Numerical analysis of inlet flows at different altitudes in the upper atmosphere

A comprehensive numerical study is performed to investigate gas flows inside the inlet of an atmosphere-breathing electric propulsion (ABEP) system operating in the upper atmosphere ranging from 120 to 300 km using the direct simulation Monte Carlo method. Gas pressure, mass flux, and aerodynamic drag are analyzed in depth in order to gain a deep understanding of the effects of operation altitude and the assumption of free molecular flow (FMF) on gas flows within the inlet. Computational results show that both the gas pressure and mass flux in the compression and ionization sections decrease with increasing altitude, indicating weaker compression and collection performances at higher altitudes. Therefore, careful attention should be paid to compression and collection performances of the inlet when it operates at higher altitudes. At altitudes smaller than 180 km, gas flows within the inlet are fully or partly characterized by transitional flows, so the FMF assumption tends to overestimate the gas pressure and underestimate the mass flux within the inlet resulting from the neglect of the collisions between the oncoming and reflected molecules. However, FMFs predominate within the inlet and even fill the entire inlet at altitudes larger than 180 km, so it is fairly reasonable to assume an FMF in the aerodynamic design of the inlet of an ABEP system.