An experimental study on flow boiling heat transfer of R410A in parallel two mini-channels heated unequally by high-temperature fluid

•Unequal heating between parallel channels caused deterioration of heat transfer rate.•The decline of heat transfer coefficient became significant at a high mass velocity.•Suppression of nucleate boiling in a lower heated channel caused the large decline. A novel high performance refrigerant-air heat exchanger employing multiport extruded tubes which have parallel refrigerant mini-channels has recently been developed for heat pump systems. In evaporators, the inequality in heat loads among parallel refrigerant channels causes also inequalities in flow rates of refrigerant. This flow maldistribution induced by unequal heating not only makes it difficult to predict heat exchange performance accurately but also often leads to the deterioration of the refrigerant heat transfer. In the present study, an experiment was conducted on flow boiling heat transfer of a refrigerant R410A flowing in parallel two mini-channels individually heated by hot water flows. When the parallel channels were heated unequally, the heat transfer rate from the hot water to the refrigerant in the channel with higher degree of wall superheat did not change so much, while that with lower degree of wall superheat remarkably decreased from the equally-heated condition. Consequently, the two-channels-mean heat transfer rate dropped off with expanding the inequality in degree of wall superheat between the channels. The deterioration of the mean heat transfer coefficient under the unequally-heated condition was significant at a high mean flow rate condition which required a large heat transfer rate for evaporation.