| 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, which provided a uniform zone of combustion products of CH4/NF 3/O2/N2 mixtures. In fluorinated environments, no clear distinction was observed to define a two-stage combustion process, a characteristic feature of boron oxidation without fluorine. The burning trajectories of boron particles in fluorinated environments showed pronounced jetting and spinning phenomena. At 1,780 K, boron ignition required a higher oxidizer concentration in non-fluorinated environments than in fluorinated environments. HF was found to increase the total burning times (t b) of boron particles; whereas F significantly reduced tb. 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 considers vaporization process of B2O3/(BO) n mixture and four global surface reactions of oxide layer with O 2, H2O, F, and HF. The major products during the oxide removal process were found to be OBF, EBOH, HBO2, and BO2. The "clean" boron combustion model includes four global surface reactions of O2, H2O, F, and HF with boron. BF3, OBF, HBO2, and B2O2 are the major products during the "clean" boron combustion stage. Predicted tb are in good agreement with the measured data in the current study and other published experimental data in the literature. The calculated results show that both oxide layer removal and "clean" boron burning rates increase significantly in the presence of atomic fluorine. In agreement with measured data, the calculated burning times of boron particles decrease with increasing ambient temperature or pressure. This study shows that fluorine-containing species can indeed increase the overall efficiency of boron combustion. |
