Thermal Nonequilibrium Characterization of Expanding CO ₂ Flows in a Reflected Shock Tunnel

A time-resolved study of thermochemical nonequilibrium [Formula: see text] expanding flows was conducted using tunable diode laser absorption spectroscopy (TDLAS) in the JF-10 reflected shock tunnel. [Formula: see text] was used as the driver gas and [Formula: see text] as the test gas, generating a nondissociative low-enthalpy flow. The TDLAS system used mid-infrared and near-infrared diode lasers to probe multiple rovibrational transitions of [Formula: see text] and [Formula: see text], thereby enabling extraction of rotational and vibrational temperatures, as well as species concentrations. Water vapor served as a tracer to identify driver-gas contamination, enabling the determination of the effective test window. The results indicated a quasi-steady test period of approximately 0.6 ms by minimal contamination and high thermochemical stability. A two-temperature computational fluid dynamics simulation was performed using the Eilmer4 solver, and the predicted vibrational relaxation behavior of [Formula: see text] was compared with the experimental data. Among the models considered, the Simpson relaxation model provided the best agreement with the observed vibrational temperature evolution. This study demonstrates the capability of TDLAS in validating nonequilibrium flow models and provides benchmark data for [Formula: see text] test flows relevant to planetary entry conditions.