Experimental burning rates and combustion mechanisms of single beryllium particles

Abstract

Single spherical beryllium particles from two powder samples having average particle diameters of 32 and 25 μ, respectively, were injected into oxidizing gases, both moist and dry, at atmospheric pressure and at temperatures ranging from 2600°–2960°K. The purpose was accurate measurement of metal burning rates and a study of parameters affecting these rates. Ignition efficiencies, burning rates, and flame diameters of particles were found to vary both with particle diameters and with ambient gas properties. It is concluded that beryllium particles may burn by several distinct modes. One is rapid vapor-phase diffusion flame, favored by high partial pressures of oxygen and high temperatures of the gaseous environment. The burning times corresponding to this mode, ranging from 1.3–4.5 msec, were found to be proportional to the square of the particle diameter and inversly proportional to a power of oxygen pressure slightly less than unity. As the partial pressure of oxygen is decreased below a certain value which is usually between 0.1 and 0.2 atm, the metal flame temperature decreases and vaporphase combustion gradually changes over into slow surface reaction. There is also evidence of a third mode in which combustion is only moderately slower than the rapid vapor-phase burning. This mode, favored by low temperature and by the presence of water vapor in the environment, may be vapor-phase combustion hindered by a heavy coating of beryllia accumulated on the particle during the pre-ignition surface reaction.