Abstract
Recently, CO_2 heat pump water heaters have already been developed and commercialized from the viewpoint of global environmental protection and energy-saving. However, the heat transfer performance deteriorates because the water flow is laminar in gas cooler of CO_2 heat pump water heater. In this study, four kinds of heat transfer tubes are tested to clarify the effects of the tube forms on the heat exchange performance between CO_2 and water in gas cooler experimentally. Pressure drops of the water flow passage in gas cooler are also evaluated. The tested tube forms are as follows: (1) straight smooth, (2) straight dimple, (3) twisted smooth, and (4) twisted dimple. It is found that the twisted dimple tube is the most effective to enhance the heat transfer performance in both water and CO_2 flow passages, though the pressure drop in water flow passage is increased.
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