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 Phase Change Materials for Vehicle and Electronic Transient Thermal Systems(2020) Jankowski, Nicholas Robert; McCluskey, F. Patrick; Mechanical Engineering; Digital Repository at the University of Maryland; University of Maryland (College Park, Md.)Most vehicle operating environments are transient in nature, yet traditional subsystem thermal management addresses peak load conditions with steady-state designs. The large, overdesigned systems that result are increasingly unable to meet target system size, weight and power demands. Phase change thermal energy storage is a promising technique for buffering thermal transients while providing a functional thermal energy reservoir. Despite significant research over the half century, few phase change material (PCM) based solutions have transitioned out of the research laboratory. This work explores the state of phase change materials research for vehicle and electronics applications and develops design tool compatible modeling approaches for applying these materials to electronics packaging. This thesis begins with a comprehensive PCM review, including over 700 candidate materials across more than a dozen material classes, and follows with a thorough analysis of transient vehicle thermal systems. After identifying promising materials for each system with potential for improvement in emissions reduction, energy efficiency, or thermal protection, future material research recommendations are made including improved data collection, alternative metrics, and increased focus on metallic and solid-state PCMs for high-speed applications. Following the material and application review, the transient electronics heat transfer problem is specifically addressed. Electronics packages are shown using finite element based thermal circuits to exhibit both worsened response and extreme convective insensitivity under pulsed conditions. Both characteristics are quantified using analytical and numerical transfer function models, including both clarification of apparently nonphysical thermal capacitance and demonstration that the convective insensitivity can be quantified using a package thermal Elmore delay metric. Finally, in order to develop design level PCM models, an energy conservative polynomial smoothing function is developed for Enthalpy and Apparent Capacity Method phase change models. Two case studies using this approach examine the incorporation of PCMs into electronics packages: substrate integrated Thermal Buffer Heat Sinks using standard finite element modeling, and direct on-die PCM integration using a new phase change thermal circuit model. Both show effectiveness in buffering thermal transients, but the metallic phase change materials exhibit better performance with significant sub-millisecond temperature suppression, something improved cooling or package integration alone were unable to address.Item DEVELOPMENT OF A COMPACT HEAT EXCHANGER WITH BIFURCATED BARE TUBES(2017) Huang, Zhiwei; Radermacher, Reinhard; Mechanical Engineering; Digital Repository at the University of Maryland; University of Maryland (College Park, Md.)Heat transfer enhancement of air-to-fluid heat exchangers by novel surface or geometry design and optimization is a major research topic. The traditional way of reducing airside thermal resistance is to extend airside heat transfer area by adding fins and the conventional method of reducing fluid side thermal resistance is to use enhanced inner surfaces. These approaches have limitations in further reducing the thermal resistance. Small diameter (4 and 5 mm) fin-and-tube heat exchangers, louvered fin mini-channel heat exchangers (MCHX), newly studied round bare tube heat exchangers (BTHX) and shape optimized bare tube heat exchangers (sBTHX) with diameter of 0.8~1.0 mm were experimentally investigated using air and water to gain the fundamental understanding of heat transfer and the current technology limitations. Correlations of air-side heat transfer coefficient and pressure drop were then developed for BTHX and sBTHX. To improve current technologies, a novel bifurcated bare tube heat exchanger (referred as bBTHX, hereafter) was proposed in this study. It was numerically investigated and optimized using Parameterized Parallel Computational Fluid Dynamics (PPCFD) and Approximation Assisted Optimization (AAO) techniques. The most unique feature of bBTHX is the addition of bifurcation, which enhances airside heat transfer by creating 3D flow and waterside heat transfer by boundary layer interruption and redevelopment. The airside and waterside pressure drop can also be reduced by proper design and optimization, resulting in smaller fan and pumping power. Compared to MCHX with similar capacity and frontal area, the optimal bBTHX design has 38% lower total power and 83% smaller volume and 87% smaller material volume. Compared to BTHX with similar capacity and frontal area, the optimal design has 28% lower total power and 11% smaller volume and 10% smaller material volume. The bBTHX design can be widely applied in industry such as automotive radiators, oil coolers, condenser and evaporator. Two applications of this heat exchanger were discussed in detail: car radiator and indoor coil for Hybrid Variable Refrigerant Flow (HVRF) system. The bBTHX car radiator has 30% lower pumping power, 68% smaller heat exchanger volume and 67% less water weight than those of baseline. Moreover, refrigerant charge of HVRF systems with bBTHX is reduced by 40~70%.Item Thermal and Manufacturing Design of Polymer Composite Heat Exchangers(2014) Cevallos, Juan Gabriel; Bar-Cohen, Avram; Mechanical Engineering; Digital Repository at the University of Maryland; University of Maryland (College Park, Md.)Polymer heat exchangers, using thermally-enhanced composites, constitute a "disruptive" thermal technology that can lead to significant freshwater and energy savings. The