Abstract
Recently, CO_2 heat pump water heaters have already been developed and commercialized from the viewpoint of energy-saving. However, further improvement on heat transfer performance is needed because the water side tends to be laminar flow in the gas-coolers of CO_2 heat pump water heaters. In this study, two kinds of heat transfer tubes are tested to clarify the effects of the tube geometries on the heat exchange performance and the pressure drop. The tested tube geometries are as follows:(1)twisted smooth (TS) and (2) twisted dimple (TD). In the C02 flow passages, the heat transfer performance though of twisted dimple tube is nearly equivalent to that of the TS tube although the pressure drop of the TD tube is higher than that of TS tube.
