Experimental Burning Rates of Single Laser-Ignited Beryllium Droplets

Abstract

Beryllium droplet combustion has been studied using an advanced laser heating technique. Characteristic burning times of single burning droplets were measured as functions of both oxygen mole fraction and droplet diameter for various mixtures of O₂/Ar at atmospheric pressure. Precise burning times have been measured photometrically with a scatter typically of ±7% The droplet combustion is shown to occur in three distinct stages: (1) predominantly vapor phase reaction but with attendant solid nonprotective surface oxide (in the form of whiskers), (2) predominantly condensed phase reaction through a molten oxide layer at the droplet surface, and (3) predominantly condensed phase reaction through a solid oxide crust at the particle surface. These experiments indicate that the dominant mechanism changes from vapor phase to condensed phase at oxygen concentrations in the neighborhood of 60%. A major product of reaction of burning beryllium droplets is a large solid BeO sphere. The sphere size increases as oxygen concentration increases and equals original droplet size at oxygen mole fractions in the range 0.55−0.7. These quantitative droplet burning experiments show that theoretical droplet burning models extant are still inadequate. The best theoretical estimates fail to properly account for back-diffusion of oxide to the droplet surface. Theoretical calculations of droplet burning times are shown to be in error by at least a factor of 2.