Mechanical Engineering
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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 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 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.Item An Innovative Thermal Management Solution for Cooling of Chips with Various Heights and Power Densities(2007-08-09) McMillin, Timothy Walter; Ohadi, Michael; Mechanical Engineering; Digital Repository at the University of Maryland; University of Maryland (College Park, Md.)The challenges and benefits of using a liquid-cooled cold plate to cool a multi-processor circuit board with complex geometry were explored. Two cold plates were designed, fabricated, and tested experimentally. Thermal interface resistance was experimentally discovered and confirmed with numerical simulations. A circuit board simulator was constructed. This simulator was meant to mimic a multi-processor circuit board with heat sources of different surface areas, heights, and heat dissipations. Results and discussions are presented in this thesis.Item Characterization of Fire Induced Flow Transport Along Ceilings Using Salt-Water Modeling(2006-04-27) Yao, Xiaobo; Marshall, André W.; Mechanical Engineering; Digital Repository at the University of Maryland; University of Maryland (College Park, Md.)This research provides a detailed analysis of turbulent mixing and heat transfer in canonical fire plume configurations by using a quantitative salt-water modeling technique. The methodology of quantitative salt-water modeling builds on the analogy between salt-water flow and fire induced flow, which has been successfully used in the qualitative analysis of fires. Non-intrusive laser diagnostics, Planar Laser Induced Fluorescence (PLIF) and Laser Doppler Velocimetry (LDV), have been implemented to measure the dimensionless density difference and velocity in salt-water plumes. In the implementation of the PLIF technique, the salt-water concentration is measured through tracking a fluorescent dye tracer within the entire spatial domain of a planar section of the salt-water flow, which is diluted at the same rate as the salt water. The quantitative salt-water modeling technique has been validated by comparing it with real fire experiments and theoretical data. The scaling laws are also proved by varying the initial source strength or ceiling height in the impinging plume configuration. The detailed salt-water measurements provide insight into of the wall interactions and laminarization effects in the impinging plume configuration. Additionally, highly resolved measurements provide mean profiles and turbulent statistics which will be useful for validating and developing sub-grid scale models in Computational Fluid Dynamics (CFD) codes. Furthermore, an engineering heat transfer model is developed to predict the convective ceiling heat transfer from impinging plumes using the quantitative salt-water modeling technique along with an adiabatic wall modeling concept. The successful application of the adiabatic wall heat transfer model illustrates a well controlled method for studying the heat transfer issues in more complex fire induced flow configurations by using the quantitative salt-water modeling technique.Item Characterization, Modeling, and Optimization of Polymer Composite Pin Fins(2005-08-24) Bahadur, Raj; Bar Cohen, Avram; Mechanical Engineering; Digital Repository at the University of Maryland; University of Maryland (College Park, Md.)Increasing electronic product manufacturing volumes and cooling requirements necessitate the use of new materials and innovative techniques to meet the thermal management challenges and to contribute towards sustainable development in the electronic industry. Thermally conductive polymer composites, using high thermal conductivity fillers such as carbon fibers, are becoming commercially available and provide favorable attributes for electronic heat sinks, such as low density and fabrication energy requirements. These polymer composites are inherently anisotropic but can be designed to provide high thermal conductivity values in particular directions to address application-specific thermal requirements. This Thesis presents a systematic approach to the characterization, analysis, design, and optimization of orthotropic polymer composite fins used in electronic heat sinks. Morphological characterization and thermal conductivity measurements of thermally conductive Poly-Phenylene Sulphide composites are used to determine the significant directional thermal conductivity in such composites. An axisymmetric orthotropic thermal conductivity pin fin equation is derived to study the orthotropic thermal conductivity effects on pin fin heat transfer rate and temperature distribution. FEM simulation and water cooled experiments, focusing on the radial temperature variations in single pin fins, are used to validate the analytical model. Theoretical models, CFD modeling, and experiments are used to characterize the thermal performance of heat sinks, fabricated of PPS composite pin fins, in air natural convection and forced convection modes. Simplified solutions, for the orthotropic fin heat transfer rate that are easy to use and can be easily implemented in a heat sink design and optimization scheme, are presented.