Experimental Determination of CO₂ Recombination Rate Coefficient in Non-equilibrium Flow

Abstract

For future Mars entry missions, accurate prediction of infrared radiative heating from CO₂ in the aftbody aeroshell of the capsule is required in order to optimize the thermal protection system. CO₂ in the aftbody region is mainly produced by recombination of CO--O system. In general, recombination rate coefficients are calculated by the principle of detailed balancing using dissociation rate coefficients and the equilibrium constant. However, in the flow of the aftbody region, where strong chemical nonequilibrium conditions are expected, the recombination rate coefficient calculated by the principle of detailed balancing is questionable. Therefore, in this study, infrared radiation from CO₂ in nonequilibrium flow was measured using a shock tube with divergent nozzle in JAXA Chofu Aerospace Center. It was shown that the recombination rate coefficient of CO--O system is dominant by sensitivity analysis. Tests at different conditions for shock wave velocity suggested that the apparent recombination coefficient was larger at lower temperatures. By comparing the measured radiation intensity time histories with JONATHAN and SPRADIAN calculation, it was shown that recombination rate coefficient of CO--O system under nonequilibrium flow field is larger than that calculated by the principle of detailed balancing.