Improved non-boltzmann modeling for nitrogen atoms

At high entry velocity, a significant contribution of the afterbody heating comes from the radiation of atomic nitrogen. Large uncertainties exist in the prediction of this radiative heating, mainly due to the poor theoretical basis of the underlying physical models. In particular, the modeling of electron-ion recombinaison processes, which is known to have a large impact on radiation, is highly sensitive to the distribution of the upper electronic states. However, the widespread approach of assuming a Boltzmann distribution of these electronic states is actually unable to capture non-equilibrium effects in the internal population, preventing such models to provide an accurate description of the thermodynamic state of the gas. Indeed, the large overpopulation of the high-lying bound electronic states in expanding flow, such as in wakes and nozzles, leads to a very large departure of the internal levels’ population from the equilibrium Boltzmann distribution. Such a modeling deficiency is expected to have a large influence on the rate of electron-ion recombinaison, and, consequently, on the predicted magnitude of radiation reaching the afterbody surface. In order to address this issue, this papers investigates the non-Boltzmann modeling of the atomic nitrogen’s electronic states, focusing on electronic excitation through electron-impact processes. These processes constitute a very efficient means of exciting electronic states due the small electron mass and to the long-range nature of the interaction potential associated to charge/neutral collisions. A high fidelity representation of the internal structure of N is considered by adopting a state-to-state description of its electronic states. A new set of electron-impact excitation rates is constructed based on a combination of approximate formulas and recent data computed from detailed quantum calculations. In order to fulfill the needs of practical multi-dimensional simulations, a reduced-order models is developed based on the detailed nitrogen atomic model and the full excitation rate database. The improvements in the modeling of electron-impact processes is discussed using results from both time-dependent and quasi-steady state simulations. The accuracy of the grouping strategy is also assessed by analyzing the error introduced by the reduced-order models for several flow conditions.