Chemical relaxation phenomena behind normal shock waves in a dissociated freestream.

The results of a study of the effect of freestream dissociation and vibrational excitation on relaxation phenomena behind normal shock waves in air are presented. Included are shocktube measurements of the relaxation time behind reflected shock waves which were obtained by monitoring the radiative emission time-history at initial driven tube pressures of 0.20 and 0.50 mm Hg and for incident shock speeds from 4.9 to 6.8 mm///sec. These conditions correspond to reflected shock-wave equilibrium temperatures ranging from 7300° to 10,300°K and freestream dissociation levels from 30 to 60%. Numerical calculations of normal shockwave nonequilibrium relaxation phenomena have also been carried out using a coupled vibration-dissociation model including preferential dissociation from excited vibrational levels. From a comparison of these numerical results and the experimental measurements, it has been concluded that the effect of freestream dissociation and vibrational excitation on normal shock-wave relaxation phenomena is small and that the particle relaxation times behind shock waves with identical downstream equilibrium thermodynamic states are approximately the same. The calculated relaxation profiles, obtained over large ranges of pressure, and measured relaxation times are also compared, together with the use of binary scaling, and these results are discussed.