Modeling of supersonic plasma flows in the VKI minitorch by means of a collisional radiative model

The analysis of the plasma during the expansion in the nozzle of the VKI Minitorch facility was studied using an electronically specific collisional radiative model for air plasmas. The collisional-radiative model accounts for thermal nonequilibrium between the roto-translational and vibrational energy modes of individual molecular species and deals with electronic states of atoms and molecules as separate species. Therefore, non-Boltzmann distributions of the electronic state populations of atoms and molecules are allowed. Deviations from Boltzmann distributions are expected to occur in a rapidly ionizing regime behind a strong shock wave, depleting the high lying bound electronic states. A separate temperature characterizes the energy contained in the translational mode of the free electrons. Radiative processes are accounted for using escape factors and the influence of the results to the optical thickness of the medium is also discussed. In order to quantify the extent of departure from equilibrium of the electronic state populations, results are compared with those obtained assuming a Boltzmann distribution. Departures of the electronic state distribution function of atoms and molecules from the Maxwell- Boltzmann distribution is found at the outlet of the nozzle.