Heat transfer and capillary performance of dual-height superhydrophilic micropost wicks

Abstract

We propose dual-height superhydrophilic (DHS) micropost evaporator wicks with improved heat transfer performance for thermal management applications such as micro heat pipes. The heat transfer coefficient and capillary performance are characterized with a numerical model that accounts for the finite curvatures of liquid menisci by varying solid fraction of micropost arrays from 0.18 to 0.54. The DHS wicks vertically stretch a thin (<2μm) evaporative film region with low thermal resistance, which enhances the heat transfer coefficient up to ∼300% compared with the previously reported single-height superhydrophilic (SHS) micropost wicks with the same solid fraction. The stretch of thin film region does not significantly affect the capillary pressure and permeability determined from microscopic menisci. The optimum solid fraction maximizing the heat transfer coefficient occurs since the maximum height ratio between tall and short posts satisfying the pinning criteria decreases with increasing solid fraction, while the total perimeter of microposts increases. The vertical stretch of thin film also lowers the sensitivity of performance to liquid fill charge over 60%. This work suggests that DHS micropost wicks can provide significantly higher heat transfer coefficient in more robust way compared with previous SHS wicks without scarifying the maximum heat transfer performance.