Ignition and combustion characteristics of boron particles with multilayer oxide structures considering knudsen transition effects

Boron powder is a promising fuel candidate for powder-fueled scramjet engines due to its high energy density. In this study, a semi-empirical model is developed to describe the ignition and combustion behavior of boron particles by accounting for a multilayer oxide structures and Knudsen transition effects. Ignition characteristics are investigated under constant ambient temperature and oxidizer concentration, with emphasis on the effects of pressure (0.1–10 atm) and particle size (1–40 μm). The ignition delay increases exponentially with decreasing pressure. The pressure exponent exhibits a non-monotonic dependence on the Knudsen number (Kn), attributed to the competition among dominant reaction pathways over varying pressure regimes. For combustion, the mode transition diameter (D_tr), defined via the Damköhler number (Da), decreases with increasing pressure and temperature. This trend reflects a shift in the controlling mechanism between kinetic and diffusive processes, influenced by particle size, ambient temperature, and Langmuir-layer temperature. Novelty and significance statement: This study presents a novel semi-empirical model that integrates multilayer oxide structures and Knudsen transition effects to capture the ignition and combustion behavior of boron particles under various conditions. Unlike existing models, it includes bidirectional diffusion across oxide layers and the shift between heat transfer regimes. The model explains the non-monotonic pressure dependence of ignition characteristics based on mechanistic analysis. A transition diameter (Dₜᵣ) based on the Damköhler number serves as a new criterion to classify combustion modes. The model achieves high predictive accuracy across a wide range of particle sizes and ambient conditions. These results enhance the understanding of boron combustion mechanisms and support the design and optimization of powder-fueled scramjet propulsion systems.