| The multi-phase and multi-stage combustion of particulate boron 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 ignition sub-model describing the oxide layer removal consists of detailed chemistry (36 species and 195 reversible elementary reactions) and multi-component molecular transport in the surrounding gas phase, surface reactions and physical absorptions at the interface of boron oxide/gas, heterogeneous reactions at the interface of boron/boron oxide and condensed-phase transport within the oxide layer. The subsequent combustion sub-model invokes a different surface mechanism on the "clean" surface of a boron particle. Classical governing equations incorporating these complex chemistry and transport phenomena are solved by applying the moving finite element method and the backward differentiation formulas.; The most important conclusions drawn from the modeling studies include: (1) The rate controlling steps in gasification of the boron oxide coating are chemisorption and desorption reactions for kinetically-controlled systems and gas-phase transport for diffusive-controlled systems in high temperature environments; the condensed-phase transport is only important for self-sustained, low temperature ignition; (2) The ignition delay time is found to be a linear function of the oxide layer thickness for both kinetically- and diffusive-controlled systems; the oxide layer thickness is proportional to boron particle radii; (3) Fluorine, predominately in the form of hydrogen fluoride, is found to significantly enhance the kinetically-controlled ignition of boron particles; the overall burning times for large boron particles are insensitive to fluorine concentrations; the presence of fluorine could improve the overall heat release by producing stable OBF(g) and {dollar}BFsb3(g){dollar} rather than HOBO(g), which converts to more stable {dollar}Bsb2Osb3(g){dollar} with great difficulty; (4) Boron nitride is formed on and near the particle surface by the surface reaction of NO(g); the condensation of BN(g) could play an important role in boron/nitramine flames; and (5) Dominant reaction pathways are different for diffusive- and kinetically-controlled particulate boron reacting systems. Further experimental measurements on the rate parameters of important surface reactions, such as B(s) + {dollar}Osb2(g),{dollar} B(s) + {dollar}Hsb2O(g),{dollar} B(s) + {dollar}Bsb2Osb3(g),{dollar} and B(s) + HF(g) are suggested.; Furthermore, the model results indicate that ignition of large particles (for example 500{dollar}mu{dollar}m diameter) in oxygen environments may be prevented by the condensation of boron oxide at the low temperature particle surface. Thus, dispersion is important for inhibiting small particle agglomeration and assuring particle ignition. The modeling study also supports the utilization of fluorinated oxidizers by showing a tremendous suppression of the concentrations of HOBO(g) and {dollar}Bsb2Osb3(g).{dollar} The benefits from fluorine are greater for small particles than for large ones in terms of reducing overall burning times. Finally, although nitramine and fluorinated nitramine oxidizers are receiving more and more research interests, the modeling study raises the concern about the condensation of boron nitride, which is the major nitrogen product for particulate boron combustion in nitramine flames. |
