Three-temperature collisional-radiative model of ionization and recombination in hypersonic air plasmas

A three-temperature electronic state-resolved kinetic model is developed to study nonequilibrium ionization and electron recombination in shock-heated and expanding hypersonic air-argon plasmas. Leveraging a recently published semiclassical analytic theory, a novel set of rate coefficients for heavy particle impact electronic excitation in atomic collisions involving N and O are determined and incorporated into the model. The state-resolved kinetics are then coupled with the one-dimensional steady Euler equations to study ionizing flows behind 3–14 km/s shock waves and recombining flows in supersonic nozzles. Electron number density predictions are evaluated using experimental data for both of these flow configurations. Next, leveraging the high-quality rate data for electronic excitation, relaxation times characterizing translational to electronic energy exchange are computed for 41 collider pairs. In most cases, the relaxation times are slower than comparable vibrational relaxation times; however, for N2–N and O2–O, electronic excitation and vibrational relaxation proceed on similar timescales, indicating that molecular excited states may become populated during, and contribute to, dissociation. The impact of nonequilibrium atomic metastable state populations on the net rate of associative ionization is then assessed. When electronic nonequilibrium effects are neglected, the ionization distance is underpredicted by up to 50% at 9 km/s. Such errors can be mitigated by adopting Ttr0.5Tvib0.5 as the rate controlling temperature for associative ionization. Finally, the nonequilibrium behavior of electron impact ionization is studied in detail. Results support the validity of the quasi-steady-state (QSS) assumption for ionizing air mixtures behind strong shock waves.