Evidence for the transition from the diffusion-limit in aluminum particle combustion

This work presents experimental evidence that the transition from gas-phase diffusion-limited combustion for aluminum particles begins to occur at a particle size of 10 μm at a pressure of 8.5 atm. Measurements of the particle temperature by AlO spectroscopy and three-color pyrometry indicate that the peak temperature surrounding a burning particle approaches the aluminum boiling temperature as particle size is decreased to 10 μm when oxygen is the oxidizer. This reduction indicates that reactions are occurring at or near the particle surface rather than in a detached diffusion flame. When CO₂ is the oxidizer, the combustion temperatures remain near the aluminum boiling temperature for particles as large as 40 μm, indicating that the flame is consistently near the surface throughout this size range. Burn time measurements of 10 and 2.8 μm powders indicate that burn time is roughly proportional to particle diameter to the first power. The burn rates of micron- and nano-particles also show strong pressure dependence. These measurements all indicate that the combustion has deviated from the vapor-phase diffusion limit, and that surface or near-surface processes are beginning to affect the rate of burning. Such processes would have to be included in combustion models in order to accurately predict burning characteristics for aluminum with diameter less than 10 μm.