Ignition and combustion of aluminium particles in hot ambient gases

Abstract

Observations have been made of individual aluminium particles entrained in combustion gases under controlled conditions, at atmospheric pressure. Effects of ambient gas temperature and oxygen content and of aluminium particle size on ignition and subsequent combustion phenomena have been determined by flame photography and by microscope examination of collected particles. Ignition occurs only when the ambient gases are above 2210° to 2360° K, this temperature being quite insensitive to free oxygen content and particle size. This result is consistent with an ignition theory which assumes a substantial change in the rate of the surface reaction at the melting temperature of alumina (2300°K). When ignition occurs, the delay time, which varies with the square of particle diameter, is in excellent agreement with simple heat-transfer calculations when the initial lag of the particle velocity relative to the gas is taken into account. Once ignition occurs, combustion phenomena are complex, involving unsymmetrical burning and fragmentation; a qualitative discussion is provided.