Infiltration-controlled combustion of magnesium for power generation in space

Abstract

Chemical heat integrated power systems can potentially provide high energy densities and long lifetimes compared to the best batteries. They are of interest for space missions where the use of solar or nuclear energy is impractical. In a new concept of such a system, a metal combustor is coupled with a chemical oxygen generator, and the metal powder bed inside the combustor burns with the infiltrated oxygen. However, the infiltration-controlled combustion of metal powders is not well understood. In the present work, a magnesium powder poured in a vertical quartz tube was ignited by a laser inside a chamber filled with oxygen at pressures of 44 – 90 kPa. The ignition resulted in the propagation of a thermal wave followed by a second ignition at the other end and propagation of a thermal wave backward. High-speed and infrared video recordings as well as thermocouple measurements were used for diagnostics. The results demonstrate that if a metal with a low Pilling-Bedworth ratio (0.81 for magnesium) is used, coflow combustion is possible. The coflow combustion wave propagates at incomplete conversion, with an axial velocity being as low as 0.1 mm/s and the zone of the highest temperature traveling along a helical path. An increase in the axial velocity of the combustion wave is accompanied by an increase in the flame thickness and a decrease in the extent of conversion.