Experiments on magnesium aerosol combustion in microgravity

An experimental study of the combustion of an aerosol of coarse magnesium p articles in microgravity is reported. Particles with sizes between 180-250 mu m were aerosolized in a 0.5-L combustion chamber and ignited in a consta nt-pressure, microgravity environment. Two flame images were produced simul taneously using interference filters separating adjacent MgO and black body radiation bands at 500 and 510 nm, respectively. The characteristic MgO ra diation 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 microgr avity 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 sho wed an oxide coating on the surfaces of particles collected after experimen ts 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 fla me propagation was observed. However, the particles collected after such ex periments contained significant amounts of dissolved oxygen. Strong MgO rad iation and production of dense MgO smoke clouds were observed in all the ex periments, including those with the slowly propagating flames. Therefore, i t 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 th e burning particles. (C) 2000 by The Combustion Institute.