Experimental study of aluminum particle flame evolution in normal and micro-gravity

This research addresses the flame structure of single aluminum particles burning in air with the emphasis on the transition from spherically symmetric to non-symmetric combustion regime. The unique feature of this work is that free motionless aluminum particles were produced and ignited in both normal and microgravity environments. That allowed us to observe whether the particle flame non-symmetry develops when effects of convection and buoyancy are minimized. The particles were produced using a novel micro-arc device ensuring repeatable formation and ignition of uniform metal droplets with controllable initial temperatures and velocities. For the microgravity experiments the device was modified to produce motionless metal particles that made it possible to study the flame structure unaffected by particle motion or buoyancy. Burning droplet temperature was measured in real time with a three wave-length pyrometer. The evolution of flame shape at normal and microgravity was studied using high-speed video imaging and correlated with the heterogeneous combustion processes. It was found that combustion times and temperatures are similar for normal and microgravity environments. A non-symmetric flame structure and brightness oscillations were observed to develop at the same combustion times around nearly motionless aluminum particles burning in air in both normal and microgravity environments. Therefore, flame non-symmetry is an intrinsic feature of aluminum particle burning rather than the result of forced or natural convection flows. It was observed that in addition to the particle spinning, actual periodic changes in the flame emission occur during aluminum particle combustion in air. It was also observed that the onset of the non-symmetric burning is accompanied by the formation of highly radiative condensed products in several locations within the particle flame zone. This was found to be consistent with an aluminum combustion mechanism in which AI-O solution forms inside burning aluminum particles shortly after ignition.