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
More information is available at Theses and Dissertations at University of Maryland Libraries.
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Item ADVANCED PACKAGING AND THERMAL MANAGEMENT OF DC-DC CONVERTERS AND NOVEL CORRELATIONS FOR MANIFOLD MICROCHANNEL HEATSINKS(2021) Yuruker, Sevket Umut; Ohadi, Michael; Mechanical Engineering; Digital Repository at the University of Maryland; University of Maryland (College Park, Md.)An advanced packaging configuration of a dual-active-bridge 10 kW DC-DC converter module is introduced in this dissertation. Through utilization of novel heatsinks for the power switches and the transformer assembly, ~20 kW/Lit converter volumetric power density based on numerical and experimental analysis is obtained. Through a unique placement of the high power/high frequency SiC switches on the printed circuit board, many beneficial features such as double-sided cooling, complete elimination of wirebonding, and circumvention of the need for TIM layers between the switches and the heatsinks, and multi functioning heatsinks as electrical busbars is achieved. A Vertically Enhanced Manifold Microchannel System (VEMMS) cooler is developed to address the thermal challenges of a pair of power switches, simultaneously. Both air and liquid cooled versions of VEMMS cooler is presented, thermal resistances of 1.1 K/W and 0.3 K/W for the air and liquid cooled versions, respectively, at reasonable flow rates and pressure drops was obtained. Besides the power switches, thermal management of the transformer assembly is accomplished via Combined Core and Coil (C3) Coolers, where both the magnetic core and coils are liquid cooled simultaneously with electrically insulating but thermally conductive 3D printed Alumina heatsinks, where thermal resistances as low as 0.3 K/W for the magnetic core and 0.09 K/W for the transformer windings is experimentally demonstrated. Furthermore, a system level model was built to investigate the effect of various components in the cooling loop on each other, and what are the limiting factors to prevent a possible thermal runaway failure. Lastly, using a metamodeling approach, closed form pressure drop and heat transfer correlations are developed for thermo-fluidic performance prediction of manifold microchannel heatsinks. Due to complexity and vastness of design variables present in manifold microchannel systems, adequate CFD analysis and optimization require significant computational power. Through utilization of the developed correlations, orders of magnitude reduction in computational time (from days to milliseconds) in prediction of pressure drop and heat transfer coefficient is demonstrated. Extensive mesh independence and residual convergence algorithms are developed to increase accuracy of the created database. Between the correlation and mesh independent CFD results, a mean error of 3.9% and max error of 24% for Nusselt number, and a mean error of 4.6% and max error of 37% for Poiseuille Number predictions are achieved.Item HOLDING DECISIONS FOR CORRELATED VEHICLE ARRIVALS AT INTERMODAL FREIGHT TRANSFER TERMINALS(2016) Sun, Yanshuo; Schonfeld, Paul M.; Civil Engineering; Digital Repository at the University of Maryland; University of Maryland (College Park, Md.)A vehicle holding method is proposed for mitigating the effect of service disruptions on coordinated intermodal freight operations. Existing studies are extended mainly by (1) modeling correlations among vehicle arrivals and (2) considering decision risks with a mean-standard deviation optimization model. It is shown that the expected value of the total cost in the proposed formulation is not affected by the correlations, while the variance can be miscomputed when arrival correlations are neglected. Some implications of delay propagation are also identified when optimizing vehicle holding decisions in real-time. General criteria are provided for determining the boundary of the affected region and length of the numerical search, based on the frequency of information updates. Theoretical analyses are supported by three numerical examples.Item Atom-trapping and photon-counting experiments with optical nanofibers(2015) Grover, Jeffrey Aaron; Orozco, Luis A; Rolston, Steven L; Physics; Digital Repository at the University of Maryland; University of Maryland (College Park, Md.)New effects can arise in quantum physics when there is strong coupling, either between atoms and light or between different quantum systems. This thesis examines an optical nanofiber atom trap as a mediator of atom-light interactions and a potential element of a hybrid quantum system. The evanescent field around the sub-wavelength waist of an optical nanofiber possesses a small mode area that increases the cooperativity between atoms and the mode, in a manner analogous to traditional cavity QED. We demonstrate trapping of $^{87}$Rb atoms with an optical nanofiber, confining hundreds of atoms with typical trap lifetimes of tens of milliseconds. We then employ single photon counting techniques to study untrapped ensembles of cold atoms around the nanofiber. A first experiment uses intensity autocorrelations of resonance fluorescence emitted into the nanofiber mode to observe a transition from classical to nonclassical photon statistics. Measuring the correlations on longer timescales reveals the motion of atoms through the optical mode, and we develop a correspondence between the transit time and atomic cloud temperature. A second experiment measures Purcell enhancement of spontaneous emission of atoms near the nanofiber by correlating their fluorescence with a known trigger event. The spontaneous decay rate of an atom near a dielectric is modified by the induced dipole and by a change in the modes of the vacuum electromagnetic field. Our observed enhancement of $6.5\pm0.9\%$ over the free-space rate matches well with what one finds from mode simulations in our system.Item PREDICTION OF HEAT TRANSFER AND PRESSURE DROP OF CONDENSING REFRIGERANT FLOW IN A HIGH ASPECT RATIO MICRO-CHANNELS(2009) Al-Hajri, Ebrahim Saeed Abdulla; Ohadi, Michael; Mechanical Engineering; Digital Repository at the University of Maryland; University of Maryland (College Park, Md.)This thesis presents a detailed study of parametric characterization of two-phase condensing flow of two selected refrigerants R134a and R-245fa in a single water-cooled micro-channel of 0.4 mm X 2.8 mm cross-section (0.7 mm hydraulic diameter and 7:1 aspect ratio) and 190 mm in length. To avoid flow mal-distribution associated with typical micro-channel tube banks, a single micro-channel was fabricated utilizing an innovative approach and used for the present study experiments. The study investigated the effects of variations in saturation temperature ranging from 30 oC to 70 oC, mass flux from 50 to 500 kg/m2s, and inlet super heat from 0 oC to 15 oC on the average heat transfer and overall pressure drop coefficient of the micro-channel condenser. In all cases the inlet vapor quality was kept at 100% quality (saturated vapor) and the outlet condition was always kept at 0% quality (saturated liquid). Accuracy of the fabricated channel geometry with careful design and choice of instrumentation of the test setup resulted in energy balance and average heat transfer coefficient uncertainties within +/-11% and +/-12%, respectively. It is observed that saturation temperature and mass flux have a significant effect on both heat transfer coefficient and overall pressure drop coefficient, where as the inlet super heat has little effect. This study provides further understanding of the potential micro-scale effects on the condensation phenomenon for the tube geometry and the dimensions investigated in the present study combined with flow visualization study. No previous study has addressed the unique single micro-channel geometry being investigated in the present work combined with the two-phase flow visualization of the flow regimes in the present micro-channel geometry. The letter was a major undertaking of the present work and represents one of the main contributions of the present work. The results of the present work shall prove useful in contributing to better understanding of any micro-scale effects on the condensation flow of the two selected refrigerants (one commonly used high pressure refrigerant, R134a) and the other a new low pressure refrigerant (R245fa). It is also expected that the results of this study will lead to future work in this area, realizing the fast penetration of the micro-channel technology in various compact/ultra compact heat exchangers, including refrigeration, petrochemical, electronics, transportation, and process industries.