Measurements of temperatures and species in CO2 post-shock thermochemical non-equilibrium

Accurate characterization and prediction of shock-induced CO2 thermochemical non-equilibrium are crucial for current and future Mars entry missions. To investigate these phenomena, measurements of CO2 rotational temperature, mode-specific vibrational temperatures, and partial pressure were conducted behind incident shock fronts at intermediate velocities (3.2-5 km/s) at the ϕ800 shock tube at the Institute of Mechanics, Chinese Academy of Sciences, via a newly developed high-frequency mid-infrared beam-splitting fiber-coupled tunable diode laser absorption spectroscopy technique. Possible shock-tube nonidealities, including shock deceleration, water contamination, flow-cutter-induced two-dimensional effects, and boundary-layer mass removal, were examined and found to have limited influence on the present post-shock test flows. Notably, to the best of the authors' knowledge, these measurements may provide the first evidence that distinct CO2 vibrational modes exhibit similar relaxation rates at the centimeter spatial scale under highly dissociating conditions. When compared with one-dimensional post-shock calculations, the modified Stellar CO2 vibrational state-to-state database was found to reasonably predict the post-shock relaxation process but overestimate the mode-specific vibrational temperatures at the early post-shock stage. The measured post-shock temperature profiles are generally bounded by the Johnston and Cruden predictions, with a tendency toward the Cruden mechanism, whereas the measured CO2 partial-pressure profiles show good agreement with the Johnston mechanism. The obtained results provide benchmark data for validating and improving CO2 thermochemical models.