Experimental study on flow boiling heat transfer for pure and zeotropic refrigerants in multi-microchannels with segmented configurations

Abstract

In this paper, the flow boiling heat transfer characteristics for pure refrigerants of R134a, R245fa and their binary zeotropic mixtures with three blending ratios (R134a/R245fa: 10/90, 30/70 and 70/30 by wt%) were experimentally investigated in the aluminum multi-microchannels with segmented configuration. Each multi-microchannel test plate consisted of seven parallel channel passages with the same total length of 110 mm and cross-sectional area of 2 mm × 1 mm (width × height), while the 10 mm-long interconnected area was arranged every 30 mm along the channel length in segmented one. For each refrigerant working fluid, the experiments were performed at the same inlet evaporating temperature of 26 °C under conditions of the heat flux and mass flux ranging from 20 to 350 kW/m2 and 300 to 400 kg/m2 s, respectively. According to the comparative study, R134a working fluid presented higher heat transfer coefficient than R245fa, but it was more prone to local dry out spots in advance. The small addition of volatile component of R134a was found to be beneficial for improving the heat transfer coefficient at higher heat flux as well as the flow boiling CHF, despite the common feature of degraded heat transfer coefficient for test zeotropic mixtures in most conditions. Compared with continuous microchannels, the heat transfer performance was enhanced for pure refrigerants but suppressed for zeotropic mixtures in segmented microchannels, however, in which the interconnected area was helpful to delay the occurrence of the transfer deterioration and improve the flow boiling CHF regardless of pure or zeotropic refrigerants.