Vibrational and electronic collisional-radiative model in air for Earth entry problems

Abstract

The two-temperature collisional-radiative model CoRaM-AIR, working over a wide range for pressure and temperatures, has been developed for the flow conditions around a space vehicle entering the Earth's atmosphere. The species N2, O2, NO, N, O, Ar, N2+, O2+, NO+, N+, O+, Ar+, and free electrons are taken into account. The model is vibrationally specific on the ground electronic state of N2, O2, and NO, and electronically specific for all species, with a total of 169 vibrational states and 829 electronic states, respectively. A wide set of elementary processes is considered under electron and heavy particle impact given the temperatures involved (up to 30 000 K). This set corresponds to almost 700 000 forward and backward elementary processes. The relaxation from initial thermal or chemical nonequilibrium is studied for dissociation-ionization situations in conditions related to the FIRE II flight experiment. Boltzmann plots clearly prove that the vibrational and electronic excitation distributions are far from being Boltzmanian. In particular, high-lying vibrational levels remain underpopulated for most of the duration of the relaxation. This relaxation can be separated in a first phase characterized by the dissociation and the excitation of the molecular species, and a second phase leading to the excitation and the ionization of the dissociation products. Owing to the vibrational relaxation, the time scales are slightly higher than the ones predicted by former kinetic mechanisms usually used in flow simulations. In the present FIRE II conditions, radiation does not play a significant role.