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 DEVELOPMENT OF VARIABLE TUBE GEOMETRY HEAT EXCHANGERS USING ADJOINT METHOD WITH PERFORMANCE EVALUATION OF AN ADDITIVELY MANUFACTURED PROTOTYPE(2023) Klein, Ellery; Radermacher, Reinhard; Mechanical Engineering; Digital Repository at the University of Maryland; University of Maryland (College Park, Md.)Air-to-refrigerant heat exchangers are a key component for heating, ventilation, air conditioning, and refrigeration (HVAC&R) systems. The performance of these heat exchangers is limited by their air-side thermal resistance. Finless non-round bare tube designs have the potential to improve the air-side thermal-hydraulic performance over their finned counterparts and consequently improve the coefficient of performance (COP) of air-conditioning systems. Previous researchers have used heuristic methods such as multi-objective genetic algorithms (MOGA) with approximation-assisted optimization (AAO) methods utilizing computational fluid dynamics (CFD) based metamodels to shape and topology optimize non-round bare tube heat exchangers. A rather unexplored optimization technique used for heat exchanger optimizations is the gradient based adjoint method. CFD solvers utilizing discrete adjoint methods can be used to shape optimize bare tube heat exchangers and can reveal unintuitive, organic, and potentially superior designs. Additionally, additive manufacturing technology has the capability of building these previously unrealizable heat exchanger designs.The objectives of this dissertation are to experimentally evaluate the performance of shape and topology optimized compact bare tube heat exchangers with non-round bare tubes on a 1) component level, and 2) system level integrated into an air conditioner. Plus, 3) develop new shape optimized variable geometry compact bare tube heat exchangers using discrete adjoint methods for HVAC&R applications. First, a comprehensive experimental investigation of multiple shape and topology optimized compact non-round bare tube heat exchangers was conducted under dry and wet evaporator, condenser, and radiator conditions. For all heat exchangers, air-side pressure drop and heat transfer capacity were predicted within 37% and 15%, respectively. Next, an experimental test facility capable of evaluating the system level performance of a 7.03-8.79 kW commercial packaged air conditioning unit was designed and constructed. The performance of the air conditioning unit was evaluated before and after its conventional tube-fin evaporator was replaced with a shape and topology optimized bare tube evaporator. Results are presented and discussed. Lastly, an ε-constraint and penalty method optimization scheme was implemented which utilizes a commercial CFD software with a built-in discrete adjoint solver to perform multi-objective shape optimizations of 2D bare tube heat exchangers. Critical solver/mesh set-up to best optimize heat exchangers with 0.5-10.0 mm diameter bare tubes were identified and established. The optimized designs can achieve a 40-50% reduction in air-side pressure drop with at least the same heat transfer capacity compared to the initial circular bare tube geometry. An adjoint shape optimized 500 W bare tube radiator was additively manufactured in polymer and experimentally tested. Air-side pressure drop and heat transfer capacity were predicted within 15% and 10%, respectively. The experimental performance confirms the adjoint method shape optimized designs improve the thermal-hydraulic performance over the initial circular bare tube geometry.Item THERMAL AND HYDRAULIC PERFORMANCE OF SPINE FIN TUBE HEAT EXCHANGERS AT LOW REYNOLDS NUMBER CONDITIONS(2017) Herrera, Carlos; Hwang, Yunho; Mechanical Engineering; Digital Repository at the University of Maryland; University of Maryland (College Park, Md.)The goal of this work is to present the air-side thermal and hydraulic performance of heat exchangers with spine-fin surface augmentation. Although not as common as plain / plate fin, spine-fin heat exchangers have been used for decades in household refrigeration evaporators and in the outdoor coils of household air-conditioning systems. Of particular interest in this study, was the performance at low air-side Reynolds numbers (500 – 900). Heat transfer coefficients for this geometry were evaluated for samples of varying fin pitch, fin height and tube diameter in both parallel and angled bank arrangements. Water was selected as the hot fluid operating in the turbulent regime with mass flow rates varying at each airflow rate test point. Static cold and hot stream temperatures were maintained for all tests. Air-side heat transfer coefficient (AHTC) is highest for the lower diameter tube heat exchangers and increases in fin pitch lowered the AHTC. This behavior is not seen in plain fin, microchannel and other heat exchangers.Item Analysis of Heat Pump Clothes Dryer(2015) Zhang, Zhilu; Hwang, Yunho; Mechanical