Electron heating and cooling in hypersonic flows

Using recently developed advanced numerical methods for plasma flows and sheaths, the first detailed study of electron cooling and heating taking place within hypersonic non-neutral flows is presented here. The numerical simulations fully couple the Navier-Stokes equations for the neutrals to the drift-diffusion model for the electrons and ions and include a 11-species finite-rate chemical solver along with a transport equation for the electron temperature in non-equilibrium. Results for Mach 18 airflow around a wedge with a sharp leading edge show that at low flight dynamic pressure the electron temperature remains close to the freestream temperature in the stagnation region. Such is attributed to the product of the electric field and the electron current being dominantly negative within the plasma sheaths and acting as an electron energy sink. This cooling effect leads to a significant portion of the flow downstream of the shock exhibiting electron temperatures much lower than expected. This study is the first to show a large impact of the non-neutral plasma sheaths on the post-shock electron temperature. This study also shows that the common approach to set the electron temperature equal to the vibrational temperature can result in the electron temperature being over-predicted by one order of magnitude or more in hypersonic flows.