Ignition and combustion of boron particles in fluorine-containing environments

The ignition and combustion of isolated boron particles in fluorine-containing environments were investigated both experimentally and theoretically. Boron particles (1-mum amorphous and 3-mum crystalline) were ignited and burned completely in the post-flame region of a multi-diffusion flat-flame burner. The high-temperature gas environment was generated by the combustion of CH4/NF3/O-2/N-2 gases at atmospheric pressure. From the recorded real-time images of burning boron particles in fluorine-containing environments, no clear distinction was observed to define a two-stage combustion process, which is a characteristic feature of boron oxidation without fluorine. The burning trajectories of boron particles in fluorine-containing environments showed pronounced jetting and spinning phenomena. At 1,780 K, the self-sustained ignition of boron particles required a higher oxidizer concentration in non-fluorinated environments than in fluorinated environments. From the experimental data, HF was found to increase total burning times (t(p)) of boron particles; whereas, F significantly reduced t(p). A theoretical model was developed for simulating the combustion of an isolated boron particle in fluorine-containing environments. The oxide layer removal process was modeled using a reaction mechanism, which considered the vaporization process of the liquid B2O3/(BO)(n) mixture and four global surface reactions of oxide layer with O-2, H2O, F, and HF. The clean boron combustion model included four global surface reactions of O-2, H2O, F, and HF with boron. Numerical calculations showed that the oxide layer removal rate significantly increased in the presence of F and HF, as much as four times when the combustion is kinetics-controlled. In agreement with measured data, the calculated first- and second-stage combustion times decreased with increasing the ambient gas mixture temperature or the total pressure. The comparison between the current model predicted characteristic times of boron combustion and the measured data in the current study and other published experimental data in the literature is generally in good agreement. (C) 2001 by The Combustion Institute.