Numerical study on the ignition and flame propagation of micron-sized boron particles in the O₂-H₂O-N₂ atmosphere

This study established ignition and combustion models for both single micron-sized boron particles and its dust clouds in the O₂-H₂O-N₂ atmosphere. The model accounted for multiple processes of boron particle and its oxide layer, including surface heterogeneous reactions, evaporation, and boiling. The heat transfer involved heat conduction ranging from the free-molecular to continuum regimes, and thermal radiation. Employing the Euler-Lagrange simulation method and considering drag force acting on particles, the flame structure and flame propagation speed of micron-sized boron dust clouds were numerically studied in a semi-open rectangular tube filled with premixed two-phase flow (B-O₂-H₂O-N₂). It was found that the heat conduction rate predominates in the particle preheating and ignition stages, while the reaction heat release rate becomes significantly pronounced during the combustion stage. A parametric study was further conducted to evaluate the influences from the viewpoint of particle properties and ambient conditions. The particle diameter, oxide layer thickness, and ambient temperature had the greatest impact on the propagation speed. The sub-significant promotion effect was oxygen mole fraction, while the effect of boron dust cloud concentration was relatively weaker. Under various operating pressures, the maximum flame propagation speed lied within 1 - 1.5 atm, and by reducing or elevating the pressure, the propagation speed was reduced. The results provided a useful guide for the designing or performance improvement of solid-fuel ramjet engines.