An Experimental Study of CO₂ Thermochemical Nonequilibrium

An experimental study of CO₂ flows subjected to expanding conditions relevant to Mars entry is presented. A condition with a nominal velocity of 4.0 km∕s was generated in the X2 expansion tunnel. Midwavelength infrared emission measurements of the 4.3 and 2.7 μm bands were made over both a two-dimensional wedge model and for the freestream. Consistent results in the characterization of the freestream were obtained from pressure and spectroscopic measurements. The rotational and vibrational temperatures estimated in the wedge flow using the 2.7 μm band were consistent with those estimated from the 4.3 μm band. The difference between the CO₂ translational–rotational temperature and the CO₂ vibrational temperature was shown to be small in the freestream and expanding flow over the wedge model: less than 10%. Using vibrational relaxation rates tuned for postshock conditions, computational fluid dynamics simulations significantly overestimated the thermal nonequilibrium in the expanding flow, but not in the postshock (compressive) flow. This observation is consistent with past results for N₂ and CO, as well as CO₂ at lower temperatures.