused by combustible dust, which has led to increased interest in this area of research. Although scholars have performed some research in this field, they often ignored interesting phenomena in their experiments. In this paper, we established a 2D numerical method to thoroughly investigate the particle motion and distribution before ignition. The optimal time for the corn starch dust cloud to ignite was determined in a semi-closed tube, and the characteristics of the flame propagation and temperature field were investigated after ignition inside and outside the tube. From the simulation, certain unexpected phenomena that occurred in the experiment were explained, and some suggestions were proposed for future experiments. The results from the simulation showed that 60–70 ms was the best time for the dust cloud to ignite. The local high-temperature flame clusters were caused by the agglomeration of high-temperature particles, and there were no flames near the wall of the tube due to particles gathering and attaching to the wall. Vortices formed around the nozzle, where the particle concentration was low and the flame spread slowly. During the explosion venting, particles flew out of the tube before the flame. The venting flame exhibited a “mushroom cloud” shape due to interactions with the vortex, and the flame maintained this shape as it was driven upward by the vortex.     

Experiment-based investigations on the effect of ignition energy on dust explosion behaviors

Explosion behaviors of typical light metal and carbonaceous dusts induced by different ignition energies were investigated based on systematic experiments in a Siwek 20 L vessel. Comparative analysis reveals that the explosion mechanism of carbonaceous dust is the volatile combustion, whereas the mechanism for light metal dust mainly features the surface heterogeneous oxidation. Influences of ignition energy on severity and flammability limit are much more significant for carbonaceous dust than light metal, especially for the powder with less volatile. An innovative approach was introduced to derive flame thickness from the pressure–time trace. The relation between explosion induction time and combustion duration of ignitor was also analyzed. Results show inappropriate ignition energy will cause under-/over-driving in the thermodynamic/kinetic characteristic measurements. In this way, a dimensionless parameter pressure ratio was introduced to evaluate the under-driving, while two methods by using flame thickness and induction time respectively, were proposed to evaluate over-driving. To improve the accuracy of dust explosion tests, authors advocate that explosion severity determination should be conducted at the critical ignition energy. Moreover, a comparison between the European and Chinese flammability limit determination procedures was also conducted, indicating that EN 14034-3 is suitable for light metal but not for carbonaceous, while GB/T 16425 appears to be slightly conservative for both carbonaceous and light metal dusts.     

Effects of premixed methane concentration on distribution of flame region and hazard effects in a tube and a tunnel gas explosion

Study of flame distribution laws and the hazard effects in a tunnel gas explosion accident is of great importance for safety issue. However, it has not yet been fully explored. The object of present work is mainly to study the effects of premixed gas concentration on the distribution law of the flame region and the hazard effects involving methane-air explosion in a tube and a tunnel based on experimental and numerical results. The experiments were conducted in a tube with one end closed and the other open. The tube was partially filled with premixed methane-air mixture with six different premixed methane concentrations. Major simulation works were performed in a full-scale tunnel with a length of 1000 m. The first 56 m of the tunnel were occupied by methane–air mixture. Results show that the flame region is always longer than the original gas region in any case. Concentration has significant effects on the flame region distribution and the explosion behaviors. In the tube, peak overpressures and maximum rates of overpressure rise (dp/dt)_max for mixtures with lower and higher concentrations are great lower than that for mixtures close to stoichiometric concentration. Due to the gas diffusion effect, not the stoichiometric mixture but the mixture with a slightly higher concentration of 11% gets the highest peak overpressure and the shock wave speed along the tube. In the full-scale tunnel, for fuel lean and stoichiometric mixture, the maximum peak combustion rates is achieved before arriving at the boundary of the original methane accumulation region, while for fuel rich mixture, the maximum value appears beyond the region. It is also found that the flame region for the case of stoichiometric mixture is the shortest as 72 m since the higher explosion intensity shortens the gas diffusion time. The case for concentration of 13% can reach up to a longest value of 128 m for longer diffusion time and the abundant fuel. The “serious injury and death” zone caused by shock wave may reach up to 3–8 times of the length of the original methane occupied region, which is the widest damage region.     

