Atomistic Simulations of High Temperature Air Chemistry Including Three-Body Collisions and Recombination
2023-05
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Atomistic Simulations of High Temperature Air Chemistry Including Three-Body Collisions and Recombination
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2023-05
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This dissertation studies chemical reactions relevant to modeling high-speed flight through Earth’s atmosphere. Shock-heated mixtures of nitrogen and oxygen undergo gas phase chemical reactions that control the concentrations of O2, O, N2, N, and NO (aptly named five-species air) that impact the heat shielding of hypersonic craft. Predicting the extreme gas state surrounding the heat shield material is extremely complex because the time scales of fluid flow, internal energy relaxation, and chemical reactions all become similar. Experiments studying this process are difficult to perform, expensive, do not exactly reproduce flight conditions, and require assumptions to interpret the measured data. First-principles computational simulations are now being used to understand these chemical processes. Advances in computational chemistry techniques and high-performance parallel computing have made the comprehensive study of these processes in dilute gases possible. To date, these computational studies have focused heavily on reactions of oxygen and nitrogen and have focused on the dissociation of diatomic molecules. Because it is highly reactive, atomic oxygen is incredibly important to model correctly. The formation and destruction of nitric oxide (NO) is also important to understand because NO is liable to emit energy as radiation. This thesis uses first-principles molecular simulation in several ways to complete the study of reactions in five-species air. Potential Energy Surfaces (PESs) describing interatomic forces in oxygen are used to simulate reactions of oxygen atoms and diatoms, and the predictions are validated against new molecular beam experimental data. This study shows that the new PESs used to describe these reactions do a better job of reproducing experimental results than earlier efforts, and that this improvement results from a more complete description of the relevant chemical states of the oxygen system. A QuasiClassical Trajectory (QCT) study of reactions that form and destroy nitric oxide is performed using new PESs and compared to Direct Molecular Simulation (DMS). These comparisons show that dissociation reactions in air mixtures are biased towards high energy vibrational states of reactant diatoms, but exchange reactions (referred to as Zeldovic reactions) are not as strongly biased. This suggests that while reduced-order chemistry models for hypersonic flow simulations should account for vibrational bias in dissociation, they do not need to account for any vibrational bias in the Zeldovic reactions. A major contribution of this thesis is the development of a new QCT theory and simulation framework to study recombination processes that involve the collisions of three neutral heavy particles. This ternary kinetic approach is based on a definition of the lifetime of binary collisions that is consistent with hard-sphere models in the limit of instantaneous reactions, and does not require an explicit appeal to the principle of detailed balance. The aspects of different sampling strategies for this method and the convergence of recombination rate constants are investigated. The results of this method are compared to the predictions of detailed balance, the results of dissociation QCT simulations, and a reduced-order chemistry model formulated using dissociation data combined with detailed balance. These comparisons show that the ternary kinetic method reproduces similar trends to those seen in dissociation studies and a reduced-order model built from them. Further analysis shows that while long-lived binary collisions are more likely to be hit by a third particle when they occur, they occur extremely rarely, are not any more likely to cause recombinations than short-lived binary collisions, and do not produce molecules in vastly different states than those predicted by dissociation data and the principle of detailed balance.
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University of Minnesota Ph.D. dissertation. May 2023. Major: Aerospace Engineering and Mechanics. Advisor: Thomas Schwartzentruber. 1 computer file (PDF); xii, 152 pages.
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Geistfeld, Eric. (2023). Atomistic Simulations of High Temperature Air Chemistry Including Three-Body Collisions and Recombination. Retrieved from the University Digital Conservancy, https://hdl.handle.net/11299/264306.
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