Engineering; Digital Repository at the University of Maryland; University of Maryland (College Park, Md.)Clothes dryers (CD) offer a rapid means to dry laundry in households, but consume a large portion (4%) of residential electricity. Heat pump clothes dryers (HPCD) can be much more energy-efficient than conventional electric CDs, but have not emerged in the U.S. market yet. In this study, experiments were conducted for a state-of-the-art commercial hybrid HPCD from the European market with two different operational modes followed by Department of Energy’s test procedure. The HPCD’s system performances were analyzed through measurements on humidity ratio (HR), temperature and power consumption for both Eco and Speed Modes. About 70% energy consumption reduction potential was observed as compared with a typical electric CD in the United States. The heating and cooling capacities during the Eco Mode were 1.48 kW and 1.18 kW, respectively, and the dehumidification rate was 0.372 g/s. The heat exchangers were modeled with CoilDesigner and their performances were simulated. The UA of the evaporator was mainly affected by the air flow rate (AFR), inlet air HR and refrigerant MFR while that of the condenser was mainly affected by the condensing temperature, AFR, and refrigerant MFR. The air leakage was estimated to be 24% to 45% in which the water vapor leakage was 26% and the energy loss was 5%. The mass transfer process through the drum was discussed and the mass transfer coefficient k between the cloth surface and air was calculated to be 0.237 g/m²·s. This study provides the performances of HPCDs and their design analysis, which can be used for developing improved HPCDs.Item CHARACTERIZATION OF HEAT TRANSFER AND PRESSURE DROP OF NORMAL FLOW HEAT EXCHANGERS IN COUNTER FLOW CONFIGURATION(2014) Andhare, Rohit Subhash; Ohadi, Michael M; Mechanical Engineering; Digital Repository at the University of Maryland; University of Maryland (College Park, Md.)In today's times, successful technology advancement lies in making systems that are highly compact, offer superior energy efficiency, while sustainable and cost effective . There is interest in developing small heat exchangers having better flow distribution control rather than bulky heat exchangers which are energy intensive. Microchannels and microreactors controlled by microprocessors are slowly taking over energy conversion, transportation and process industry. The nature inspired - Fractal arrangement of manifold-microchannels has the potential to provide enormous heat transfer capabilities at an attractive coefficient of performance. However majority of such fractal flow manifolds are very short and operate with short counterpart microchannel. They have not been completely adopted for counter flow configuration required by majority of the industrial processes. The work covered under this thesis is focused on adopting of high performance fractal microchannel arrangement to counter flow configuration heat exchangers that are required by industrial processes. Two single phase solution heat exchangers were developed using this approach. The solution heat exchanger is an essential component in absorption refrigeration cycle to convert waste heat into cooling. The study also utilized the novel additive manufacturing process of 3D printing to develop a tubular manifold in order to promote the fractal normal flow on tubular surfaces. The heat exchangers developed as a part of this thesis show enhancement in the overall performance and demonstrate high potential of the proposed technology.Item Manufacturability Analysis of Thermally-Enhanced Polymer Composite Heat Exchangers(2011) Hall, Timothy; Gupta, Satyandra K; Mechanical Engineering; Digital Repository at the University of Maryland; University of Maryland (College Park, Md.)Thermally-enhanced polymer composite heat exchangers are an attractive alternative for applications such as the use of seawater as a cooling medium and other corrosive environments that traditionally use expensive exotic metallic alloys, but a number of manufacturing challenges exist. The goal of this thesis is to develop an understanding of the manufacturing feasibility, in particular mold filling and fiber orientation, of utilizing thermally-enhanced polymer composites and injection molding to manufacture polymer heat exchangers. To best predict mold filling feasibility, this thesis proposes developing an explicit construction of the boundary, represented as a surface based on the parameter space, which separates the feasible and infeasible design space. The feasibility boundary for injection molding in terms of the design parameters is quite complex due to the highly nonlinear process physics, which, consequently, makes molding simulation computationally intensive and time consuming. This thesis presents a new approach for the explicit construction of a moldability-based feasibility boundary based on intelligent Design