Boiling heat transfer and bubble distribution on inhomogeneous wetting surface patterned with Sierpinski carpet

During the chips working, heat is mainly generated from its central area and shows a typical inhomogeneous distribution. To meet the chip's cooling requirements, a novel Sierpinski hydrophobic/hydrophilic fractal pattern (SHFP) was designed and fabricated on copper surface in the present study. This fractal pattern can match with the real heat flux distribution and the bubble sizes distribution, fully utilizing the enhancement potential of hydrophobic spots, so it can well avoid the disadvantage of a sharp reduction in the CHF. The dynamic behavior of the bubbles on this SHFP surface was studied theoretically and experimentally. The results show that small bubbles nucleated, grew and merged with neighboring bubbles at the preset high-order hydrophobic spots and their distribution presents typical fractal characteristics. Then small bubbles started moving from the high-order hydrophobic region to central low-order region spontaneously and a large bubble was finally formed and departed rapidly in the central first-order hydrophobic region after bubbles coalesced. Through observation and analysis, we found that the nucleation sites of the bubbles and their movement and merging can be designed and planned in advance on the well-designed SHFP surface, so the distribution of the bubbles can precisely match the actual distribution of heat flux. The comparison shows that the well-designed SHFP surface presented a low wall superheat at ONB and the heat transfer coefficient was even better than the mono-hydrophobic surface after the secondary pool boiling enhancement. The critical heat flux also reached the same level (i.e., about 95%) of natural spatial distribution, which is greatly superior to the surface with regular hydrophobic spots. Therefore, the SHFP surface designed in this study is a very suitable surface for utilizing the distribution of hydrophobic spots to enhance boiling heat transfer.