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 INTEGRATED, MULTI-PHYSICS ANALYSIS AND DESIGN OPTIMIZATION FRAMEWORK FOR AIR-TO-REFRIGERANT HEAT EXCHANGERS WITH SHAPE-OPTIMIZED TUBES(2022) Tancabel, James M; Radermacher, Reinhard; Aute, Vikrant; Mechanical Engineering; Digital Repository at the University of Maryland; University of Maryland (College Park, Md.)Air-to-fluid Heat eXchangers (HX) are fundamental components of many systems we encounter in our daily lives, from Heating, Ventilation, Air-Conditioning and Refrigeration (HVAC&R) systems to electronics cooling, automotive, power plants, and aviation applications. The importance of HXs is evident in the level of investment devoted to HX innovation in recent years. While current state-of-the-art HXs have adequately addressed past challenges, ever-increasing energy demands and increasingly stringent global energy standards require novel tools and methodologies which can quickly and efficiently develop the next generation of high-performance HXs. In recent years, advancements in computational tools and advanced manufacturing technologies have enabled engineers to consider small characteristic diameter HX tubes with novel shapes and topologies which were not feasible even a decade ago. These small diameter, shape-optimized tubes have been shown to perform the same job as existing HXs while offering significant and desirable improvements in performance metrics such as envelope volume, face area, weight, and refrigerant charge. However, the structural integrity of shape-optimized tubes was often guaranteed by utilizing conservative tube thicknesses to ensure equipment safety, prevent refrigerant leakages, and satisfy product qualification requirements, resulting in increased material consumption and manufacturer costs while reducing the likelihood of industry acceptance for the new technology. Additionally, the actual HX operating conditions are often different from design conditions, resulting in significant performance degradations. For example, novel HX design is typically assumes uniform normal airflow on the HX face area even though HXs in HVAC&R applications rarely experience such flows, and compact HXs have been shown to experience water bridging under dehumidification conditions, which greatly impacts HX performance. This research sheds light on the next generation of air-to-refrigerant HXs and aims to address several practical challenges to HX commercialization such as novelty, manufacturing, and operational challenges through the use of comprehensive multi-physics and multi-scale modeling. The novelty of this research is summarized as follows: i. A new, comprehensive and experimentally validated air-to-refrigerant HX optimization framework with simultaneous thermal-hydraulic performance and mechanical strength considerations for novel, non-round, shape- and topology-optimized tubes capable of optimizing single and two-phase HX designs for any refrigerant choice and performance requirement with significant engineering time savings compared to conventional design practices. The framework was exercised for a wide range of applications, resulting in HXs which achieved greater than 20 improved performance, than 20% reductions in size, and 25% reductions in refrigerant charge. ii. Development of a fundamental understanding of performance degradation for HXs with shape- and topology-optimized tubes under typical HX installation configurations in practical applications such as inclined and A-type configurations. New modeling capabilities were integrated into existing HX modeling tools to accurately predict the airflow maldistribution profiles for HXs with shape- and topology-optimized tubes without the need for computationally-expensive CFD simulations. iii. Development of a framework to model and understand the impact of moist air dehumidification on the performance of highly compact HX tube bundles which utilize generalized, non-round tubes. Correlations for Lewis number were developed to understand whether traditional HX dehumidification modeling assumptions remained valid for new HXs with generalized, non-round tube bundles. Such an understanding is critical to accurately and efficiently modeling HX performance under dehumidifying (i.e., wet-coil) conditions.