An experimental investigation of the ignition and combustion phenomena of laser-ignited boron agglomerates in a flowing oxygen environment

An experimental investigation of the ignition and combustion phenomena of laser-ignited boron agglomerates in a flowing oxygen environment is presented. Individual agglomerates with initial diameters ranging from 350-1100 μm were supported by silicon carbide fibers and ignited by a cw CO₂ laser. A near-infrared two-color pyrometer and a broadband radiometer were used to measure the surface temperature and the radiation emitted during the combustion event. From the outputs of the pyrometer and radiometer, it was possible to determine the ignition delay time and the burning time of the boron agglomerate. High-speed motion picture photography was also used to record the phenomenological events and to correlate the instrument data. For the agglomerate diameter range used in the study, a linear relationship was found between the square of the agglomerate diameter and the burning time, signifying the burning rate of boron agglomerates is controlled(limited) by the diffusion of oxidant to the surface. Increased oxygen flow rate was found to significantly increase the burning rate, thus significantly decrease the burning time. It was also found that the surface temperature of the agglomerate exceeded 2450K (i.e. melting point of boron) in all cases, therefore it appears that the melting of boron agglomerates does occur during combustion. In addition, from the results of the high speed cinematography, the existence of BO₂ (boric oxide) has been confirmed in boron agglomerate combustion. To obtain an accurate model of boron combustion, the chemical reactions involving BO₂ must be considered. In general, the techniques used to measure burning time in the present study appear more efficient than those used in previous studies.