Ignition and Combustion of Boron Slurry Agglomerates

Abstract

The behavior of agglomerates o f boron particles, formed during the combustion of boron slurry fuels, was studied in the post-flame region of a Rat-flame burner. Agglomerates having initial diameters of 175-800 μ_m were supported on a probe and placed in the hot gas environment. Measurements were made of the variation of agglomerate size with time. Quenched agglomerate surface morphology was studied using a scanning electron microscope. Burner operating conditions were varied to give fuel equivalence ratios in the range 0.3-0.7 and gas temperatures in the range 1690-1975 K at atmospheric pressure. Flame environments both with and without water vapor were considered. Observations of surface morphology of partiallyreacted slurry agglomerates suggest that the ignition and combustion process consists of the heat-up of a relatively porous agglomerate composed of individual boron particles covered with a solid oxide coating followed by oxide melting to form a liquid coating of low porosity. The oxide layer then gasifies. leaving an open, porous structure of reacting boron particles. Finally, the boron melts and transforms the agglomerate to a droplet. Measurements of ignition times and burning rates were interpreted in this framework for agglomerates formed from three different slurries. Burning rates computed from a diffusion-limited, equilibrium analysis were in reasonable agreement with measurements.