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 The Effects of Gravity on Flow Boiling Heat Transfer(2021) Hammer, Caleb Franklin; Kim, Jungho; Mechanical Engineering; Digital Repository at the University of Maryland; University of Maryland (College Park, Md.)Flow boiling is a method of phase change heat transfer used widely in electronics cooling, refrigeration, air conditioning, and other areas where stable temperatures are needed. An area of interest is spaceflight systems, where efficient heat transfer is desired to minimize mass, power requirements, and cost. When compared to terrestrial gravity conditions, the heat transfer of flow boiling in microgravity typically depreciates. This depreciation has been documented across multiple experimental studies performed by teams using different fluids, tube geometries, and flow regimes over the past three decades. Though select experimental microgravity flow boiling heat transfer data are available in the literature, holistic results are sparse due to the cost and limited availability of microgravity research. The two-phase heat transfer mechanisms responsible for the depreciation are therefore not well known, and so heat transfer models for variable gravity flow boiling do not exist. The goal of the proposed study is to develop models for flow boiling heat transfer through a tube as a function of gravity by identifying the effect of gravity on different heat transfer mechanisms. The scope of this proposal involves modeling three microgravity flow regimes (bubbly, slug, and annular flow) to serve as baseline predictions for flow boiling heat transfer without the influence of gravity. Additional gravity effects can be identified using partial and hyper-gravity data. Experiments have been performed aboard parabolic flights and on the ground at various flow rates, heating rates, and inlet subcoolings in microgravity, hyper-gravity, Lunar gravity, Martian gravity, and terrestrial gravity. Results from the experiments showed that negligible slip velocity plays an important role in modeling flow boiling heat transfer. Simulations using modified single-phase models of an accelerating flow were performed which predicted microgravity flow boiling heat transfer well in the nucleate boiling regime.Additional experiments concerning terrestrial gravity quenching heat transfer have been performed to address research gaps in microgravity cryogen chilldown studies. Quenching heat transfer coefficients were recorded in the nucleate boiling regime and compared with correlations. The correlations were able to predict heat transfer for room temperature fluids much more accurately than for cryogenic fluids. Scaling parameters must be tuned to match cryogen data to examine the large disparity between cryogenic quenching heat transfer data and correlations observed in the literature.Item An Experimental Study of Static and Oscillating Rotor Blade Sections in Reverse Flow(2015) Lind, Andrew Hume; Jones, Anya R; Aerospace Engineering; Digital Repository at the University of Maryland; University of Maryland (College Park, Md.)The rotorcraft community has a growing interest in the development of high-speed helicopters to replace outdated fleets. One barrier to the design of such helicopters is the lack of understanding of the aerodynamic behavior of retreating rotor blades in the reverse flow region. This work considers two fundamental models of this complex unsteady flow regime: static and oscillating (i.e., pitching) airfoils in reverse flow. Wind tunnel tests have been performed at the University of Maryland (UMD) and the United States Naval Academy (USNA). Four rotor blade sections are considered: two featuring a sharp geometric trailing edge (NACA 0012 and NACA 0024) and two featuring a blunt geometric trailing edge (ellipse and cambered ellipse). Static airfoil experiments were performed at angles of attack through 180 deg and Reynolds numbers up to one million, representative of the conditions found in the reverse flow region of a full-scale high-speed helicopter. Time-resolved velocity field measurements were used to identify three unsteady flow regimes: slender body vortex shedding, turbulent wake, and deep stall vortex shedding. Unsteady airloads were measured in these three regimes using unsteady pressure transducers. The magnitude of the unsteady airloads is high in the turbulent wake regime when the separated shear layer is close to the airfoil surface and in deep stall due to periodic vortex-induced flow. Oscillating airfoil experiments were performed on a