Ignition model of boron particle based on the change of oxide layer structure

The oxide layer of boron particle is generally regarded as a two-layer structure the inner layer is B₂O₃ and the outer is (BO)_n. However, under lower temperature, a tiny layer of B₂O₃ can be generated at the surface of the (BO)_n and form a three-layer structure during the ignition process, which has been proven by experimental phenomenon. Accordingly, a parameter xo is adopted to represent the thickness of the outer B₂O₃(l) layer (outermost layer), and a simplified kinetic and diffusion model for the ignition process of boron particles in dry and wet atmosphere is developed with considering the generation and consumption process of the outermost layer. The ignition process is divided into two parts (ignition delay and first-stage combustion) by the parameter xo. The ignition temperature is defined as the particle temperature at the moment that xo reaches 0. When xo > 0, the particle is under the ignition delay process, and the evaporation product is B₂O₃(g). When xo = 0, the particle turns to the first-stage combustion process. (BO)_n is exposed to the environment, the evaporation products are B₂O₂(g) and B₂O₃(g), and the particle is under the two-layer structure. The oxygen diffusion inward is available during these two processes. The ignition time which is predicted by this model is in good agreement with published experimental data. Under a real ramjet condition, higher ambient temperature and concentration of water vapor can reduce both the maximum value of xo and the ignition time. The ignition temperature decreases with higher water vapor concentration, but increases with the higher ambient temperature.