Browsing by Subject "Thermophysics"
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Item Modeling and Analysis of Chemical Kinetics for Hypersonic Flows in Air(2018-11) Chaudhry, RossGas-phase chemical kinetics are relevant for hypersonic flows, but they are currently modeled in CFD using empirical assumptions and decades-old experimental data. Recent advances in quantum chemistry have enabled the construction of accurate potential energy surfaces (PESs) for diatom-diatom interactions in air. Using these PESs, a database of simulated interactions is generated and analyzed; N2 + N2, N2 + N, N2 + O2, O2 + O2, and O2 + O reactions are considered. The conditions studied range from 4000 K to 30,000 K and include thermal equilibrium and nonequilibrium test sets. The nitrogen dissociation rate is found to be similar for collision partners N2, N, and O2. The oxygen dissociation rate, in contrast, is moderately dependent on partner species; O2 is approximately 2 to 3 times more effective than partner N2. Oxygen dissociation with partner N2 is therefore found to be substantially overpredicted by current CFD models, which is consistent with the limited experimental data available for this reaction. The presence of N is known from experiments to promote nitrogen dissociation; this augmentation is found to be due to increased vibrational relaxation, rather than an increased dissociation rate as described by current CFD models. Similar observations are made for oxygen dissociation with partner O, due to a combination of vibrational and electronic energy relaxation. Using only the shock tube data that informed popular CFD models, it was impossible to isolate the effect of increased relaxation from increased dissociation. The change in vibrational energy per dissociation, a necessary input to CFD, is found to be very sensitive to the degree of thermal nonequilibrium. This dependence is not well predicted by any existing chemical kinetics models; correctly describing this term fundamentally changes the thermal evolution of a gas in CFD. The mechanics of dissociation are similar for all reactions studied, so a series of aggregate analyses on all dissociation reactions is performed. Vibration is found to have a more pronounced effect on dissociation than rotation, due to rotation increasing the centrifugal barrier. The classic Marrone-Treanor preferential dissociation model is found to accurately describe all data in the nonequilibrium test sets, but it neglects the effect of rotational energy on dissociation. A modified model is proposed that describes rates to within 22% and vibrational energy changes to within 4% of the dissociation energy, for all dissociation reactions and conditions. For this work, we have considered Boltzmann or approximately Boltzmann distributions, but the population of high-energy molecules is known to be depleted in a dissociating gas ensemble. Various kinetics models based on Boltzmann distributions are implemented in US3D, a production CFD solver designed for hypersonic flows. As expected, the dissociation rate is overpredicted compared to the benchmark data. Work remains, therefore, to account for the non-Boltzmann distributions that exist in reality. These data and insights about dissociation can form the basis for next-generation chemical kinetics models for CFD.Item Nano-scale Heat Transfer in Nanostructures: Toward Understanding and Engineering Thermal Transport(2017-05) Ma, JihongHeat transfer is vital throughout research and industry. This thesis focuses on heat transfer in nanostructures and amorphous materials, in which the arrangement of atoms is crucial for the effectiveness of heat transport. Defects and mechanical deformations in a material which cause displacement or reconfiguration of atoms relative to that material’s “normal” or “pristine” condition can dramatically influence its heat transport efficiency. Since the 1950’s, there has been little progress in understanding the defects–thermal transport property relationship. Using novel numerical techniques and large-scale computations performed on modern supercomputers, I have studied heat transport in nanomaterials containing various defects and mechanical deformations. From the properties of atomic vibrations in my simulations, the effects these deformations have on heat transport can be deduced. Three research projects are presented in this thesis. The study of heat transport in screw-dislocated nanowires with low thermal conductivities in their bulk form represents the knowledge base needed for engineering thermal transport in advanced thermoelectric and electronic materials. This research also suggests a new potential route to lower thermal conductivity, which could promote thermoelectricity. The study of high-temperature coating composite materials helps with the understanding of the role played by composition and the structural characterization, which is difficult to be approached by experiments. The method applied in studying the composition-structure-property relationship of amorphous Silicon-Boron-Nitride networks could also be used in the investigation of various other similar composite materials. Such studies can further provide guidance in designing ultra-high-temperature ceramics, including space shuttle thermal protection system materials and high-temperature-resistance coating. The understanding of the impact of bending and collapsing on thermal transport along carbon nanotubes is important as carbon nanotubes are excellent materials candidates in a variety of applications, including thermal interface materials, thermal switches and composite materials. The atomistic study of carbon nanotubes can also provide crucial guidance in multi-scale study of the materials to enable large-scale thermal behavior prediction.