Electronic excitation of atoms and molecules for the FIRE II flight experiment

An accurate investigation of the behavior of electronically excited states of atoms and molecules in the postshock relaxation zone of a trajectory point of the Flight Investigation of Reentry Environment 2 (FIRE II) flight experiment is carried out by means of a one-dimensional flow solver coupled to a collisional-radiative model. The model accounts for thermal nonequilibrium between the translational energy mode of the gas and the vibrational energy mode of individual molecules. Furthermore, electronic states of atoms and molecules are treated as separate species, allowing for non-Boltzmann distributions of their populations. In the rapidly ionizing regime behind a strong shock wave, the high-lying bound electronic states of atoms are depleted. This leads to the electronic energy level populations of atoms departing from the Boltzmann distributions. For molecular species, departures from Boltzmann equilibrium are limited to a narrow zone close to the shock front. A comparison with the recent model derived by Park (Park, C., "Parameters for Electronic Excitation of Diatomic Molecules 1. Electron-Impact Processes," 46th AIAA Aerospace Sciences Meeting and Exhibit, Reno, NV, AIAA Paper 2008-1206, 2008.) shows adequate agreement for predictions involving molecules. However, the predictions of the electronic level populations of atoms differ significantly. Based on the detailed collisional-radiative model developed, a reduced kinetic mechanism has been designed for implementation into two-dimensional or three-dimensional flow codes.