Effect of particle size on boron combustion in air

Mathematical modeling of combustion of micron-sized and nanometer boron particles in air with allowance for changes in the heat and mass transfer mechanism and a decreasing particle size is carried out. The Knudsen number is taken as an indicator of the transition from one regime to another: a continuous medium assumption is valid for describing the heat and mass transfer mechanisms in the case where $\mathrm{Kn}<0.01$, the free molecular regime takes place for $\mathrm{Kn}>10$, and a transition regime is realized for $0.01<\mathrm{Kn}<10$. Particle sizes at which different types of heat and mass transfer occurs with respect to boron combustion in air at pressures of 0.1–4 MPa are estimated. The time it takes for boron particles to combust within the framework of the assumption of a continuous medium and in a free molecular mode is determined. It is shown that calculation models for determining the burning time of boron particles with initial sizes close to micron-sized and nanodispersed ones should take into account a change in the heat and mass transfer mechanism with variation in the current particle radius during burnout.