widespread use of seawater as a coolant can be made possible by the favorable qualities of thermally-enhanced polymer composites: good corrosion resistance, higher thermal conductivities, higher strengths, low embodied energy and good manufacturability. Polymer composites can bridge the gap between unfilled polymers and corrosion-resistant metals, and can be applied to a variety of heat exchanger applications. However, thermally enhanced polymer composites behave differently from more conventional polymers during the molding process. The desired thin walled large structures are expected to pose challenges during the molding process. This dissertation presents a design methodology that integrates thermo-fluid considerations and manufacturing issues into a single design tool for thermally enhanced polymer heat exchangers. The methodology shows that the choice of optimum designs is restricted by moldability considerations. Additionally, additive manufacturing has the potential to be a transformative manufacturing process, in which complex geometries are built layer-by-layer, which could allow for production and assembly of heat exchangers in a single step. In this dissertation, an air-to-water polymer heat exchanger was made by fused deposition modeling and tested for the first time. This dissertation also introduces a novel heat exchanger geometry that can favorably exploit the intrinsic thermal anisotropy of filled polymers. A laboratory-scale air-to-water polymer composite heat exchanger was made by injection molding. Its performance was verified empirically, and modeled with numerical and analytical tools.Item Forced Convective Boiling via Infrared Thermography(2012) Kommer, Eric; Kim, Jungho; Mechanical Engineering; Digital Repository at the University of Maryland; University of Maryland (College Park, Md.)Multiphase heat transfer is an important mechanism across wide variety of engineering disciplines. The prediction of the heat transfer rate as a function of flow conditions and temperature has been based almost exclusively on experimentally derived correlations. The quality of these correlations depends on the accuracy and resolution of the measurement technique. In addition to the complexities of flow boiling phenomenon in earth gravity, engineering design of space systems requires knowledge of any gravity dependence for heat transfer characteristics. Current research has shown significant variation in the heat transfer characteristics during pool boiling as a function of gravity magnitude. Research into flow boiling in variable gravity environments is extremely limited at this time, but necessary before multiphase systems can be designed for space. The objective of this study is to develop, validate, and use a unique infrared thermometry method to quantify the heat transfer characteristics of flow boiling in earth gravity, prior to use of the apparatus in variable gravity environments. This new method allows high spatial and temporal resolution measurements, while simultaneously visualizing the flow phenomenon. Validation of this technique will be demonstrated by comparison to accepted correlations for single and multiphase heat transfer in earth gravity environments.Item Development of a Model for Flaming Combustion of Double-Wall Corrugated Cardboard(2012) McKinnon, Mark; Stoliarov, Stanislav I; Fire Protection Engineering; Digital Repository at the University of Maryland; University of Maryland (College Park, Md.)Corrugated cardboard is used extensively in a storage capacity in warehouses and frequently acts as the primary fuel for accidental fires that begin in storage facilities. A one-dimensional numerical pyrolysis model for double-wall corrugated cardboard was developed using the Thermakin modeling environment to describe the burning rate of corrugated cardboard. The model parameters corresponding to the thermal properties of the corrugated cardboard layers were determined through analysis of data collected in cone calorimeter tests conducted with incident heat fluxes in the range 20-80 kW/m2. An apparent pyrolysis reaction mechanism and thermodynamic properties for the material were obtained using thermogravimetric analysis (TGA) and differential scanning calorimetry (DSC). The fully-parameterized bench-scale model predicted burning rate profiles that were in agreement with the experimental data for the entire range of incident heat fluxes, with more consistent predictions at higher heat fluxes.Item Experimental Study of Hybrid Cooled Heat Exchanger(2011) Tsao, Han-Chuan; Radermacher, Reinhard; Mechanical Engineering; Digital Repository at the University of Maryland; University of Maryland (College Park, Md.)A test system for a hybrid cooled heat exchanger was designed, and the test facility was constructed based on ASHRAE Standard 41.2-1987. A conventional air-cooled tube-fin heat exchanger was tested with and without application of wetting water. The baseline tests were conducted to investigate the heat exchanger performance improvement by applying evaporative cooling technology. The heat exchanger capacity and air side pressure drop were measured while varying operating conditions and heat exchanger inclination angles. The results show the heat exchanger capacity increased by 170% with application of the hybrid cooling technology, but the air side pressure drop increased by 130%. Additional research investigating air fan power was also conducted, which increased 120% from the dry condition to the hybrid cooled condition. In summary, the potential for improving the heat exchanger performance by applying hybrid cooling is shown in this research.Item Green Facade Energetics(2010) Price, Jeffrey; Tilley, David R; Biological Resources Engineering; Digital Repository at the University of Maryland; University of Maryland (College Park, Md.)Rising energy costs and a warming climate create the need for innovative, low-carbon technologies