Constant pressure combustion of aerosol of coarse magnesium particles in microgravity

The combustion mechanisms of clouds of metal particles are addressed in this research. A microgravity environment was used to create a 'stationary model aerosol' consisting of relatively large (100-300 μm diameter), initially motionless particles. The development of individual particle flames, motion of individual particles, and overall aerosol combustion process could be observed simultaneously. The experiments used the 2.2-s Drop Tower at the NASA Lewis Research Center. Various image analysis procedures were employed to extract information on the flame structure from the high- speed movie and video records. Mg particle aerosol combustion at constant pressure was addressed. The observed flame structure contained preheat and combustion zones typical of the volatile type aerosol flames. The preheat and combustion zones were identified by differences in intensity and spectral content of the emitted radiation. The velocity of propagation of the preheat zone into the unburnt mixture was in the range of 0.15-0.30 m/s, consistent with the microgravity flame speed measurements reported in the literature. The combustion zone propagated at a slower rate of less than 0.1 m/s. The width of the preheat zone increased and the width of the combustion zone decreased during the flame propagation. Particle inertia caused significant velocity lag relative to the cold gas that was pushed ahead of the flame. The particles were, however, efficiently entrained by the hot gas in the preheat and combustion zones. Thus, the particle number density in the preheat and combustion zones increased as the flame propagated, eventually resulting in flame quenching due to oxygen deficiency. Also, collective particle motion was observed in the direction opposite to that of the flame propagation; the nature of this motion needs further investigation. Unburnt metal particles were observed to reignite when fresh air from the constant pressure ballast reservoir returned to the combustion chamber as it cooled.