Spectrophotometric Measurements of the Vibrational Relaxation of CO in Shock-Wave and Nozzle Expansion-Flow Environments

Abstract

Infrared and Na line-reversal spectrophotometric methods have been employed for the investigation of the vibrational relaxation of pure CO and 5% CO+95% Ar test-gas mixtures behind normal shock waves and in quasisteady expansion flows. The shock-wave studies were conducted over a range of equilibrium shocked-gas temperatures of about 1400°—3200°K and pressures of a few atmospheres. Vibrational relaxation times obtained by these two techniques were found to agree quite well and are consistent with other available shock-wave data for both test-gas mixtures. The expansion-flow results for 5% CO+95% Ar were obtained in a shock-driven conical nozzle for a range of reservoir temperatures of about 3500°—5000°K and reservoir pressures of about 45 atm. For these experiments, the Na line-reversal and CO infrared diagnostic techniques indicated vibrational temperatures much lower than those predicted by the Landau—Teller theory using the shock-wave measured rate data. In addition, the infrared measurements indicated somewhat lower vibrational temperatures than the Na line-reversal results. In terms of vibrational relaxation rates, the expansion-flow data indicate that the de-excitation process in the nozzle proceeds about two orders of magnitude faster than anticipated on the basis of the rates measured behind shock waves. These observations are consistent with previous results obtained for N2. While a conclusive explanation of these results is not presently available, the results of recent theoretical studies are discussed which suggest that the detailed mechanism of the relaxation process may depend on the gas-dynamic environment.