Numerical study on nonequilibrium state of high enthalpy air plasmas using a collisional-radiative model

Abstract

In this thesis, detailed numerical investigation on nonequilibrium state of high-enthalpy air plasma flows is carried out by devising an accurate collisional-radiative approach including a line-by-line radiation model. The state-of-the-art electronic transition rates are compiled with comparisons of existing data, and it is coupled with a viscous-shock layer method and a post-shock flow solver to analyze nonequilibrium phenomena in the typical hypersonic Earth reentry conditions. Two-different applications are made, and they are the prediction of stagnation-point heating of Fire II flight experiment and the analysis of nonequilibrium radiation measured in the recent electric-arc shock tube experiments. In the first case, influence of collisional excitation modeling including electron and heavy-particle impacts on the stagnation-point radiative heat flux is investigated in detail, and the escape factors of the strongest atomic lines with the non-local absorption effect are proposed to more efficiently consider the non-local nature of the radiative transition. When compared with the experimental data from the Fire II trajectories, it is found that the present collisional-radiative model with the non-local absorption improves the ability to predict non-Boltzmann radiative heating, and it is also found that the conventional two-temperature model can be an appropriate option in prediction of nonequilibrium radiative heat flux to the stagnation-point. In the second case, the electronic-state-resolved analysis of nonequilibrium air radiation is performed to assess accuracy of the conventional two-temperature model with the quasi-steady state assumption when the free stream flow is in a highly nonequilibrium condition. In the comparison with the measured nonequilibrium spectrum, the present electronic master equation coupling method is more accurate than the conventional two-temperature approach when used to estimate the initial rising rate and peak value of the diatomic intensity and small amounts of atomic radiation when the diatomic nonequilibrium condition is dominant. Moreover, the spatial distributions of the intensity and electron number density are more accurately predicted by the electronic master equation coupling methods when the flow-fields are dominated by atomic nonequilibrium. From these results, it is found that the conventional two-temperature model should be improved to more accurately predict nonequilibrium radiative transition in high-enthalpy air plasma flows. Based on the results obtained in the second case, an improvement of thermochemical nonequilibrium model is made by modifying chemical reaction rate coefficients and thermal energy exchange rates, and modeling the rotational nonequilibrium energy transfers. In the rotational nonequilibrium modeling, the state-resolved master equation analysis is carried out, and the rotational-translational energy relaxation time is newly determined. In the wavelength ranges from VUV to IR, the present nonequilibrium model represents improved accuracy in comparison with the previous thermochemical nonequilibrium parameters, based on the two-temperature description. The improvement is mainly attributed by: 1) Modification of vibrational energy relaxation consists of chemical dissociation and vibrational-translational energy transfer. 2) Modeling of rotational nonequilibrium resolving the overestimation of high-lying rotational states in nonequilibrium region.