Heat transfer and flow characteristics of condensing flow in a quadrilobed tube.

Abstract

A double tube heat exchanger is usually used in heat pump water heater to obtain large water temperature rise. Condensing heat transfer coefficient decreases with decreasing vapor quality due to increase in liquid film thickness. Non-circular small diameter tube has possibility to improve the condensing heat transfer, because liquid film thickness becomes non-uniform by surface tension. This study deals with condensing heat transfer of HFC-134a in quadrilobed tube with the crosssection consisting of four leaves and core. Condensing heat transfer and pressure loss in horizontal tubes were experimentally evaluated. From the results, it was confirmed that deterioration in the condensing heat transfer coefficient was suppressed under moderate wetness for the low refrigerant mass flux less than 300 kg×m-2×s-1, because condensed liquid might be accumulated in the narrow part by the surface tension. However, pressure loss increased due to maldistribution of refrigerant flow.