Effects of heterogeneous surface reactions on the ignition of aluminum particles

A mechanistic model for the ignition and combustion of an aluminum particle in air is presented. The model consists of two stages, ignition and combustion. In the ignition stage, melting and heterogeneous surface reactions (HSR) are assumed to occur until the melting temperature of the oxide is attained. In the combustion stage, a quasi-steady state diffusion flame is assumed to exist allowing for the use of commonly employed flame sheet approximations. A system of non-linear ordinary differential equations is developed to describe each stage. Simulations are conducted for several particle sizes and surrounding gas-phase temperatures, with, and without HSR. The latter cases correspond to an often used approximation that ignition may modeled solely using an ignition temperature criterion. Results indicate that accounting for the effects of HSR significantly reduces overall ignition and burnout times for temperatures ranging from 1500 to 3000 K over a wide range of particle sizes.