Experimental study and visualization of flow boiling in open microchannels with laser-etched micro-nanoscale composite fins

Flow boiling of HFE-7100 in conventional open microchannels (OMs), open microchannels with parallel micro-straight fins (OMs-PMSFs), and open microchannels with micro-pin fins (OMs-MPFs) were experimentally studied. The mass flux (G) ranges from 169.1 to 394.6 kg/(m²s), the effective heat flux is up to 192.27 W/cm², the outlet pressure is from 102 kPa to 156 kPa. The heat transfer behaviors in the three microchannels are analyzed. The enhanced heat transfer mechanisms are explained according to the observed flow patterns. In the nucleate boiling dominant regime, the bubble departure diameters in the OMs-PMSFs and the OMs-MPFs are smaller than those in the OMs, therefore, the heat transfer is enhanced. The local heat transfer coefficient (HTC) initially increases and then decreases with both the heat flux and the outlet vapor quality for all three open microchannels. Additionally, the maximum HTCs of the OMs-PMSFs and the OMs-MPFs show increase of 21.92 % and 90.24 %, respectively, at G = 394.6 kg/(m²s) compared to the maximum HTC of the OMs. It is observed through flow visualization that the enhanced capillary effect of the micro-nanoscale composite structures can accelerate the re-wettability of the microchannel surfaces, which reduces the period of the bubble growth cycle and delays the occurrence of the dry-out. As a result, the intensified nucleate boiling and liquid evaporation significantly enhance flow boiling heat transfer in the OMs-PMSFs and the OMs-MPFs as compared to that in the OMs. Moreover, the micro-nanoscale composite fins located on the bottom of the microchannel effectively reduce the growth rate of pressure drop under two-phase conditions.