Flame Propagation Mechanisms in Hybrid-Mixture Explosions

混合混合物，即可燃气体和可燃粉尘的混合物，经常出现在化学和加工操作中，如油漆制造、制药工业、煤矿和谷物升降机。对粉尘和气体-空气爆炸的研究已经很广泛了，所以我们对它们很了解，但是对混合-混合爆炸的研究还没有那么广泛，所以关于它们的数据相当稀少。它们构成重大危险，因为它们的组合爆炸下限（LEL）已被证明低于单个组件的下限。******本研究经费的总体目标是：1)阐明混合爆炸中火焰传播的机制；2)更好地理解和量化影响粉尘火焰最小爆炸浓度（MEC）和混杂混合物LEL的因素；3)预测爆炸与不爆炸的边界。******意大利研究人员确定了混合燃料的五种不同模式：1)双燃料爆炸，气体浓度高于其易燃下限（LFL），粉尘浓度高于其MEC；2)粉尘在MEC以上，气体在LFL以下的粉尘驱动爆炸；3)粉尘低于MEC，气体高于LFL的气体驱动爆炸；4)粉尘和气体均低于其下限的协同爆炸；5)尘埃和气体均低于其下限且不发生爆炸的区域。******气体和粉尘的火焰传播都得到了广泛的研究。然而，目前还没有对四种爆炸模式（粉尘驱动、气体驱动、双燃料和协同爆炸）中的混合火焰传播进行全面的研究。同样，对协同爆炸也没有给出明确的解释，也没有确定协同爆炸和不爆炸之间的界限。粉尘的MEC是复杂的，因为它取决于颗粒大小分布，粉尘的性质，粉尘云中的湍流程度，点火类型和强度以及测量设备。更复杂的是，除了影响粉尘MEC的所有因素外，杂化混合物的爆炸下限（LEL）还取决于气体的性质。******火焰在不同状态下的传播机制将使用两种新技术进行研究。第一个是一个独特的爆炸室，通过升华产生接近混合物LEL的不同气体/粉尘浓度的混合混合物，即蒸汽直接变成固体，就像二氧化碳灭火器排放并形成干冰一样。第二种是一种特殊的喷雾室，通过快速冷却冷可燃气体中的液滴来形成混合混合物。更清楚地了解混合-混合爆炸将有助于更好地估计风险并改进缓解方案，从而提高加工工厂的安全性。

Hybrid mixtures, i.e. mixtures of flammable gas and combustible dust, often occur in chemical and processing operations like paint manufacturing, pharmaceutical industries, coal mines, and grain elevators. Research into dust- and gas-air explosions has been extensive so they are well understood, but hybrid-mixture explosions have not been as extensively investigated so data on them is rather scarce. They pose a significant hazard because their combined Lower Explosion Limit (LEL) has been shown to lie below that of the single components.******The overall objectives of this research grant are to: 1) elucidate the mechanisms of flame propagation in hybrid mixture explosions; 2) better understand and quantify the factors that influence the minimum explosible concentration (MEC) of dust flames and the LEL of hybrid mixtures; and 3) predict the boundary between explosion and no-explosion.******Italian researchers identified five distinct regimes for hybrid mixtures: 1) dual-fuel explosions where the gas concentration is above its lower flammable limit (LFL) and the dust concentration is above its MEC; 2) dust-driven explosions where the dust is above its MEC and the gas is below its LFL; 3) gas-driven explosions where the dust is below its MEC and the gas is above its LFL; 4) synergistic explosions in which both dust and gas are below their lower limits; and 5) a region where both dust and gas are below their lower limits and no explosion occurs.******Both gas- and dust flame propagation have been extensively studied. However, no comprehensive study of hybrid-mixture flame propagation in each of the four explosion regimes – dust-driven, gas-driven, dual fuel, and synergistic – has been undertaken. Likewise, no definitive explanation of synergistic explosions has been given nor has the boundary between synergistic explosions and no explosion been conclusively defined. The MEC of dusts is complex since it depends upon the particle size distribution, nature of the dust, turbulence level in the dust cloud, and ignition type and strength as well as the equipment in which it is measured. To further complicate matters, the lower explosion limit (LEL) of hybrid mixtures also depends upon the nature of the gas in addition to all the factors that influence the MEC of dusts.******Flame propagation mechanisms for the different regimes will be investigated using two novel techniques. The first is a unique explosion chamber that creates a hybrid mixture of varying gas/dust concentrations close to the LEL of the mixture by sublimation, i.e. where the vapour turns directly to solid just like when a CO2 fire extinguisher is discharged and forms dry ice. The secondly is a special spray chamber to form hybrid mixtures by rapid cooling of liquid droplets in a cold flammable gas. A clearer understanding of hybrid-mixture explosions will lead to better estimates of risk and improved mitigation protocols thereby leading to enhanced safety in process plants.