Vibrational relaxation of carbon monoxide in an unsteady expansion wave

Abstract

The vibrational relaxation of carbon monoxide (purity 99.62%) has been investigated under both excitation and de-excitation conditions by means of emission and emission-absorption techniques. The experimental emission data show that the vibrational relaxation rate behind shock fronts is well described by the Landau-Teller theory. The emission-absorption technique has been used, at both the fundamental and first overtone bands of CO, to infer the vibrational temperature distribution through an unsteady expansion wave which has been generated in shock heated gas. The structure of the expansion wave has been investigated experimentally using thin film gauges and pressure transducers while the diaphragm motion has been observed by a sequence of Schlieren photographs. All measurements indicated that the expansion wave was well centred. The emission-absorption data results for pure CO showed that the rate of de-excitation of vibrational energy proceeded twice as fast as the relaxation rate measured behind normal shocks. Assuming that the lower harmonic states of CO and N2 are in a Boltzmann distribution vibrational temperatures of N2 have been inferred from the emission-absorption data from 1% CO+ 99% N2 mixture, in an unsteady expansion wave. Results indicate that N2 also proceeds about twice as fast under de-excitation conditions.