Disruptive events in combustion of metal particles

Foundational metal combustion studies characterize combustion of individual particles. In experiments, disruptive events occur commonly, including microexplosions, rapid changes in brightness and color of the burning particle, and sudden changes in its trajectory. This paper reviews related literature and discusses the current level of our understanding the mechanisms and processes leading to such disruptive phenomena. It is concluded that the observed disruptions are associated with phase changes occurring in the burning metal particles. In most cases, a metastable phase is produced during combustion and its subsequent relaxation results in a disruptive event. The mechanisms of formation of the metastable phases are complex and can differ substantially for different metals and different combustion environments. Three main reaction pathways are considered leading to disruptive combustion events. Different boiling points of the metals in an alloy or a burning metal and its combustion products can cause selective nucleation and boiling of the low-boiling component. A phase change involving a supersaturated metal-gas solution involving release of excess gas (often, nitrogen) can expand and explode a burning particle. Finally, an asymmetry in the standoff flame and in the particle structure itself can generate temperature and composition gradients in the burning particle disrupting its combustion.