Theses and Dissertations from UMD
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New submissions to the thesis/dissertation collections are added automatically as they are received from the Graduate School. Currently, the Graduate School deposits all theses and dissertations from a given semester after the official graduation date. This means that there may be up to a 4 month delay in the appearance of a give thesis/dissertation in DRUM
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Item An Investigation of Flames, Deflagrations, and Detonations in High-Speed Flows(2018) Goodwin, Gabriel Benjamin; Oran, Elaine S; Aerospace Engineering; Digital Repository at the University of Maryland; University of Maryland (College Park, Md.)A comprehensive understanding of the fundamental physics underlying combustion and detonations in turbulent and high-speed flows is crucial to the design of robust ramjet, scramjet, and detonation engines. This work uses high-fidelity, multidimensional numerical simulations to investigate flame stability and deflagration-to-detonation transition (DDT) mechanisms in supersonic reactive flows. The study consists of four major sections. The first section discusses the acceleration of a flame in a channel with obstacles and its transition from a laminar, expanding flame to a turbulent deflagration and eventual detonation. As the flame accelerates, a highly dynamic, shock-heated region forms ahead of the flame. Shock collisions and reflections focus energy in localized volumes of unburned gas at timescales that are small relative to the acoustic timescale of the unburned gas. The rapid deposition of energy causes the unburned gas to detonate through an energy-focusing mechanism that has elements of both direct initiation and detonation in a gradient of reactivity. The second section describes how the blockage of a channel with regularly spaced obstacles, analogous to the igniter in a detonation engine, affects flame acceleration and turbulence in the region ahead of the accelerating flame. The rate of flame acceleration, time and distance to DDT, and detonation mechanism are compared for channels with high, medium, and low blockage ratios. Stochasticity and uncertainty in the numerical solutions are discussed. In the third section, the stability of premixed flames at high supersonic speeds in a constant-area combustor is investigated. After autoignition of the fuel-oxidizer mixture in the boundary layer at the combustor walls, the flame front eventually becomes unstable due to a Rayleigh-Taylor (RT) instability at the interface between burned and unburned gas. The turbulent flame front transitions to a detonation through the energy-focusing mechanism when a shock passes through the flame and amplifies its energy release. The final section discusses the effect of inflow Mach number in the supersonic combustor on ignition, flame stability, and transition to detonation of a premixed flame. Timescales for growth of the RT instability and detonation initiation increase rapidly with flow speed, but, qualitatively, flame evolution is independent of Mach number.Item EXPERIMENTAL AND COMPUTATIONAL ANALYSIS OF AN ELECTROHYDRODYNAMIC MESOPUMP FOR SPOT COOLING APPLICATIONS(2004-11-22) Shoushtari, Amir H.; Ohadi, Michael M.; Mechanical Engineering; Digital Repository at the University of Maryland; University of Maryland (College Park, Md.)As electronic products become faster, more compact, and incorporate greater functionality, their thermal management becomes increasingly more challenging as well. In fact, shrinking system sizes, along with increasing circuit density, are resulting in rapid growth of volumetric heat generation rate and reduction in surface area for adequate heat dissipation. Moreover, system miniaturization by employing microfabrication technology has had a great influence on thermal and fluid research Smaller systems have many attractive characteristics and can be more conveniently fabricated using batch production technologies. One of the fields showing promising potential in microsystems and electronics cooling is the use of the phenomenon of electrohydrodynamics or EHD defined as a direct interaction between the electric and hydrodynamic fields where the electric field introduces fluid motion. The objectives of the present study were to identify the physics of these phenomena as related to the present study, to simulate it numerically, and to verify the modeling through experiments. More specifically, the goals were to develop a novel numerical methodology to simulate the highly complex interaction between fluid flow and electrical fields. Next, to verify the model a mesoscale ion-injection pump was designed and fabricated, followed by a set of experiments that characterized the pump's performance. The experiments will also demonstrate the application potential of the concept in electronics cooling and particularly for spot cooling applications. Experimental tests were conducted on an EHD ion-injection mesopump to measure the flow rates and generated pressure heads with HFE -7100 as working liquid. It is shown that the results of two different flow rate measurement techniques that were employed, are in agreement. The experimental results also show that maximum flow rate of about 30 ml/min and pressure head of 270 Pa for the electrode gap of 250 m and voltage of 1500 V are achievable. A novel numerical modeling method was developed that incorporates both the injection and dissociation of ions. This modeling method is used to simulate the EHD mesopump. The numerical results show a fairly good agreement with experimental data.