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 Hydraulic Performance of Heat Exchangers for Low Temperature Lift Heat Pump Systems(2012) Lee, Hoseong; Radermacher, Reinhard; Mechanical Engineering; Digital Repository at the University of Maryland; University of Maryland (College Park, Md.)The work presented in this dissertation focused on investigating and understanding the hydraulic and thermal design space and tradeoffs for low temperature difference high performance heat exchangers for a low temperature lift heat pump (LTLHP) system, which benefits from a small difference between the condensing and evaporating temperatures of a working fluid. The heat exchangers for the LTLHP application require a larger heat transfer area, a higher volume flow rate, and a higher temperature of heat source fluid, as compared to the typical high temperature lift heat pump system. Therefore, heat exchanger research is critical, and it needs to be balanced between the heat transfer and pressure drop performance of both fluids in the heat exchanger. A plate heat exchanger (PHX) was selected to establish a baseline of a low temperature lift heat exchanger and was investigated experimentally and numerically. The traditional PHX is designed to have the identical surface area and enhancements on both fluid sides for ease of production. However, fluid side heat transfer coefficients and heat transfer capacities can be drastically different, for example, single-phase water versus two-phase refrigerant. Moreover, the PHX needs to have a large cross sectional flow area in order to reduce the heat-source fluid-side pressure drop. In the experimental test, the PHX showed a relatively low overall heat transfer performance and a large pressure drop of the heat source fluid side under LTLHP operating conditions. The CFD simulation was carried out to further improve the potential of the PHX performance. However, there were limitations in the PHX. It was concluded that the PHX was restricted by two main factors: one was a large pressure drop on the heat source fluid-side due to corrugated shape, and the other was low overall heat transfer performance due to the low refrigerant-side mass flux and resulting low heat transfer performance. A concept of a novel low temperature lift heat exchanger (LTLHX) has been developed based on the lessons learned from the PHX performance investigation for the application to the LTLHP. Geometries were newly defined such as a channel width, channel height, channel pitch, and plate flow gap. Two design strategies were applied to the novel heat exchanger development: the flow area ratio was regulated, and plates were offset. The design parameters of the novel heat exchanger were optimized with multi scale approaches. After developing the laboratory heat exchanger test facility and the prototype of the novel LTLHX, its performance was experimentally measured. Then the thermal and hydraulic performance of the novel LTLHX was validated with experimental data. The heat transfer coefficient correlations and the pressure drop correlations of both the water-side and refrigerant-side were newly developed for the novel LTLHX. The overall heat transfer performance of the novel LTLHX was more than doubled as compared to that of the PHX. Moreover, the pressure drop of the novel heat exchanger was drastically lower than that of the PHX. Lastly, the novel heat exchangers were applied to a water source heat pump system, and its performance was investigated with parametric studies.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 Thermomechanical Behavior of Polymer Composite Heat Exchangers(2011) Robinson, Franklin Lee; Bar-Cohen, Avram; Bruck, Hugh A; Mechanical Engineering; Digital Repository at the University of Maryland; University of Maryland (College Park, Md.)Industrial cooling with seawater, particularly at elevated temperature and salinity, shortens the useful lives of conventional metallic heat exchangers. Cost effective, corrosion-resistant heat exchangers are required to fully utilize available saline water resources. Polymer composites, which use carbon fibers for thermal and mechanical reinforcement, are a promising material for such heat exchangers. The present thesis provides a characterization and thermomechanical analysis of heat exchangers fabricated using thermally conductive polymers. The change in mechanical properties resulting from exposure to saltwater at elevated temperature is characterized for raw and reinforced polymers. Then, thermal performance of such heat exchangers is compared to that of heat exchangers fabricated from conventional corrosion-resistant materials. Finally, the mechanical and combined thermomechanical response of such heat exchangers to conditions typical of LNG operations is studied and compared to that of heat exchangers fabricated from conventional corrosion-resistant materials.Item Thermal integration of tubular solid oxide fuel cell with catalytic partial oxidation reactor and anode exhaust combustor for small power application(2010) Maxey, Christopher; Jackson, Gregory S; Mechanical Engineering; Digital Repository at the University of Maryland; University of Maryland (College Park, Md.)In the current study, a system configuration of a tubular