Experimental study on pool boiling enhancement by unique designing of porous media with a wettability gradient

Abstract

High heat flux dissipation can be achieved in pool boiling heat transfer through regulating the bubble escape and liquid replenishment paths over the heated surfaces. In this work, a unique design of copper foam with a wettability gradient is presented to generate separate liquid–vapor pathways for pool boiling enhancement. The effect of copper foam with a wettability gradient on bubble dynamics during nucleate boiling period is evaluated using bubble visualization experiment. Results demonstrated that the liquid replenishment and bubble escape flow is determined by copper foam with different wettability gradient. Comparing with the super-hydrophilic (SHPi) copper foam and super-hydrophobic (SHPo) copper foam, the super-hydrophilic top with super-hydrophobic bottom (SHPiT-SHPoB) copper foam structure and super-hydrophobic top with super-hydrophilic bottom (SHPoT-SHPiB) copper foam structure can achieve the critical heat flux (CHF) of 113.3 W/cm2 and 108.3 W/cm2, corresponding to the maximum heat transfer coefficient (HTC) of 5.93 W/cm2·K and 5.28 W/cm2·K, respectively. Two bubble escape forms and liquid replenishment models are established to give detailed explanations on the pool boiling enhancement. It is evident that when bubbles escape from sides, the bubble escape path is shorter, resulting in a faster bubble escape rate and a smaller bubble departure diameter. The experimental results also reveal that liquid replenishment rate of the SHPiT-SHPoB copper foam is also the best amongst all samples, providing a trade-off between the bubble escape and liquid replenishment in diverse conditions.