Experimental characterization of combustion regimes for micron-sized aluminum powders

Abstract

This work presents an experimental study of combustion characteristics of micron-sized aluminum particles in the transition regime under constant volume combustion experiments. Burning velocities were estimated from the measured pressure traces using both a simplified model for aerosol combustion on closed spherical bombs and a semi-empirical correlation, and compared to previous literature. Flame temperatures were measured by bi-color pyrometry and indicate that for particles smaller than 12 μm, the flame moves closer to the particle’s surface, since flame temperatures were close to aluminum boiling point. For 17.9 μm particles, flame temperatures were close to predicted adiabatic flame temperature and alumina vaporization-dissociation temperature, indicating a classical vapor phase flame under a diffusion-controlled mechanism. However, spectroscopy measurements did not detect significant reductions on molecular AlO emissions for finer particles. This indicates a still very significant presence of vapor phase reaction for powders with a Sauter mean diameter at least as large as 7 μm, which is further supported by the presence of nanometric spheres in the combustion residues, since alumina formed under a vapor phase reaction is expected to condensate into nanometric droplets.