Effect of phase changes on metal-particle combustion processes

This paper summarizes a series of experimental studies addressing combustion of single metal particles. Sets of free-falling monodisperse molten metal droplets were formed at repeatable initial temperatures and velocities in a pulsed microarc discharge ignited between a cold tool cathode and a consumable wire anode. The droplets formed in oxygenated environments immediately ignited and burned, while their temperature histories were studied using optical pyrometry. Burning particles were quenched at different combustion times using techniques providing variable cooling rates. Analyses of the quenched samples were used to recover the evolution of burning particle compositions for different metals. Experiments were conducted with Al, Mg, Zr, Ti, Ta, W, Mo, Fe, and Cu particles. In addition, similar combustion experiments were carried out with boron particles produced using an oxygen-acetylene torch melting an edge of a vibrating boron filament. Most of the combustion experiments were conducted in air, while argon–oxygen, helium–oxygen, and carbon-dioxide environments were also used in some tests. A limited number of experiments on aluminum-particle combustion were conducted in microgravity. The experiments were aimed at identifying correlations between the burning particle temperature and composition histories. Dissolution of oxygen and other gases was observed to occur within the burning metal, leading to phase changes coinciding with sudden changes in metal combustion regimes. Equilibrium metal–gas phase diagrams were used to interpret the experimentally observed metal combustion behavior. Based on the experimental results, an expanded mechanism of metal combustion was suggested, emphasizing reactions and phase changes occurring within the burning metal in addition to reactions occurring on and above the metal surface.