Radiative-Collisional Models in Non-Equilibrium Aerothermodynamics of Entry Probes

Abstract

To design space vehicles aimed for returning payloads from a geostationary orbit, the Moon and other large or small planets of Solar system, a knowledge of the total (convective and radiative) heating from an environment is required. It is well known that the radiative heat load on a space vehicle moving through the atmosphere increases as the speed and the size increase, therefore, in many of these missions the large part of the trajectory will pass at high altitude, where the low atmospheric density can lead to significant thermal, chemical and physical nonequilibrium effects. Physical models and computational codes used to predict the aerothermodynamics must account for not only high temperature equilibrium thermodynamics (as a rule, within the framework of the local thermodynamic equilibrium (LTE) approach), but also for nonequilibrium one. Therefore, an accurate prediction of radiative heating as well as convective one under both equilibrium and nonequilibrium conditions becomes important to designers and space mission planners. To develop a prediction computational fluid dynamics (CFD) tool for reentry flows, where dissociation, ionization and radiation are important, some major areas are addressed. The most significant of them are following: (1) physical-chemical kinetics of high temperature dissociated and ionized gases, (2) transport properties of the gas mixtures, (3) spectral radiation properties of high temperature gases and low-temperature plasmas, (4) numerical simulation algorithms for prediction of nonequilibrium gas mixtures dynamics and radiation heat transfer in volumes of various geometry, and (5) models of physical and chemical processes accompanied by interaction of gas flows and radiation with thermoprotection systems (TPS) of space vehicles (including their thermochemical destruction, ablation, sublimation, etc.). In literatures (See Refs. (Park, C, 1990, Nonequilibrium Hypersonic Aerothermodynamics, Willey-Interscience Publication, J. Wiley & Sons, New York; Park, C., 1993, “Review of Chemical Kinetic Problems of Future NASA Missions. I: Earth Entries,” J. Thermophys. Heat Transfer, 7 (3), pp. 385–398; Park, et al. , 1994, “Review of Chemical-Kinetic Problems of Future NASA Missions, II: Mars Entries,” J. Thermophys. Heat Transfer, 8 (1), pp. 9–23; Sarma, G., 2000, “Physico-Chemical Modelling in Hypersonic Flow Simulation,” Prog. Aerosp. Sci., 36 , pp. 281–349; Huo, and Thuemmel, 1995, Electron-Air Molecule Collisions in Hypersonic Flows. Molecular Physics and Hypersonic Flows, Capitelli M., ed., Kluwer Academic Publishers, pp. 115–138.)) one can find reviews of governing equations used in the aerophysics, boundary conditions and the associated inputs using the physical-chemical models and their partially successful applications. This article presents the states of the art of models of electronic kinetics in the nonequilibrium low-temperature plasma of complex chemical compositions (air and carbon dioxide mixtures) widely met in various aerospace applications. Special attention is given to electronic kinetics of atoms and diatomic molecules within the framework of the radiative-collisional models.