Thermal nonequilibrium models for high-temperature reactive processes
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This dissertation examines how thermal nonequilibrium affects mixing and combustion in high-enthalpy, high-speed systems such as reentry vehicles, scramjets, and detonation-driven engines. Specifically, the focus is in the development of physical models that accurately describe thermal nonequilibrium in a continuum-scale ow and its relative effect on mixing and reaction processes. To this end, the quasi-classical trajectory method is utilized, in which bimolecular collisions (or trajectories) are individually simulated. The aggregate of the outcomes from many trajectories is then used to calculate the macroscopic reaction and scattering rates of the system. A QCT program is presented for massively parallel simulations, which includes an algorithm for calculating and tabulating the potential energy surface throughout the QCT simulation. Using the QCT program, chain-reactions in hydrogen combustions are simulated, and the subsequent rates are used directly in CFD simulations as well as to develop vibrational nonequilibrium reaction rate models. Also, nitrogen dissociation is simulated to calculate the dissociation rate as a function of independent translational, rotational, and vibrational temperatures, thus extending the conventional two-temperature model. This simulation is made tractable via a new method for selectively sampling trajectories. Finally, the QCT program is utilized to calculate N₂-O₂ inelastic cross-sections. This work was motivated by CFD simulations of experimental observations which indicated that the conventional N₂-O₂ vibrational exchange rates were invalid at moderate temperatures. The QCT-calculated rates support these observations. In addition to QCT-based simulations, 1D and 2D simulations of detonation waves with vibrational nonequilibrium (modeled using the aforementioned data) are analyzed. It is observed that nonequilibrium only marginally affects the induction zone of the detonation wave. However, in the 2D simulations, it is observed that vibrational nonequilibrium plays a critical role in determining detonation cell sizes. In summary, vibrational nonequilibrium is analyzed using QCT for a variety of systems, and the resulting data is utilized to develop CFD-scale models. We have high confidence in the resulting models because they are derived from first principles and microscopic observations as opposed to simplified models or empirical fits.