of Experiments and adaptive control techniques to minimize the number or computation experiments needed to build an accurate model of the feasibility boundary. Additionally, to improve the flexibility of the mold filling prediction framework to changes in overall heat exchanger design, a model simplification approach is presented to predict mold filling for general finned-plate designs by determining an equivalent flat plate representation and utilizing a developed flat plate mold filling metamodel to estimate mold filling. Finally, a fiber orientation measurement methodology is presented for experimentally determining fiber orientation behavior for sample heat exchanger geometries that develops both a local and global understanding of the fiber orientation behavior and compares thesis findings to simulation predictions. The work presented in this thesis significantly advances the understanding of manufacturability considerations for utilizing thermally-enhanced polymer composites in heat exchanger applications and is useful in design exploration, optimization, and decision-making approaches.Item Development of Multi-Scale, Multi-Physics, Analysis Capability and its Application to Novel Heat Exchanger Design and Optimizaiton(2009) Abdelaziz, Omar A.; Radermacher, Reinhard; Mechanical Engineering; Digital Repository at the University of Maryland; University of Maryland (College Park, Md.)Heat exchanger development using enhanced heat transfer surface designs and optimization techniques is a continuing effort that is constrained by current state of the art technology. Assessment of novel geometries and concepts are currently limited to experimental and numerical investigations on discrete levels. This dissertation aims at the advancement of the heat exchanger technology through the development of multi-scale multi-physics simulation tools for conventional and novel heat exchanger designs. A unified heat exchanger design and optimization framework was developed. This framework integrates the multi-scale multi-physics simulation capabilities with previously developed approximation assisted optimization techniques. The optimized designs are then interpreted in order to provide design guidelines for next generation air-to-water heat exchangers. These capabilities required the development of: (a) generic ε − NTU solver capable of analyzing the performance under geometrical variability, (b) systematic integration approach for CFD simulation at the segment level with the ε − NTU solver at the heat exchanger level, (c) refrigerant distribution analysis tool. The developed simulation tools were verified numerically using systematic techniques adopted from literature and validated experimentally using measured data from a prototype heat exchanger. The structural integrity under conventional operating pressures of the novel heat exchanger design was analyzed using FEM for different tube materials and different wall thicknesses. Finally, existing single phase water flow in microtubes correlations were investigated numerically. The best matching correlation was selected for incorporation within the multi-scale simulation tool. The approach described in this dissertation for the design and optimization of novel and conventional heat exchanger designs resulted in significant improvements over the current state of the art. Example performance improvements achieved in this dissertation show potential for 84 percent material savings and 61 percent volume savings for the same airside and refrigerant side pressure drop. The experimental investigations were in good agreement with the simulation results and demonstrated the superior performance of the novel design.Item Advances to a Computer Model Used in the Simulation and Optimization of Heat Exchangers(2005-08-11) Schwentker, Robert Andrew; Radermacher, Reinhard; Mechanical Engineering; Digital Repository at the University of Maryland; University of Maryland (College Park, Md.)Heat exchangers play an important role in a variety of energy conversion applications. They have a significant impact on the energy efficiency, cost, size, and weight of energy conversion systems. CoilDesigner is a software program introduced by Jiang (2003) for simulating and optimizing heat exchangers. This thesis details advances that have been made to CoilDesigner to increase its accuracy, flexibility, and usability. CoilDesigner now has the capability of modeling wire-and-tube condensers under both natural and forced convection conditions on the air side. A model for flat tube heat exchangers of the type used in automotive applications has also been developed. Void fraction models have been included to aid in the calculation of charge. In addition, the ability to model oil retention and oil's effects on fluid flow and heat transfer has been included. CoilDesigner predictions have been validated with experimental data and heat exchanger optimization studies have been performed.