Experiments on magnesium aerosol combustion in microgravity

An experimental study of the combustion of an aerosol of coarse magnesium particles in microgravity is reported. Particles with sizes between 180-250 μm were aerosolized in a 0.5-L combustion chamber and ignited in a constant-pressure, microgravity environment. Two flame images were produced simultaneously using interference filters separating adjacent MgO and black body radiation bands at 500 and 510 nm, respectively. The characteristic MgO radiation was used as an indicator of the gas-phase combustion. Comparison of the two filtered flame images showed that preheat and combustion zones can be distinguished in the flame. Experiments have also shown that in microgravity the flame speed depends on the initial particle speeds varied in the range of 0.02-0.4 m/s. This dependence is, most likely, due to the role the moving particles play in the heat transfer processes. Product analyses showed an oxide coating on the surfaces of particles collected after experiments in which the flame speeds were higher than 0.1 m/s. No oxide coating was detected in the products collected after experiments in which a slower flame propagation was observed. However, the particles collected after such experiments contained significant amounts of dissolved oxygen. Strong MgO radiation and production of dense MgO smoke clouds were observed in all the experiments, including those with the slowly propagating flames. Therefore, it has been suggested that the MgO produced in the vapor-phase flame is not the primary source of the MgO coating found on the burnt particle surfaces. An alternative mechanism of forming the oxide coating is, consistent with the earlier single metal particle combustion studies, via the formation of a metal-oxygen solution followed by a phase separation occurring within the burning particles. (C) 2000 by The Combustion Institute.