Chemiluminescence and oxygen molecular radiation in hypersonic carbon dioxide expanding flow

This study investigates the radiative properties of hypersonic carbon dioxide (CO2) expanding flows. Spatially resolved spectral measurements for the expansion region of a two-dimensional model were conducted in the JF14 shock tunnel. Experiments were performed in a shock tube mode, where the shock wave with velocities ranging from 3.37 to 3.88 km/s directly propagates to the test section, forming a stable flow field around the model. Results showed that the radiation emission within the 250–710 nm range is primarily dominated by CO2 chemiluminescence and Schumann–Runge bands of molecular oxygen (O2). The intensity of the Schumann–Runge spectrum was found to be significantly higher than the O2 thermal radiation calculated under the two-temperature (2-T) model, indicating that the O2 excited state (O2(3B)) was overpopulated relative to the Boltzmann distribution in the expansion region. In regions with lower degrees of expansion, the CO2 chemiluminescence intensity was overestimated by about 3 to 6 times, which may be attributed to an overprediction of the atomic oxygen (O) and carbon monoxide (CO) number densities in the current thermochemical model. As the degree of expansion intensified, the CO2 chemiluminescence intensity gradually exceeded the predicted values, suggesting that the expansion process can accelerate the recombination of O and CO, leading to stronger CO2 chemiluminescence. This work enhances the understanding of radiative processes in hypersonic CO2 expanding flow and provides benchmarks for the development of thermochemical and radiative models.