UMD Theses and Dissertations

Permanent URI for this collectionhttp://hdl.handle.net/1903/3

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 given thesis/dissertation in DRUM.

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    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.
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    Single and Multiresponse Adaptive Design of Experiments with Application to Design Optimization of Novel Heat Exchangers
    (2009) Aute, Vikrant Chandramohan; Azarm, Shapour; Mechanical Engineering; Digital Repository at the University of Maryland; University of Maryland (College Park, Md.)
    Engineering design optimization often involves complex computer simulations. Optimization with such simulation models can be time consuming and sometimes computationally intractable. In order to reduce the computational burden, the use of approximation-assisted optimization is proposed in the literature. Approximation involves two phases, first is the Design of Experiments (DOE) phase, in which sample points in the input space are chosen. These sample points are then used in a second phase to develop a simplified model termed as a metamodel, which is computationally efficient and can reasonably represent the behavior of the simulation response. The DOE phase is very crucial to the success of approximation assisted optimization. This dissertation proposes a new adaptive method for single and multiresponse DOE for approximation along with an approximation-based framework for multilevel performance evaluation and design optimization of air-cooled heat exchangers. The dissertation is divided into three research thrusts. The first thrust presents a new adaptive DOE method for single response deterministic computer simulations, also called SFCVT. For SFCVT, the problem of adaptive DOE is posed as a bi-objective optimization problem. The two objectives in this problem, i.e., a cross validation error criterion and a space-filling criterion, are chosen based on the notion that the DOE method has to make a tradeoff between allocating new sample points in regions that are multi-modal and have sensitive response versus allocating sample points in regions that are sparsely sampled. In the second research thrust, a new approach for multiresponse adaptive DOE is developed (i.e., MSFCVT). Here the approach from the first thrust is extended with the notion that the tradeoff should also consider all responses. SFCVT is compared with three other methods from the literature (i.e., maximum entropy design, maximin scaled distance, and accumulative error). It was found that the SFCVT method leads to better performing metamodels for majority of the test problems. The MSFCVT method is also compared with two adaptive DOE methods from the literature and is shown to yield better metamodels, resulting in fewer function calls. In the third research thrust, an approximation-based framework is developed for the performance evaluation and design optimization of novel heat exchangers. There are two parts to this research thrust. First, is a new multi-level performance evaluation method for air-cooled heat exchangers in which conventional 3D Computational Fluid Dynamics (CFD) simulation is replaced with a 2D CFD simulation coupled with an e-NTU based heat exchanger model. In the second part, the methods developed in research thrusts 1 and 2 are used for design optimization of heat exchangers. The optimal solutions from the methods in this thrust have 44% less volume and utilize 61% less material when compared to the current state of the art microchannel heat exchangers. Compared to 3D CFD, the overall computational savings is greater than 95%.
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    Development of a Simulation and Optimization Tool for Heat Exchanger Design
    (2003) Jiang, Haobo; Digital Repository at the University of Maryland; University of Maryland (College Park, Md.); Mechanical Engineering
    Heat exchangers have been used extensively and play an important role in the capital cost, energy efficiency and physical size of refrigeration and air conditioning systems. In this dissertation, a simulation and optimization tool to improve effectiveness and efficiency in design, rating, and analysis of air to refrigerant heat exchangers including conventional finned tube coils and emerging microchannel heat exchangers, Coil Designer, is developed and investigated using a general-purpose modeling concept and user-friendly interface. It is applicable to design of condensers, evaporators, and heating and cooling coils under any operating conditions. A network viewpoint was adopted to establish the general-purpose model and allow for analysis of arbitrary tube circuitry and mal-distribution of fluid flow inside the tubes. Comprehensive evaluation of solutions to the highly nonlinear system of equations in the local thermal/hydraulic performance within the tube network was conducted and a new solution method to successively approximate the physics of heat and fluid flow was developed to enhance the solution convergence capability. A segment-by-segment approach within each tube was implemented, to account for two-dimensional non-uniformity of air distribution across the exchanger, and heterogeneous refrigerant flow patterns through a tube. A further sub-dividable-segment model was created in order to address the significant change of properties and heat transfer coefficients in the single-phase and two-phase regime when a segment experiences flow regime change. The effectiveness-NTU method for cross-flow configuration was used also for combined heat and mass transfer problem under dehumidification, by defining equivalent thermal resistance and heat capacity. Object-oriented programming techniques were applied in developing Coil Designer to facilitate flexible and customizable design platform and building graphic user-friendly interface. Coupled heat exchangers with multiple fluids inside different subsets of tubes can be modeled and analyzed simultaneously. A wide variety of working fluids and correlations of heat transfer and pressure drop are available at the user’s choice. The tabular and graphic representation of performance simulation results provides convenience in comprehensive and detailed parametric analysis. The model prediction with Coil Designer was verified against experimentally determined data collected from a number of sources. The simulation tool was shown to be able to predict the heat transfer rate for a variety of coils with good accuracy. Parametric studies were conducted to confirm the capability of the program in exploring all aspects of heat exchanger performance under a wide variation of design and operating conditions. A genetic algorithm is introduced and integrated with the simulation tool for single and multi-objective optimization design of heat exchanger to accomplish the following goals quickly and accurately: achieve optimum circuitry selection, minimize volume, minimize the amount of material utilized in the coil and thus minimize overall cost of the coil while achieving the best possible performance.