• Boltzmann equation
• hypersonic plasma flow
• non-Boltzmann distribution
• collisional-radiative model

The study of high speed, unsteady non-equilibrium flows are one of the main challenges in the hypersonic research. They are found in different space applications: for instance the wake region of a capsule during the reentry phase and the converging-diverging nozzle. In particular, the non-equilibrium effects control the back shell heating in high-speed entries. They play an important role in thermal protection system design. Moreover, the non-equilibrium vibrational and electronic kinetics affects the high-enthalpy nozzle flow with significant changes in chemical kinetics and rate coefficients. The knowledge of the nature and extent of the non-equilibrium phenomena in an expansion allows predicting those effects.
The primary objective of this work is to devise a framework for studying the non-equilibrium modeling of the electron kinetics in rarefied molecular gases, under the action of an expanding flow. In these conditions, the electron energy distribution function is non-equilibrium. First of all, a collisional-radiative model is analyzed to evaluate the concentration of the gas species, the distribution of the species populations and the flow proprieties for an ionizing regime treating each internal energy level as an independent pseudo-species with its kinetics. Second, a Boltzmann equation solver was implemented to evaluate the electron nonequilibrium distribution function. The rate and transport coefficients are used to couple the Boltzmann solver with the zero-dimensional flow solver to remove the temperature dependence of the species distribution.