On the mechanism of asymmetric aluminum particle combustion

Abstract

The results of an experimental study of the combustion of single aluminum particles in N 2/O 2, Ar/O 2, and He/O 2 gas mixtures, and in pure O 2 are presented and interpreted. This research focuses on identifying conditions under which asymmetric combustion of the aluminum particles develops. It also illustrates the relationship between aluminum particle combustion behavior and particle internal and surface structure and composition. The experimental technique used in this work is based on a micro-arc generator of monodisperse metal droplets and has been previously employed for metal particle combustion experiments. Liquid aluminum particles of 90 and 250 μm diameter were produced and ignited in a transparent chamber containing controlled gas atmospheres. The burning times, radiation histories, and color temperatures of the free falling particles were measured using optical sensors. Partially burned particles were rapidly quenched and their surfaces and interiors were examined by electron microscopy. It was found that brightness oscillations indicative of asymmetric particle burning developed reproducibly in the N 2/O 2 gas mixtures, consistent with the previous observations. Similar brightness oscillations developed occasionally in both Ar/O 2 and He/O 2 mixtures. Oxide caps were found on the surface of particles burning in all the environments; however the size of the oxide caps detected on the particles quenched in the Ar/O 2 and He/O 2 mixtures was markedly smaller than that found in the N 2/O 2 gas mixtures. Dissolved oxygen was detected in the interiors of all the partially burned particles independent of the gas environment in which the particles burned. A qualitative mechanism of diffusive oxygen transport to the surface of burning aluminum particles is discussed in which a higher rate of oxygen transport is expected in the N 2/O 2, as compared to the Ar/O 2 and He/O 2 gas mixtures, due to production of significant amounts of NO.