Experimental study on boiling heat transfer and bubble characteristics in counterflow minichannel heat sink with vapor transport structures

Micro/minichannel flow boiling often suffers from non-uniform phase distribution due to excessive vapor generation in downstream regions, which hinders liquid replenishment and leads to degraded heat transfer and increased temperature non-uniformity. In this study, a counterflow minichannel heat sink was developed by integrating vapor transport structures into the channel ribs. In contrast to previous membrane-vented microchannel designs that required external venting, the present design enables downstream-generated vapor to pass through a porous membrane and reenter the upstream region of an adjacent counterflowing channel, thus realizing internal vapor redistribution and suppressing bubble overgrowth. To facilitate vapor-liquid separation using anhydrous ethanol as the working fluid, surface-modified electrospun poly(vinylidene difluoride-cohexafluoropropylene) membranes were employed. Experiments were conducted under mass fluxes (G) of 86.11 and 172.22 kg/(m2 & sdot;s), and effective heat fluxes (qeff) ranging from 14.3 to 106.2 kW/m2. Three venting layouts were tested and compared to a baseline configuration without vapor transport structures. The optimal design improved the local heat transfer coefficient (htp)-particularly in the slug flow regime-from 3.71 to 4.41 kW/m2 & sdot;K at a heat flux of 68.2 kW/m2, and reduced the heat sink wall temperature standard deviation (6w) by up to 10.4 %, enhancing temperature uniformity. High-speed imaging revealed that vapor from downstream elongated bubbles penetrated the porous membrane and condensed in the upstream subcooled region, effectively limiting axial bubble growth. Quantitative image analysis at qeff= 87.29 kW/m2 and G = 86.11 kg/(m2 & sdot;s), within the 195-213 mm downstream region from the channel inlet, showed a reduction in the average aspect ratio of confined bubbles (alpha) from 4.90 to 4.31, and a standard deviation (6b) reduction from 0.89 to 0.77, indicating a more uniform bubble morphology. These results suggest that vapor redistribution mitigates bubble elongation and reveal the underlying mechanism governing vapor transport and its influence on flow boiling heat transfer.