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，经常出现在油漆制造、制药工业、煤矿和谷物升降机等化学和加工作业中。对粉尘和气体-空气爆炸的研究已经很广泛，因此人们对它们有很好的理解，但混合混合物爆炸还没有得到广泛的研究，因此关于它们的数据相当稀缺。它们构成了重大危险，因为它们的组合爆炸下限(LEL)已被证明低于单一组分的爆炸下限。 这项研究拨款的总体目标是：1)阐明混合混合物爆炸中的火焰传播机制；2)更好地了解和量化影响粉尘火焰的最小可爆炸浓度(MEC)和混合混合物的能级的因素；以及3)预测爆炸和非爆炸之间的边界。 意大利研究人员为混合混合物确定了五种不同的模式：1)双燃料爆炸，其中气体浓度高于其可燃下限(LFL)，而粉尘浓度高于其MEC；2)粉尘爆炸，其中粉尘高于其MEC，而气体低于其LFL；3)气体驱动爆炸，其中粉尘低于其MEC，气体高于其LFL；4)协同爆炸，其中粉尘和气体均低于其下限；以及5)粉尘和气体均低于其下限且未发生爆炸的区域。 气体和尘埃火焰的传播已经得到了广泛的研究。然而，尚未对粉尘驱动、气体驱动、双燃料和协同作用四种爆炸模式中的混合燃料火焰传播进行全面的研究。同样，对于协同爆炸也没有给出明确的解释，也没有明确界定协同爆炸和无爆炸之间的界限。粉尘的MEC是复杂的，因为它取决于粉尘的粒度分布、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它的设备。更复杂的是，混合混合物的爆炸下限(LEL)除了取决于影响粉尘MEC的所有因素外，还取决于气体的性质。 将使用两种新技术来研究不同区域的火焰传播机制。第一种是独特的爆炸室，它通过升华在混合物的LEL附近创造出一种不同气体/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被释放并形成干冰一样。第二种是一个特殊的喷雾室，通过在冷的可燃气体中快速冷却液滴来形成混合混合物。更清楚地了解混合混合物爆炸将导致更好的风险估计和改进的缓解方案，从而提高加工厂的安全性。

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.