Flow boiling heat transfer and flow distribution of HFC32 and HFC134a in unequally heated parallel mini-channels

A novel high-performance heat exchanger using multiport extruded tubes which have parallel refrigerant mini-channels has recently been developed for air conditioning systems. In a multiport tube for evaporators, due to unequal heat transfer rates among parallel channels, vapor–liquid two-phase refrigerant flow tends to be distributed unevenly. This flow maldistribution makes it difficult to predict heat exchange performance accurately. Hence, it is important to clarify the characteristics of flow boiling heat transfer and flow maldistribution in unequally heated parallel mini-channels. In the present study, experiments were performed on flow boiling of HFC32 and HFC134a in unequally heated two parallel circular mini-channels connected with T-junction headers. When heat flux was unequal between two channels, mass flow and inlet quality maldistributions were recognized. These maldistributions were more intense with widening inequality in channel heat fluxes. In addition, the effects of the averaged flow rate of the two channels, the quality before branching, and refrigerant properties on flow maldistribution were examined. Based on the obtained characteristics, correlation equations were developed to reproduce the mass flow rates and inlet qualities in the respective channels. In boiling heat transfer, dryout tended to occur in a relatively wide portion of the higher-heated channel because of flow rate reduction. Then, heat transfer became degraded especially near the outlet, even at the same averaged heat flux of two channels as the value at which dryout did not occur under equally heated condition. © 2020