The characteristics of flow boiling heat transfer of R410a heated unequally between two parallel mini-channels

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, since the flow direction of refrigerant in the multiport tubes is perpendicular to that of outside heat source air, the heat transfer rate to the refrigerant channels on upstream side of air flow are larger than those on downstream. Such an inequality in heat loads of refrigerant channels causes also inequalities in flow rates and heat transfer coefficients of the refrigerant. This phenomenon not only makes it difficult to predict heat exchange performance accurately but also often induces the deterioration of refrigerant heat transfer. In this study, experiments on flow boiling heat transfer of a refrigerant R410A flowing in two parallel mini-channels heated unequally were conducted. As the inequality in heat loads of the channels increased, the heat transfer rate of higher-heated channel does not change much from that in the equally heated condition, while that of lower-heated channels decreased remarkably. As a result, the averaged heat transfer rate of the two channels reduced with expanding the difference in the heat loads. In addition, the deterioration of averaged heat transfer rate of the two channels was noticeable at a higher mass velocity.