Combustion of Aluminum particles in the transition regime between the diffusion and kinetic limits

An experimental investigation of the combustion times for micro-sized Aluminum particles in conditions interesting to solid rocket motor (SRM) conditions, pressures 4-20 atm, temperatures near 2650 K, and oxidizer compositions 15-70% for oxygen, carbon dioxide, and water vapor oxidizers are presented. Combustion in these conditions is in the transition regime between the diffusion limit which would be predicted for larger particles, and the kinetic limit, which would be predicted for nano particles. Burn times were calculated based upon emission from AlO, an aluminum combustion intermediate for all three oxidizers. These burn times are in contrast to those that could otherwise be predicted by the Beckstead correlation in the diffusion limited regime. A heterogeneous shock tube was used to create the combustion conditions for injected particles. The burn time of particles in the transition regime were shown to have a stronger dependence upon composition mole fraction and pressure, but not strong temperature dependence. A correlation is presented which should aid modelers in predicting burn times with particles down to a few micrometers.