Direct Molecular Simulation of Nitrogen and Oxygen at Hypersonic Conditions

Abstract

The objective of this thesis is to characterize the gas-phase thermochemical non-equilibrium that occurs during hypersonic flight for nitrogen and oxygen gases. This thesis uses the direct molecular simulation (DMS) method in conjunction with potential energy surfaces (PESs) to provide an in-depth molecular level analysis of internal energy excitation and dissociation of molecular nitrogen due to $N_2+N_2$ and $N_2+N$ interactions. Characteristic vibrational excitation times and non-equilibrium dissociation rate coefficients are calculated using the $ab-initio$ PESs developed at NASA Ames Research Center. Comparison of these rate coefficients and non-equilibrium vibrational energy distributions is carried out against prior work done with nitrogen using an independently developed $ab-initio$ PES at the University of Minnesota. Good agreement was found between properties predicted by the two PESs. Furthermore, comparative studies were carried out for the nitrogen system between the DMS method and the state-to-state method. The results obtained by the two different methods, are found to be in good agreement. The DMS method is used to calculate benchmark data for vibrational energy excitation and non-equilibrium dissociation due to $O_2+O$ interactions. $O_2+O$ interactions are modeled using nine PESs corresponding to. $1^1A'$, $2^1A'$, $1^1A''$, $1^3A'$, $2^3A'$, $1^3A''$ $1^5A'$, $2^5A'$ and $1^5A''$ states, which govern electronically adiabatic (ground-electronic-state) collisions of diatomic oxygen with atomic oxygen. This is the first data set in the aerospace community that incorporates all nine PESs for the $O_2+O$ system and fully describes the dynamics of ground state interactions of diatomic oxygen with atomic oxygen. Characteristic vibrational excitation times are calculated over a temperature range of $T=3000K$ to $T=15000K$. It is observed that the characteristic vibrational excitation time for $O_2+O$ interactions is weakly dependent on temperature and increases slightly with increasing temperature. Vibrational excitation is slowest for interactions in the quintet spin state, with the $1^5A''$ state having the slowest excitation rate, and vibrational excitation is fastest on the $1^1A'$ potential energy surface. Non-equilibrium dissociation rate coefficients are calculated over a temperature range of $T=6000K$ to $T=15000K$ during quasi-steady state (QSS) dissociation, and the results agree well with experimental data. For the $O_2+O_2$ system interactions can occur over singlet, quintet and triplet spin states. An in-depth analysis of excitation and dissociation on the quintet and singlet surfaces is provided and bench-mark data for excitation using all three PESs for $O_2+O_2$ interactions is presented for a temperature range of $T=5000K$ to $T=12000K$ . Finally, this thesis explores internal energy exchange processes in oxygen and nitrogen. Probability distribution functions for vibrational energy change during collisions are presented (due to $N_2+N_2$ non-reactive collisions, $N_2+N_2$ exchange reactions, $N_2+N$ non-reactive collisions, $N_2+N$ exchange reactions, $O_2+O$ non-reactive collisions, and $O_2+O$ exchange reactions). It is shown that non-reactive collisions are less efficient in vibrational energy redistribution when compared to exchange reactions. Furthermore, it is observed that the probability distribution functions for vibrational energy change (for both oxygen and nitrogen) are self-similar and may be modeled by simplified functional forms.