Multi-phase model for ignition and combustion of boron particles

The multi-phase and multi-stage combustion of boron particles is studied numerically with a time-dependent spherosymmetric numerical model specifically developed for simulating the sequential ignition and combustion of an isolated boron particle in chemically reacting gases. The present model represents an extension of previous models developed by the authors in which ignition and combustion were modeled separately. In the present paper, surface nitrogen-boron chemistry is developed and integrated into the model to enable the simulation of particle ignition and combustion in nitramine- based propellant environments with and without fluorine. In addition, second- order surface reactions at the outer surface of the boron oxide coating are added to the model specifically for low temperature and/or high pressure calculations where adsorption of surface complexes can be significant. The condensed-phase transport is reported in detail with the estimation of solubility parameters. The results show that the presence of fluorine significantly decreases the overall burning time for kinetically controlled burning systems but can increase the time for diffusive-controlled systems. Boron nitride (BN), produced by the surface reaction of NO, is found to be formed near the surface in the post-flame products of a RDX flame, but is converted to B/O compounds at greater radii. Finally, model predictions are compared with new high pressure ignition and combustion time data. The comparisons show reasonable agreement with the experimental measurements. Both the ignition and combustion times are found to decrease with increasing gas temperature and pressure. While the experimental results did not show a uniform trend of oxygen effect at high pressure, model predictions indicate that in high-temperature environments (e.g., >1800 K), ignition is insensitive to the molecular oxygen mole fraction and that combustion is strongly enhanced in oxygen-rich mixtures.