Inerting effect of CaCO3 powder on flame spread of wood dust layer

The effect of CaCO₃ powder, a typical inert dust, on the flame spread characteristics of wood dust layers was studied using an experimental device to understand the ignition characteristics of and develop inert explosion-proof technology for deposited wood dust. The results showed that the flame spread velocity (FSV) of the mixed dust layer was affected by the dispersion effect of CaCO₃ powder and physical heat absorption. As the CaCO₃ powder mass fraction increased, the FSV of the dust layer first increased and then decreased, reaching a peak at a 50% mass fraction. Moreover, the front-end temperature of the flame gradually decreased, and the red spark faded. The combustion reaction of the mixed dust layer could be more completed, and the colour of the combustion residue changed from charcoal black to charcoal grey. The coupling effect of the initial temperature and wind speed can promote an increase in the FSV in the mixed dust layer. The Gauss–Amp model of the FSV of the wood dust layer and mass fraction of CaCO₃ powder showed that the peak of the FSV occurred when the mass fraction of CaCO₃ powder was between 40 and 50%. Thus, a good inerting and explosion-proof effect can be achieved by using CaCO₃ powder with a mass fraction of more than 50%; it can improve the whole inerting process. Inert explosion-proof technology should be considered when assessing fire and explosion risk of dust in real process industry situations.     

彩虹粉引燃危险性实验研究

为了研究彩虹粉引燃危险性,应用固体燃烧速率试验仪初步甄别了彩虹粉传播燃烧能力,发现堆垛状彩虹粉固体火焰传播危险性较低;采用粉尘爆炸筛选装置,判定彩虹粉具有爆炸性;应用最小点火能测定装置测定彩虹粉粉尘云的最小点火能在24~60 mJ之间,最优爆炸浓度为1 167 g/m3;应用快速筛选量热仪测试,彩虹粉在227℃开始分解;固体自燃点测试仪显示彩虹粉在250℃附近会发生自燃。向彩虹粉内添加不同比例相近粒径分布的食用盐粉体进行抑爆研究,结果证明食用盐对彩虹粉具有明显的抑爆效果。     

高炉喷吹用潞安贫瘦煤爆炸下限与返回火焰长度的试验研究

该试验通过测定爆炸下限与返回火焰长度这两个参数来确定4种煤粉的爆炸性。爆炸下限指能使喷入一定装置中的粉尘云点燃并维持火焰传播的最小粉尘浓度,是确定粉尘爆炸性重要参数,试验室通常使用20L的爆炸装置进行测定。喷吹现场广泛采用长管式煤粉爆炸性测试仪检测煤尘引燃后产生的返回火焰长度,该长度随煤粉爆炸性的强弱而显著变化:返回火焰长度大于600 mm可认定该煤粉具有强爆炸性;在400~600 mm之间则煤粉具有中强度爆炸性;小于400 mm则煤粉具有弱爆炸性。结果表明:20 L球测得4种煤粉的爆炸下限在60~85 g/m3之间;长管式煤粉爆炸性测定仪测得4种煤粉的返回火焰长度在20~50 mm之间。由测定的返回火焰长度可知,试验所用的4种煤样均属于弱爆炸性煤种。     

Law of variation for low density polyethylene dust explosion with different inert gases

To reveal the effects of different inert gases on explosion characteristics during low density polyethylene (LDPE) dust explosion and optimize the explosion-proof process, eight N₂ (CO₂)/air mixed inerting conditions were experimentally studied. Typical inerting conditions with 12 L cylindrical explosive tank were used to study the characteristics on the flame propagation. The thermogravimetric analysis with related theories were used to further explain the mechanism and quantities in low density polyethylene (LDPE) dust explosion with different inert gases. The results showed that the reduction of O₂ concentration could effectively delay the progress of flame growth process and weaken the effect of dust combustion reaction. The flame growth process of condition (N₂/air (18% O₂)) was 2.05 times slower than that of the non-inert condition. The explosion strength was obviously reduced, and the characteristic parameters such as explosion pressure and flame propagation speed were also affected by the decrease of O₂ concentration. For LDPE powder, the smaller the median diameter, the greater the explosion intensity and the lower the limiting oxygen content (LOC). The LOC with CO₂ was usually higher than that with N₂ and the effect of CO₂ was significantly better than N₂ in inerting.     

