Pool boiling heat transfer on silicon chips with non-uniform micro-pillars

This paper investigated the pool boiling heat transfer performance of non-uniform micro-pillar surfaces. Three kinds of uniform micro-pillar surfaces (named U-S, U-M and U-L) and two kinds of non-uniform micro-pillar surfaces (named R-SML and R-SM) were fabricated to be tested, and a smooth surface (named SS) was prepared as a comparison. Experiments were performed in FC-72 at the subcoolings of 0 K and 25 K in both horizontal and vertical orientations. The results showed that the heat transfer coefficient (HTC) and critical heat flux (CHF) of surfaces with micro-pillars have been greatly enhanced compared to those of the smooth surface. Active nucleation site radius was discussed to explain the advanced ONB (onset of nucleate boiling) of U-M, R-SML and R-SM, then SEM images were token to prove this theory. Boiling images of 6 W/cm² at 25 K subcooling were token by a high-speed camera to analyze the HTC of different chips. Surface area enhancement ratio and wickability were used for the explanation of CHF comparison. The results showed that surface area enhancement ratio and wickability are not the decisive factors that influences CHF for FC-72, and the uniformity of microstructures should be considered when predicting the CHF. Non-uniform micro-pillars with small surface area enhancement ratio and wickability can have larger CHF than that of uniform micro-pillars with large surface area enhancement ratio and wickability.