SOFC with a catalytic partial oxidation (CPOx) reactor and an anode exhaust catalytic combustor is explored to test the feasibility of such a system. A system level model was developed to more fully assess system design and operability issues. For the SOFC, a detailed 1-D SOFC determines local current production and is combined with down-the-channel flow models for the SOFC as well as the catalytic combustor/heat exchanger, and CPOx reactor. System model results showed that variations in fuel flow and air to fuel ratio have large impacts on temperature distribution and power out, with lower fuel flows and air-to-fuel ratios providing higher SOFC power densities (~0.64 W/cm2) at high efficiencies (~45%). The system model also shows that external heat loss greatly reduces system power and efficiency but lower air-to-fuel ratios can offset associated temperature and associate performance losses.Item DEVELOPMENT OF AN ADVANCED HEAT EXCHANGER MODEL FOR STEADY STATE AND FROSTING CONDITIONS(2009) Singh, Varun; Radermacher, Reinhard; Mechanical Engineering; Digital Repository at the University of Maryland; University of Maryland (College Park, Md.)Air-to-refrigerant fin-and-tube heat exchangers are a key component in the heating, air conditioning and refrigeration industry. Considering their dominance, the industry has focused immensely on employing computer modeling in their design and development. Recently, advances in manufacturing capabilities, heat exchanger technology coupled with the move towards new environment-friendly refrigerants provide unprecedented challenges for designers and opportunities for researchers. In addition, the field of Computational Fluid Dynamics (CFD) has assumed a greater role in the design of heat exchangers. This research presents the development of an advanced heat exchanger model and design tool which aims to provide greater accuracy, design flexibility and unparalleled capabilities compared to existing heat exchanger models. The heat exchanger model developed here achieves the following. * Account for tube-to-tube conduction along fins, which is known to degrade the performance of heat exchangers, especially in carbon dioxide gas coolers * Study and develop heat exchangers with arbitrary fin sheets, which meet performance as well as packaging goals with minimal consumption of resources * Allow engineers to integrate CFD results for air flow through a heat exchanger, which the modeling tool employs to develop its air propagation sequence leading to improved accuracy over existing models which assume normal air flow propagation * Function in a quasi-steady state mode for the purpose of simulating frost accumulation and growth on heat exchangers, and completely simulate local heat transfer degradation, as well as blockage of flow passage on air side Additionally, the heat exchanger model was used to investigate gains that are enabled due to the presence of cut fins in carbon dioxide gas coolers and develop design guidelines for engineers. Finally, this dissertation analyzes the implications of minimum entropy generation on heat exchanger performance criteria of heat capacity and pressure drop, as well as evaluates the ability of entropy generation minimization as a design objective. This also serves as the first step toward an expert knowledge-based system for guiding engineers towards better designs, during the process of heat exchanger design.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 Construction of Test Facility to Measure and Visualize Refrigerant Maldistribution in Multiport Evaporator Headers(2005-07-18) Linde, John Eric; Radermacher, Reinhard; Mechanical Engineering; Digital Repository at the University of Maryland; University of Maryland (College Park, Md.)In a refrigeration cycle, condensed liquid refrigerant is expanded to a two-phase fluid entering the evaporator. In many applications, the evaporator paths are divided into a number of parallel sections to keep the pressure drop across the evaporator within a reasonable range and to maximize overall heat exchanger performance. Since the state of the refrigerant entering the evaporator is two-phase and its quality changes depending upon the operating conditions, the proper refrigerant distribution to individual sections is not an easy task. Nonuniform distribution, or maldistribution, will cause dry out at sections of lesser mass flow by superheating the refrigerant gas. This can result in nonuniform heat exchanger surface temperature distribution. Single-phase heat transfer coefficients (HTCs) are much lower than those of two-phase HTCs. When dryout occurs, both refrigerant-side HTCs and air-side HTCs are lower than those of wet surfaces. In addition to this, the temperature difference between the air and