Combustion of boron particles: Experiment and theory

Abstract

Burning times of single boron particles were measured as functions of three experimental variables: particle diameter (d) from 37 to 124 μm, pressure (P) from 0.17 to 35.0 atm, and mole fraction of oxygen in the gas (X) from 0.1 to 1.0. Limited studies were also made in fluorine-containing gases without oxygen. A previously described laser-ignition technique was used. Measurable pre-ignition delays were observed in oxygen-containing gases, but not in the absence of oxygen. The effect of convection in these free-fall experiments was found to be significant at large values of d and P, and had to be taken into account in interpretation of the data. When d, P, and X are sufficiently large, the burning times are found to be inversely proportional to X, directly proportional to d2, and independent of P. At low values of d, P, and X, burning times deviate from these scaling laws. The flammability limit is between X=0.10 and 0.21. Quantitative agreement with a gas-phase diffusion theory is good under conditions well removed from the flammability limit. Burning rates with fluorine as oxidant are lower than rates with oxygen, consistent with diffusion considerations.