that help cool buildings. We constructed four small buildings and instrumented them to measure the cooling effect of a green façade on their south and west walls. The green façade significantly reduced the temperature of the building's ambient air, exterior surface, and interior air, and the heat flux through the vegetated wall. Using a mathematical model, we determined that the whole-building cooling load reduction (1.4 to 28.4%) depended on building construction, green façade placement, and especially whether the windows were covered. An emergy analysis of a south-facing green façade revealed that the total emergy consumed could be balanced by the electricity saved from reduced air conditioning if the cooling load was reduced by at least 14%. With thoughtful design and placement of a green façade it can sustainably and effectively help cool buildings.Item MODELING AND SIMULATION OF MIXING LAYER FLOWS FOR ROCKET ENGINE FILM COOLING(2010) Dellimore, Kiran Hamilton Jeffrey; Cadou, Christopher P; Trouvé, Arnaud; Aerospace Engineering; Digital Repository at the University of Maryland; University of Maryland (College Park, Md.)Film cooling has been selected for the thermal protection of the composite nozzle extension of the J-2X engine which is currently being developed for the second stage of NASA's next generation launch vehicle, the Ares I rocket. However, several challenges remain in order to achieve effective film cooling of the nozzle extension and to ensure its safe operation. The extreme complexity of the flow (three-dimensional wakes, lateral flows, vorticity, and flow separation) makes predicting film cooling performance difficult. There is also a dearth of useful supersonic film cooling data available for engineers to use in engine design and a lack of maturity of CFD tools to quantitatively match supersonic film cooling data. This dissertation advances the state of the art in film cooling by presenting semi-empirical analytical models which improve the basic physical understanding and prediction of the effects of pressure gradients, compressibility and density gradients on film cooling effectiveness. These models are shown to correlate most experimental data well and to resolve several conflicts in the open literature. The core-to-coolant stream velocity ratio, R, and the Kays acceleration parameter, KP, are identified as the critical parameters needed to understand how pressure gradients influence film cooling performance. The convective Mach number, Mc, the total temperature ratio, Ω0, and the Mach number of the high speed stream, MHS, are shown to be important when explaining the effects of compressibility and density gradient on film cooling effectiveness. An advance in the simulation of film cooling flows is also presented through the development of a computationally inexpensive RANS methodology capable of correctly predicting film cooling performance under turbulent, subsonic conditions. The subsonic simulation results suggest that it in order to obtain accurate predictions using RANS it is essential to thoroughly characterize the turbulent states at the inlet of the coolant and core streams of the film cooling flow. The limitations of this approach are established using a Grid Convergence Index (GCI) Test and a demonstration of the extension of this RANS methodology to supersonic conditions is presented.Item MINIMUM ENERGY DESIGN OF SEAWATER HEAT EXCHANGERS(2009) Luckow, Patrick Wass; Bar-Cohen, Avram; Rodgers, Peter; Mechanical Engineering; Digital Repository at the University of Maryland; University of Maryland (College Park, Md.)Industrial cooling with seawater, particularly natural gas liquefaction in arid environments, places large strains on existing heat exchanger designs. High temperature, high salinity water damages metals and leads to devices with a short useful life. Cost effective, corrosion resistant heat exchangers are required to fully utilize available saline water resources. Thermally conductive polymer composites, using carbon fiber fillers to enhance conductivity, are a promising material. This Thesis provides a characterization, analysis, and optimization of heat exchangers built of anisotropic thermally conductive polymers. The energy content of such polymers is compared to several other materials, and the required content of carbon-fiber fillers is studied for optimum conductivity enhancement. A methodology for the optimization of low thermal conductivity fins, and subsequently heat exchangers, is presented. Finally, the thermal performance of a prototype thermally enhanced polymer heat exchanger is experimentally verified, and compared to numerical and analytical results.Item A Self-Contained Cold Plate Utilizing Force-Fed Evaporation for Cooling of High-Flux Electronics(2007-12-11) Baummer, Thomas Buchanan; Ohadi, Michael M; Mechanical Engineering; Digital Repository at the University of Maryland; University of Maryland (College Park, Md.)In recent years, the rapid increase in the functionality, speed, and power density of electronics has introduced new challenges, which have led to demand for high heat flux electronics cooling at levels that cannot be met by conventional technologies. The next generation of high power electronics will require advanced cooling beyond the methodologies currently available. This thesis describes work done on a novel form of two-phase heat transfer, named "Force-Fed Evaporation," which addresses this need. This process utilizes evaporation of a liquid in a microchannel surface to produce high heat transfer coefficient cooling at very high heat flux while maintaining a low hydraulic pressure drop. Component level tests were conducted to demonstrate the capability of this process. This led to the development of a self-contained, two-phase cold plate suitable for cooling a high power circuit board. The results show that this technology bears promise for the future of electronics cooling.