Shock-Tube Study of Vibrational Relaxation in Carbon Dioxide

Measurements of the variation of density behind incident shock waves in carbon dioxide have been made using a Mach–Zehnder interferometer at shock speeds of up to Mach 9 (2500°K). The results show that vibrational relaxation occurs to a state in which all the vibrational modes are excited to their equilibrium values with neither the symmetric stretching nor the asymmetric stretching modes having significantly longer relaxation times than the bending mode. It has been found that the density does not change exactly exponentially with distance behind a shock front as has been assumed in the past. It has been shown that it is this assumption which led to the idea that the rate of vibrational relaxation depends not only on temperature, but also on the departure of the system from equilibrium. The present results can be accounted for if the relaxation time is a function of temperature alone. Relaxation times have been calculated taking account of the changes in the translational temperature which occur during the relaxation process. The results cover the temperature range 330°–1600°K.