ON WEAK EFFECT OF PARTICLE SIZE ON ITS BURN TIME FOR MICRON-SIZED ALUMINUM POWDERS

This article aims to verify and validate recent experiments with single Al particles demonstrating a weak effect of particle size on its burn time. Individual particles were fed through two laser beams: a low-power laser to in-situ measure particle size using light scattering, and a CO2 laser for ignition. The sensitivity and dynamic ranges of photo-detectors were selected to capture emission from particles in the size range of 1-10 mu m. The data processing method was modified. Measured color temperatures for individual particles were supported by spectroscopic measurements from multiple burning particles. The results confirmed that the burn time, t, as a function of the particle diameter, D, can be approximately described as t similar to D-n with n<1, which is a much weaker function than that expected based on classic droplet combustion models. Experimental data further confirmed that Al particles finer than 10 mu m burn in room temperature air, achieving high combustion temperatures and producing significant molecular AlO emission. Aluminum consumption rates were estimated analyzing the particle temperature histories. Estimates showed that thermal conductivity of gas surrounding burning Al particles is much less than that of the heated air, and that vapor phase reactions become non-negligible for particles greater than similar to 3 mu m.