Measurements of Carbon Dioxide Nonequilibrium Infrared Radiation in Shocked and Expanded Flows

This paper presents the first efforts in measuring carbon dioxide infrared radiation under nonequilibrium expansion, such as encountered in wakes, when carbon dioxide undergoes recombination. This work is motivated by the lack of radiation measurements in this flow and thermodynamic regimes, whereas carbon dioxide infrared radiation is deemed to contribute significantly to the afterbody heating withstood by a Martian spacecraft. The Hypervelocity Expansion Tube facility at Japan Aerospace Exploration Agency's Chofu Aerospace Center is operated at the flight binary-scaled conditions of a mission currently considered at Japan Aerospace Exploration Agency. The test flow in front and after a scaled aeroshell is characterized with a combination of analytical formulations, numerical simulations, flow measurements, and schlieren diagnostics. Pressure and shock standoff measurements are presented. The thermodynamic state of the flow and its degree of nonequilibrium are discussed. Spatial infrared radiation measurements are obtained from the shock layer to the wake of the model. A noticeable increase of carbon dioxide infrared radiation is observed in the wake of the spacecraft: the reasons of which are discussed. Shock standoff distance measurements are complemented by carbon dioxide infrared spectra measurements. The latter outline the highly complicated vibrational energy distribution of carbon dioxide in the expansion tube and the subsequent radiation, as well as the possible shortcomings of the multitemperature models. Therefore, these first measurements of carbon dioxide nonequilibrium radiation in high-enthalpy facilities pose new challenges to the community and will enable the upgrade and validation of the multitemperature and state-to-state models, as well as radiation models currently in use.