Characterization of reflected shock tunnel freestream in carbon dioxide

In this study, the JFX reflected shock tunnel freestream is characterized using pitot probes, laser absorption spectroscopy, and high-speed schlieren for shock stand-off distances over a sphere. The experiment employed two driver gases: a mixture of H₂ and CO₂, and pure He. Three lasers, operating at wavelengths around 2.0 \(\upmu \mathrm{m}\) and 1.4 \(\upmu \mathrm{m}\) with a scanning frequency of 50 kHz, were utilized to measure the properties of CO₂ and H₂O. Computational fluid dynamics simulations showed near thermo-equilibrium conditions, supporting the use of an equilibrium model to determine the temperature and partial pressure of the two species. Isentropic calculations indicate that there is no significant thermodynamic nonequilibrium in the freestream. During the effective test time, the measured and simulated results were in good agreement for both the shock stand-off distance and CO₂ partial pressure. However, the detection of H₂O indicated contamination from the driver gas, with early onset leading to an increase in the shock stand-off distance. The uncontaminated time is around 700-800 \(\upmu \mathrm{s}\) for both conditions, and the contamination onset time falls between the predicted values using different nozzle conditions, which also indicates a certain degree of contamination.