Extension of the Frozen Sonic Flow Method to mixtures of polyatomic gases

Esposito, A. and Lappa, M. (2020) Extension of the Frozen Sonic Flow Method to mixtures of polyatomic gases. AIAA Journal, 58 (1). pp. 265-277. ISSN 0001-1452 (https://doi.org/10.2514/1.J058472)

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A contemporary issue of crucial importance for further developments in the field of thermal protection systems and related arc-jet-based testing activities calls for improvements in existing abilities to measure the centerline total enthalpy. Starting from the original assumptions of Vincenti and Kruger (1965) and through the elaboration of a mathematical framework relying on a specific modelling hierarchy of balance equations for the moles of different species involved, we show that the extension of the Frozen Sonic Flow Method (FSFM) to the case of polyatomic molecules can be made well posed. Dedicated experiments have been conducted using a re-entry simulation facility and varying the mass-averaged enthalpy in the range between 5 and 30 [MJ/kg]. In particular, three different gas mixtures have been considered (using Nitrogen as hot feeding gas and adding cold Oxygen, Carbon Dioxide and Methane, respectively). The enthalpy ratios calculated by the FSFM, found to depend on the gas mixture, have been compared with the values determined using two alternate techniques, namely, 1) the Heating Rate Method and 2) the Calorimetric Probe Method. Given the extremely complex experimental conditions considered (high-enthalpy, low density, supersonic reactive flows), the agreement between the theoretical and experimental results can be considered very satisfactory.