NACA 0012 and cambered ellipse to investigate reverse flow dynamic stall characteristics by modeling cyclic pitching kinematics. The parameter space spanned three Reynolds numbers (165,000; 330,000; and 500,000), five reduced frequencies between 0.100 and 0.511, three mean pitch angles (5,10, and 15 deg), and two pitch amplitudes (5 deg and 10 deg). The sharp aerodynamic leading edge of the NACA 0012 airfoil forces flow separation resulting in deep dynamic stall. The number of associated vortex structures depends strongly on pitching kinematics. The cambered ellipse exhibits light reverse flow dynamic stall for a wide range of pitching kinematics. Deep dynamic stall over the cambered ellipse airfoil is observed for high mean pitch angles and pitch amplitudes. The detailed results and analysis in this work contributes to the development of a new generation of high-speed helicopters.Item DETERMINATION OF SIDEROPHILE ELEMENT CHARACTERISTICS THROUGHOUT LUNAR HISTORY: IMPLICATIONS FOR THE LUNAR MAGMA OCEAN AND LATE HEAVY BOMBARDMENT(2014) Sharp, Miriam; Walker, Richard J; Geology; Digital Repository at the University of Maryland; University of Maryland (College Park, Md.)Examining the chemical behavior of highly siderophile elements (HSE) in impact events and during planetary differentiation can illuminate geologic processes that have affected the Moon. This dissertation addresses impactor compositions during the putative late heavy bombardment and the chemical composition of the evolving lunar magma ocean at both the times of core segregation and crust formation. Concentrations of the HSE Re, Os, Ir, Ru, Pt, and Pd and 187Os/188Os isotopic compositions are reported for seven Apollo 17 and four Apollo 16 impact melt rocks. Most Apollo 17 samples examined here as in prior studies are characterized by very similar HSE signatures, consistent with a common impactor that had suprachondritic Ru/Ir, Pd/Ir, and Re/Os. In contrast to the Apollo 17 signature, the Apollo 16 impact melts have a wider range of Ru/Ir, Pd/Ir, and Re/Os. This compositional range might be the result of sampling at least three impactor signatures at this site. Experimentally determined plagioclase-melt partition coefficients are also presented. These partition coefficients are used to estimate the concentrations of Sr, Hf, Ga, W, Mo, Ru, Pd, Au, Ni, and Co in a crystallizing lunar magma ocean at the point of plagioclase flotation. Plagioclase-melt derived concentrations for Sr, Ga, Ru, Pd, Au, Ni, and Co are also consistent with prior estimates. Estimates for Hf, W, and Mo, however, are higher. These elements may have concentrated in the residual liquid during fractional crystallization, due to their incompatibility. Experimentally determined metal-silicate partition coefficients are used to constrain the concentrations of W, Mo, Ru, Pd, Au, Ni, and Co in the lunar magma ocean at the time of core formation. The resulting lunar mantle estimates are generally consistent with previous estimates for the concentration of these elements in the lunar mantle. Together, these new results are used to present a compositional timeline for the Moon between the crystallization of the lunar magma ocean and the late heavy bombardment.Item The Effect Of Surfactants On The Breakup Of An Axisymmetric Laminar Liquid Jet(2012) Walker, Justin Robert; Calabrese, Richard V; Chemical Engineering; Digital Repository at the University of Maryland; University of Maryland (College Park, Md.)The contacting of multiple liquid phases is a complex process, and one that is difficult to study experimentally. Liquid dispersion studies in stirred tanks and high shear mixers frequently involve the use of surfactants without a strong physical understanding of how the surfactants affect the mechanics of droplet production and breakup. In this study, experiments are performed using the axisymmetric laminar jet system. The breakup of a laminar axisymmetric jet is a well-studied fluid dynamics phenomenon. Despite the extensive literature on jet breakup, the impact of surface active agents on jet breakup has received limited attention. An extensive series of experiments with water-air and oil-water jet systems with and without surfactants has been performed, varying fluid flow rate, jet diameter, jet bulk viscosity, surfactant type, and surfactant concentration. Surfactants were found to significantly affect the breakup of laminar liquid jets. Significant effects on both the length of jets and the size of resulting droplets are reported. In general, the effect of surfactants is to