Thermal theory of aluminum particle ignition in continuum, free-molecular, and transition heat transfer regimes

Abstract

Most studies on nano- and micro- sized aluminum particle ignition have been focused on the processes occurring inside particles. In the current paper, thermal ignition of an aluminum particle in the air is simulated with different heat transfer models: continuum, free-molecular, and Fuchs model. A single parabolic oxidation law is assumed in the particle size range from nano- to millimeter diameters. A particle is considered ignited when it reaches the oxide melting point. The criterion defining the limits of validity for each model is the ratio of continuum and free-molecular heat transfer rates. The dependence of ignition temperature Ti on the particle size is in qualitative agreement with the experimental trends: Ti can have values in the range of 700–1500 K for nanoparticles due to the dominating contribution of a free-molecular heat transfer, and sharp growth of Ti with the particle size in the range of 1–100 μm diameter is due to the transitional character of heat transfer. For small values of the accommodation coefficient, ignition may occur in the critical ignition mode with the thermal runaway. The results suggest the importance of non-continuous heat transfer and, in particular, energy accommodation in ignition of nano- and micro- sized particles.