A comprehensive physical and numerical model of boron particle ignition

A comprehensive spherosymmetric numerical model has been developed to study the ignition of an isolated boron particle with a thin boron oxide layer. This model includes detailed chemistry and multicomponent molecular transport in the surrounding gas phase, heterogeneous reactions, and physical absorption on the boron oxide/gas interface, as well as transport of the dissolved gas in the oxide layer and heterogeneous reactions at the boron oxide/boron interface. The calculated times required to gasify the oxide layer are in good agreement with published experimental data, with greater disparity for small particle cases at high temperatures. The desorption of complexes (at low surface temperature) and the adsorption of gas-phase species (at high surface temperature) at the boron oxide/gas interface are found to be rate controlling during the gasification of the oxide layer, with little sensitivity to species transport within the oxide layer. The gasification rate of the oxide layer increases with gas temperature and water vapor mole fraction, but is independent of the mole fraction of oxygen. The onset of oxide layer removal also occurs at lower temperatures with moisture present. Consistent with experiment, the presence of moisture is computationally shown to lead to a reduction in the particle ignition temperature.