Study on the deflagration reaction process of oil gas in long-narrow confined space based on PLIF and flame spectra

Oil gas explosion in long-narrow confined space is a typical unsteady combustion process. To study the reaction process, two experiment techniques are adopted in this research. One is planner laser induced fluorescence, which is used to achieve the transient measurement of free radicals in unsteady premixed combustion field. The other one is the spectral testing technique, which is used to measure the luminescence spectrum characteristics of oil gas combustion flame. The distribution of OH radical and CH radical at different positions in the combustion field, the common partition of deflagration flame structure under different oil gas concentrations, and the main luminescence spectra of radicals such as OH, CH, C₂, C₃, CO₂, H₂O and HCO are obtained. By comparing the above three aspects, the combustion reaction process of premixed mixture is revealed, driven by the coupling effect of chemical reaction and fluid flow in the process of explosion propagation. The process can be described briefly as follows. In the “outer flame zone”, large hydrocarbon molecules are mainly transformed to small molecules and free radicals by means of pyrolysis, dehydrogenation and oxidation. In the “inner flame zone”, carbon particles and combustion products produced and gathered after relevant reactions.     

Inerting characteristics of ultrafine Mg(OH)2 on starch dust explosion flame propagation

Experiments on the flame propagation of starch dust explosion with the participation of ultrafine Mg(OH)₂ in a vertical duct were conducted to reveal the inerting evolution of explosion processes. Combining the dynamic behaviors of flame propagation, the formation law of gaseous combustion products, and the heat dissipation features of solid inert particles, the inerting mechanism of explosion flame propagation is discussed. Results indicate that the ultrafine of Mg(OH)₂ powders can cause the agglomeration of suspended dust clouds, which makes the flame combustion reaction zone fragmented and forms multiple small flame regions. The flame reaction zone presents non-homogeneous insufficient combustion, which leads to the obstruction of the explosion flame propagation process and the obvious pulsation propagation phenomenon. As the proportion of ultrafine Mg(OH)₂ increases, flame speed, flame luminescence intensity, flame temperature and deflagration pressure all show different degrees of inerting behavior. The addition of ultrafine Mg(OH)₂ not only causes partial inerting on the explosion flame, but also the heat dissipation of solid inert particles affects the acceleration of its propagation. The explosion flame propagation is inhibited by the synergistic effect of inert gas-solid phase, which attenuates the risk of starch explosion. The gas-solid synergistic inerting mechanism of starch explosion flame propagation by ultrafine Mg(OH)₂ is further revealed.     

多元可燃气体爆炸极限理论预测模型研究*

为准确掌握和预测多元可燃气体的爆炸极限,开展2种多元可燃气体爆炸极限的理论预测模型研究。第1种模型针对“多种可燃气体+多种惰性气体”在空气中或氧气中混合,基于求解可燃气体绝热火焰温度的总比热特性方法以及化学平衡反应中的贫燃料(富氧)反应,提出该多元可燃气体的爆炸下限预测模型;第2种模型针对“可燃气体+惰性气体+氧气”混合,基于热平衡方程及混合气体的各组分浓度、淬灭电势及燃烧潜热,提出该多元可燃气体的爆炸极限预测模型。结果表明:在预测多元可燃气体的爆炸极限时,第1种模型具有较广泛的应用性,且表现出较高的准确度;第2种模型具有使用简单的特点,且扩展了LCR（勒夏特列原理）的应用范围。     

煤尘爆炸性鉴定中瞬间火焰长度检测技术的研究

确定煤尘爆炸性质需要检测其燃烧瞬间的火焰长度,现有的大管状煤尘爆炸性鉴定系统以肉眼观察作为主要检测手段,存在测量精度差、重复性误差大以及人为干扰因素多等问题,提出将数字图像处理技术引入到煤尘爆炸性鉴定系统的方法即通过采用数字图像处理技术,对煤尘火焰图像进行预处理及图像分割与边缘检测后,得到最大火焰长度特征参数,并推导出煤尘爆炸的判断依据。     

An investigation on the dust explosion of micron and nano scale aluminium particles

A series of dust explosion were conducted to compare the flame structure between nano and micron aluminium dusts. Two-color pyrometer technique is applied to have qualitative observation of flame development. Measurement of temperature indicates that explosion in micron aluminium dust clouds start in a single spot at 3000 K, in contrast, explosion in nano aluminium dust clouds start when hot powder accumulated to a certain amount at lower temperature of 2600 K. For micron aluminium dust clouds, flame at leading edge has the highest temperature and propagates in all directions. On the other hand, flame in nano aluminium dust clouds propagate only upward with the hottest part left behind at the downside. As flame propagates, the temperature at top edge gradually decreases from 2600 K to finally 2000 K, but temperature at bottom edge maintains in 3000 K with no significant displacement. The unevenness of flame structure is considered as the consequence of different particle densities, which suggests that the reaction of nano aluminium particles stays in molten state, meanwhile, the high surface area also leads to unignorable heat loss.     

