Implications of droplet breakup and formation of ultra fine mist in blast mitigation

Blast-induced droplet breakup producing ultra fine water mist process was examined in view of assessing its implications on blast mitigation. An earlier review proposed that droplet breakup process, amongst other implications, may weaken the shock due to breakup energy absorption. In this work, droplet breakup energies for water droplets have been determined from the surface energies of both parent and child droplets. A breakup energy of 18 J/kg was required to fragment a 0.5 mm parent droplet into 10,000 mono-dispersed child droplets. Compared to the vaporization energy of 2.25 MJ/kg, the droplet breakup energy was found not significant in weakening the shock. While the droplet deformation energy and curvature effects could increase the breakup energy, its overall contribution to the total energy extraction was not as significant as the latent heat of vaporization. Further, the analysis shows about 22-fold increase in surface area of the child droplets. The study revealed the surface-to-volume ratio of the ultra fine droplets and their vaporization timescale should be well positioned for shock energy extraction.     

超细水雾影响小型火焰在固体可燃物上蔓延的数值模型

开发了CFD模型，用来预测细水雾中火焰沿固体燃料蔓延的特性。采用气体和固体燃料作为对比试验，利用不稳定两维保守公式描述自持性火焰蔓延情况。因为的分析重点主要放在火焰前锋火焰的熄灭机理（火焰前锋完全暴露在细水雾中），所以考虑了有限率（finite—rate）化学反应。火焰基本传播数据也可虑了用于细水雾和蒸气质量的公式，这包括水蒸发造成的能量消耗。还对在细水雾喷洒下聚合材料做成的厚燃料床上火焰的水平传播进行了试验。结果显示，火焰热释放区内自持性能量守恒对外部能量消耗非常敏感。在本试验中，主要是水滴蒸发造成的能量消耗。因此，在火焰前锋，火焰在细水雾的喷洒下挣扎着，要么按几乎相同的速度（没有水喷雾情况下）继续传播，要么被完全熄灭。本文还研究了水滴直径大于30μm细水雾的灭火特性。本文获得了在不同条件下火焰蔓延情况下，灭火需要的细水雾质量分率的关键浓度。