Enhanced Pool Boiling on Micro-Nano Composited Surfaces with Nanostructures on Micro-Pin-Fins

In the present study, the effect of the distribution of nanostructures on the pool boiling heat transfer performance of micro-nano composited surfaces in FC-72 was investigated. For this purpose, a micro-pin-fined surface (MPF), a micro-nano composited surface with nanoforests covering the top and bottom sides of micro-pin-fins (MPF-NF), a micro-pin-fined surface with nanostructures etched at the top corners of micro-pin-fins (MPF-TC), and a micro-pin-fined surface with nanostructures etched at the top edges of micro-pin-fins (MPF-TE) were fabricated. The pool boiling heat transfer performance was compared under the liquid subcooling of 0 K, 15 K, 25 K, and 35 K. It is found that MPF-NF has the lowest wall superheat at the onset of the nucleate boiling point due to its highest nucleation site density, while MPF-TE has the highest maximum heat transfer coefficient because its critical heat flux (CHF) is significantly higher than that of MPF-NF. The bubbles of MPF-TC and MPF-TE are mainly generated on the tops of micro-pin-fins, thereby alleviating the blockage of the capillary wicking channels by the bubbles in the high heat flux region. Furthermore, the condensation effect of the subcooled liquid on the bubbles generated on the tops micro-pin-fins is stronger compared with that generated in the microchannels. On the contrary, the bubbles of MPF are mainly generated in the microchannels, leading to a blockage of the capillary wicking channels at high heat fluxes. Therefore, the CHF of MPF-TE at 35 K liquid subcooling is increased by 25% compared to MPF. However, the wake effect dominates the liquid supply for saturated boiling and numerous lateral bubble coalescence on MPF-TE weakens the wake effect on the liquid supply, resulting in a lower CHF than that of MPF at 0 K liquid subcooling