直线管道煤尘爆炸火焰传播规律的试验研究

为提供煤尘爆炸事故预防和缓解所需的科学依据,对煤尘爆炸火焰传播过程进行试验研究。所用试验装置,其主要部分为直径0.3 m的圆形管道与断面边长为80 mm的方形管道对接形成的一个长2 m的爆炸腔体。在其中共进行9次煤尘爆炸试验。结果表明,煤尘爆炸火焰传播具有速度快,波动大,稳定性较差的特点,火焰区长度远大于扬尘区长度,最大火焰速度和传播距离与煤尘量均不存在正比例关系,但存在一个特定的煤尘质量浓度。在这个特定质量浓度处,最大火焰速度达到最大值。当煤尘质量浓度大于这个特定质量浓度时,火焰传播速度曲线整体下降,暂时缺氧被认为是导致这一情况的重要因素。     

混合金属粉尘爆炸最小点火能量影响因素研究*

为探究混合金属粉尘爆炸危险性及与单一粉体爆炸特性差异,确保车间安全生产,采用粉尘云点火能量测试系统对车间混合金属粉尘及铝粉最小点火能量在不同影响因素下的变化规律及2种粉尘火焰变化特征进行测试。研究结果表明:混合金属粉尘和铝粉最小点火能量在一定范围内（38~96 μm）与粒径呈正相关性,当混合金属粉尘粒径大于75 μm时,所需最小点火能量大于1 000 mJ,其爆炸敏感性迅速降低,此时铝粉仍有较强爆炸敏感性;2种粉尘最小点火能量随质量浓度增加呈先降低后升高的趋势,最小点火能分别为295,15 mJ,对应的敏感质量浓度为600,1 000 g/m3,混合金属粉尘在质量浓度为500~700 g/m3时具有较大爆炸危险性;同铝粉相比,混合金属粉尘点火能量更高、火焰燃烧时间更短、火焰高度更低、爆炸剧烈程度更弱。     

Experimental investigation on explosion flame propagation of wood dust in a semi-closed tube

In order to explore flame propagation characteristics during wood dust explosions in a semi-closed tube, a high-speed camera, a thermal infrared imaging device and a pressure sensor were used in the study. Poplar dusts with different particle size distributions (0–50, 50–96 and 96–180 μm) were respectively placed in a Hartmann tube to mimic dust cloud explosions, and flame propagation behaviors such as flame propagation velocity, flame temperature and explosion pressure were detected and analyzed. According to the changes of flame shapes, flame propagations in wood dust explosions were divided into three stages including ignition, vertical propagation and free diffusion. Flame propagations for the two smaller particles were dominated by homogeneous combustion, while flame propagation for the largest particles was controlled by heterogeneous combustion, which had been confirmed by individual Damköhler number. All flame propagation velocities for different groups of wood particles in dust explosions were increased at first and then decreased with the augmentation of mass concentration. Flame temperatures and explosion pressures were almost similarly changed. Dust explosions in 50–96 μm wood particles were more intense than in the other two particles, of which the most severe explosion appeared at a mass concentration of 750 g/m³. Meanwhile, flame propagation velocity, flame propagation temperature and explosion pressure reached to the maximum values of 10.45 m/s, 1373 °C and 0.41 MPa. In addition, sensitive concentrations corresponding to the three groups of particles from small to large were 500, 750 and 1000 g/m³, separately, indicating that sensitive concentration in dust explosions of wood particles was elevated with the increase of particle size. Taken together, the finding demonstrated that particle size and mass concentration of wood dusts affected the occurrence and severity of dust explosions, which could provide guidance and reference for the identification, assessment and industrial safety management of wood dust explosions.