Anharmonic Effects on the Rate of Relaxation of Vibrational Energy in Rapidly Expanding Flows

Detailed numerical results are presented for the time dependent vibrational relaxation process in the rapid expansion of anharmonic oscillators in nonequilibrium nozzle flows. Calculations for the expansion of CO, CO/Ar, and CO/N2 mixtures from high stagnation temperatures, in the range 2000–4500°K, are compared with previously reported experimental results. It is concluded that the effects of the anharmonicity of the molecules are unlikely to lead to an increase of more than a factor of 2 in the vibrational energy relaxation rate. A simple analytical result is derived in terms of the molecular constants and the expansion temperature ratio to predict the degree of distortion of the low level ``vibrational temperature'' which results from the anharmonicity.