Computational Investigation of Multiphase Phenomena in a Li-SF6 Combustion System for Spacecraft Power and Heating

Lithium-sulfur hexafluoride (Li-SF6) stored chemical energy propulsion systems (SCEPS) have been employed as self-contained, high-specific-energy, power systems for undersea vehicles in defense applications. Recent investigations have studied the potential to adapt this technology for spacecraft power and heating. The reaction between lithium and SF6 produces 4049 Wh kg-1 of heat, which can enable long duration space missions operating in cold, dark, and resource-constrained environments [1]. However, there are knowledge gaps related to the spatial distribution of corrosive intermediate products in such reactors and heat flux distribution on the reactor surface. The present study develops a multiphase reacting flow simulation approach for wick-type SCEPS reactors to predict such factors and guide system engineering for long-duration spacecraft operation. Results from this multiphase simulation are compared with a previously developed ideal gas formulation. Minimal deviations (<1%) are observed in many parameters such as the reacting oxidizer jet velocity field. Moderate deviations are identified in other fields (up to ~10%), such as the mixture density field. Overall, findings suggest that multiphase phenomena in the main combustion zone are minor and that the peak condensed phase volume fraction is <10-5. A second analysis is performed to assess impacts of two-phase SF6 oxidizer injection into the reactor. This could occur due to operation in cold environments and under reduced gravity, which could make phase separation more challenging in the oxidizer tank. An analytic state-point model is employed to predict phase distribution along the SF6 nozzle and jet flow. From this analysis, cases where liquid SF6 is present in the nozzle flow demonstrated higher velocities in the flow when compared to scenarios where solid SF6 is present. This aspect of the flow would be favorable as it could potentially push the corrosive species further downstream of the injector and hence reducing the possibility of corroding it.