reduce the interfacial tension of the system in question, which results in longer jet breakup lengths and larger diameter droplets. However, the interfacial tension alone is insufficient to explain the physics of the jet breakup phenomena. Several breakup mechanisms were identified, and the regimes in which each operates vary not only due to jet geometry and velocity, but on the interfacial properties as well. The effect of surfactants on the breakup phenomena differs in each of these distinct breakup regimes. A mechanistic model for the prediction of breakup length for surfactant laden jets is presented. This model results in good agreement between predicted and experimentally observed values over a wide variety of surfactant concentrations and jet conditions and was shown to be useful for both the oil-water and water-air systems, within the axisymmetric jetting regime.Item Metals in Arc Magmas: The Role of Cu-Rich Sulfide Phases(2011) Mengason, Michael James; Candela, Philip A; Piccoli, Philip M; Geology; Digital Repository at the University of Maryland; University of Maryland (College Park, Md.)Based on experiments performed on hydrous andesitic melts at 1000°C, 150 MPa, fO2 from the Co-CoO to Ni-NiO buffer, and log fS2 equal to -0.5 to -1.5 (bar), greater than 32 ± 4 ppm copper (all uncertainties = 1 sigma, standard deviation of the mean) in the silicate melt favors the formation of a Cu-Fe sulfide liquid (CFSL) relative to pyrrhotite at sulfide saturation. This concentration is well within the range encountered in intrusive and extrusive rocks suggesting that saturation by sulfide liquids is a common occurrence in magmatic arc systems consistent with observations in naturally occurring andesites. Nernst-type partition coefficients determined from these experiments highlight the importance of accurately modeling the composition of the sulfide phase present during partial melting or fractional crystallization: Dpyrrhotite/melt = 1320 ± 220 for Cu, 1.73 ± 0.37 for Mo, 90 ± 19 for Ag, and 500 ± 87 for Au, whereas DCFSL/melt = 7,800 ± 1,400 for Cu, 0.45 ± 0.14 for Mo, 6,800 ± 1,300 for Ag, and 84,000 ± 19,000 for Au. Data from these experiments support a direct correlation between the solubility of gold and the concentration of sulfur in the silicate melt at low fO2, as well as a dependence of the solubility of gold on fS20.25 in pyrrhotite and CFSL. As a part of this research, pyrrhotite of variable copper concentration was equilibrated at 1000°C in sealed evacuated silica tubes to determine a method that allows the equation of Toulmin and Barton (1964) to be used to calculate fS2 for Cu-bearing pyrrhotite. This method is consistent for pyrrhotite with up to 6 wt % Cu by using N=2*[(XCu+XFe)/(1.5XCu+XFe+XS)]. These data suggest that separation of CFSL from the magma along with crystalline phases during fractional crystallization can reduce the likelihood of magmatic hydrothermal ore formation. For example, modeling 30 % Rayleigh fractional crystallization (F=1.0 to F=0.7), with 0.1% sulfide among the separating phases, and an initial 65 ppm Cu in the silicate melt, would result in the sequestration of up to 50% of the initial Ag, 60 % Cu, and > 99 % Au.Item Three Quasi-Experimental and Experimental Papers in Environmental Economics(2011) Herberich, David; McConnell, Ted; Agricultural and Resource Economics; Digital Repository at the University of Maryland; University of Maryland (College Park, Md.)This dissertation utilizes quasi-experimental and experimental techniques to contribute to the literature on environmental and resource economics in three distinct ways but with the overarching goal of demonstrating the usefulness of experimental techniques to explore topics related to the environment. First, chapter one demonstrates the feasibility and draw-backs of using a quasi-experimental regression discontinuity approach to analyze the ozone regulation contained in the 1990 Clean Air Act Amendments. Chapter two explores the impacts how economic and social-psychology factors affect the adoption of an environmental technology, namely compact fluorescent light bulbs. In order to consider these factors, chapter two utilizes a large-scale field experiment informed by a theoretical model of adoption. Finally, chapter three utilizes a field experiment designed for a large apartment management company, to advance the literature on presumed (opt-out) and explicit (opt-in) consent procedures by exploring a willingness to pay to forgo the decision to opt-out of the installation of an environmental technology.