The morphology of aluminum particles affects their combustion

Micron-sized, densely packed spherical aggregates of aluminum with a narrow particle size distribution were prepared by Emulsion-Assisted Milling (EAM). Combustion of this powder was compared to that of similarly sized atomized aluminum. Single particles of each powder were fed into an air-acetylene flame. The ignited particles continued to burn in room air. Each burning particle produced an optical emission pulse. Such pulses were measured using two photomultipliers filtered at 700 and 800 nm. The measured signals were processed to recover the pulse emission intensities, durations, and color temperatures for each particle. A statistically significant dataset of descriptors for the optical emission pulses produced by burning particles was obtained and processed for each powder. In addition, burned-out particles were collected and examined using electron microscopy. The results show that the EAM-prepared aluminum powder particles generate a stronger emission than atomized particles. The particle burn times are comparable for both powders. The optical signatures of the burning particles are successfully represented as a sum of contributions from both the particle surface and the standoff flame. In both cases, the flame is assumed to be at the boiling point of alumina. The particle surface temperature appears to be lower for the milled powder particles. Despite that, the emission from the standoff flame is markedly stronger for the EAM-prepared aluminum. Consistent with their lower surface temperatures, the EAM-prepared aluminum particles do not coalesce into single droplets during combustion, retaining at least partially their initial morphology. This altered surface morphology is likely responsible for the observed differences in their combustion characteristics.