Synergistic effect of mixed wettability of micro-nano porous surface on boiling heat transfer enhancement

In this work, a simple immersion technique was created to further enhance the pool boiling heat transfer performance of micro-nano copper foam by modifying its mixed wettability. The SEM images exhibit that the entirely-covered micro/nanostructures can be observed on the super-hydrophilic copper foam, while the hydrophilic-hydrophobic copper foam is covered with nanograss partially. The experiments indicated that the hydrophilic-hydrophobic copper foam showed the best heat transfer coefficient of the all sample surfaces at a high heat flux (>40 W/cm(2)), which can be as high as 4.15 W/cm(2)center dot K. Meanwhile, the critical heat flux (CHF) was measured as 108.32 W/cm(2), which was 157.3% higher than that on the smooth surface, and 57%, 33.2% and 37.4% higher than that of untreated, super-hydrophilic, super-hydrophobic copper foams, respectively. Interestingly, due to the unique modification of the surface, the onset of boiling (ONB) on the sample was much reduced from 9 K to 0.5 K. These improvements are due to the increasing nucleation density, the reduced bubble departure diameter and the much shortened bubble period compared to other comparative samples. It is speculated that enhancement of the boiling heat transfer is attributed to the synergistic effect of improved bubble nucleation, the vapor escape, and the liquid replenishment on the designed sample with a mix wettability.