Graduate Thesis Or Dissertation
 

Experimental evaluation and numerical modeling of a water source heat pump evaporator

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  • Water source heat pumps are increasingly being used for residential and commercial space heating. Typically, these heat pumps are capable of both heating and cooling, and historically the design emphasis has been on improving the units' cooling performance. Recently, there has been more interest in improving the heating performance, however, in order to optimize the design for improved heating performance data on the heat pump components operating in the heating mode are required. These data have not been available for the waterto- refrigerant heat exchangers typically installed in unitary water source heat pumps. The purpose of this work is to provide performance data on such heat exchangers operating as evaporators (data is available for the heat exchangers operating as condensers) and to provide a numerical model by which a limited amount of evaporator data might be expanded for system modeling and optimization studies. A well instrumented water-to-water heat pump was assembled for the purpose of testing two such water-to-refrigerant heat exchangers to determine their heat transfer and refrigerant pressure drop characteristics as evaporators. The overall heat transfer coefficient for each of the two heat exchangers was found to be a strong function of both the water and refrigerant flow rates and a relatively weak function of the refrigerant evaporation temperature. The refrigerant pressure drop was determined to be highly dependent on the refrigerant flow rate and only slightly dependent on the evaporator temperature. The overall heat transfer coefficient showed an apparent difference between parallel and counter flow conditions. This coefficient, as defined by the log-mean temperature difference, was slightly higher for the counter flow condition. A numerical model of the evaporator was developed and compared with the experimental results. The model follows a numerical solution in which the governing equations (continuity, momentum, and energy) are solved for a finite control volume and an appropriate finitedifference scheme is used to continue the solution step-wise along the length of the heat exchanger. The numerical model accurately predicts the effect of flow rate and temperature on refrigerant pressure drop and the effect of water flow rate and direction on the overall heat transfer coefficient. The model does not accurately predict the effect of evaporator temperature and refrigerant flow rate on the heat transfer coefficient.
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