Phase-change heat transfer of sintered-particle wick in downward facing orientation: Particle size and wick thickness effects

Two-phase cooling systems using wicks offer reliable and high heat flux cooling capability, however, the maximum heat removal capacity is related to capillary pumping limit and wick superheat. In this study, non-uniform wicks are examined to simultaneously enhance the capillary pumping limit and minimize wick superheat. The non-uniform wick consists of a thin evaporation wick for reducing the wick superheat and a thick coolant supply wick for enhancing the coolant supply. Also, it has a phase-separating wick attached to the coolant supply wick to ensure the vapor escape channels. The thin wick is fabricated using sintered-copper particles with 30-200 mu m spherical particles, while the post and phase-separating wicks are constructed using 10 and 3 layers of 200 mu m copper particles, respectively. To minimize the gravity-driven liquid supply, the liquid reservoir is placed below the wick structure, i.e., downward facing orientation. The heat flux is measured as a function of the wick superheat using different particles sizes and thicknesses of the thin wick, i.e., 30, 60, 100, 200, 60/200, and 100/200 mu m with 1-3 layers. The results show that 60 Am particles result in the minimal wick superheat at q < 100 W/cm(2) for 1 and 2 layers of the wick, while the 100/200 mu m particles with 3 layers leads to the maximum heat flux of 223 W/cm(2), which is similar to 48% enhancement compared to the bare copper surface at the wick superheat of 38.6 degrees C. Also, it is found that the 2 and 3 layer wicks substantially decrease the wick superheat compared to the single layer wick, although the wick thickness (conductive thermal resistance) is larger. This is attributed to the 2 and 3 layer wick enhance the liquid supply, by increasing the cross-section area of the wick, thus, delaying the surface dryout. (C) 2020 Elsevier Ltd. All rights reserved.