Theses and Dissertations from UMD
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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 A Refrigerant Expansion Control Device Fabricated Through Nickel Deposition Within SU8 Micromolds(2006-03-29) Yashar, David Anthony; DeVoe, Donald; Mechanical Engineering; Digital Repository at the University of Maryland; University of Maryland (College Park, Md.)Control of refrigerant expansion offers improved system efficiency and reduced noise among other benefits. The cost of traditional active expansion devices, however, has limited their use to large HVAC&R systems. The design, fabrication and testing of a thermopneumatic microfabricated valve for controlling refrigerant mass flow rate during expansion is presented in this dissertation. This device was fabricated through nickel electroplating within very thick SU-8 molds, thereby realizing expansion devices useful for small HVAC&R applications through an inexpensive modified UV-LIGA process. This work begins with process development to make meso-scale nickel electroplating within SU8 micromolds a feasible option. Then, two test apparatuses were constructed and used to benchmark the performance of a prototype; one to control the flow of compressed air, one to control refrigerant expansion. Next, three dimensional CFD simulations were performed on the flowfield within the device at various levels of actuation to predict the device's ability to control compressed air flow. A numerical code was also developed to predict the device's temporal response and relationship between actuation level and power input. The assembled prototype was demonstrated on the air flow test bench. The prototype was able to reduce the mass flow rate of the compressed air by 22 % at the conditions used in the CFD analysis. The performance was then demonstrated in a 1.5-2 kW R134a vapor compression system. Both steady state and transient response were characterized. Steady state data showed that the mass flow rate of refrigerant could be effectively controlled using the valve. The level of refrigerant subcooling defined the magnitude of the response. Steady state data taken at 750 kPa inlet pressure shows the mass flow rate was reduced by 4.2 % at 1 ºC subcooling and up to 10.8 % at 5 ºC subcooling for a given level of actuation. Transient system response was characterized using cyclic actuation of the device in the HVAC system. The change in capacity was approximately 5 %, at the conditions used during these tests. Data from the transient response tests showed the device's time constant to be within 11 % of the value predicted in the simulations.