ELECTRON-VIBRATION ENERGY-EXCHANGE MODELS IN NITROGEN PLASMA FLOWS

This work presents an examination of the validity of the simple linear Landau-Teller-type model proposed by Lee for the electron-vibration e nergy exchange term in nitrogen [in Thermal Design of Aeroassisted Orb ital Transfer Vehicles, edited by H. F. Nelson (AIAA, New York, 1985), Vol. 96, p. 3; in Thermophysical Aspects of Re-entry Flows, edited by J. N. Moss and C. D. Scott (AIAA, New York, 1986), Vol. 103, p, 197]. Plasma flow conditions encountered in high enthalpy wind tunnels are considered. The time-dependent relaxation of the vibrational energy of nitrogen due to electron inelastic collisions is calculated. The infl uence of the anharmonicity of the molecule and of the initial vibratio nal temperature T-v is studied. With a harmonic oscillator approximati on, it is found that a linear Landau-Teller-type model is accurate to describe the vibrational energy relaxation rate for electron temperatu res T-e in the range 3000 K less than or equal to T less than or equal to 20 000 R. When T-v <T-e, our results show that the initial model p roposed by Lee overestimates by a mean factor of 3 the electron-vibrat ion (e-V) relaxation time. When T-v>T-e, the relaxation time appears t o depend on the initial vibrational distribution. When the anharmonici ty of nitrogen is taken into account, generally the relaxation rate of the vibrational energy deviates from a Linear rate equation. In this case, it appears much more difficult to model accurately the e-V energ y exchange term.