Thermal and Hydraulic Performance of Heat Exchangers for Low Temperature Lift Heat Pump Systems
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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.