An area-scalable two-layer evaporator wick concept for high-heat-flux vapor chambers

Abstract

For vapor chamber heat spreaders to operate at very high heat fluxes, the internal wick layer at the evaporator must simultaneously minimize the device temperature rise and the flow resistance to liquid resupply by capillary action. Prior investigations in the literature have reported sustained capillary-fed boiling at heat fluxes as high as 1 kW/cm2 for small hotspots of significantly less than ∼1 cm2. However, the need to provide liquid feeding to avoid dryout prevents high levels of heat fluxes from being dissipated over areas any larger than localized hotspots. Thin layers of homogeneous evaporator wicks can help reduce the thermal resistance across the layer, but fail to sustain adequate liquid supply at high heat fluxes or over large areas. Thicker evaporator wicks offer greater flow cross-sections to better feed liquid to the evaporator by capillary action, but induce unacceptably large surface superheats due to the high thermal resistance across these thick layers. This work proposes and analyzes a hybrid two-layer evaporator wick for passive, high-heat-flux dissipation. A thick cap layer of wick material evenly routes liquid to a thin, low-thermal-resistance base layer through an array of vertical liquid-feeding posts. This two-layer structure decouples the functions of liquid resupply (cap layer) and capillary-fed boiling heat transfer (base layer), making the design scalable to heat input areas of ∼1 cm2 for operation at 1 kW/cm2. A model is developed to demonstrate the potential performance of a vapor chamber incorporating such a two-layer evaporator wick design and to establish the target sizes of critical wick features that must be fabricated. The model comprises simplified hydraulic and thermal resistance networks for predicting the capillary-limited maximum heat flux and the overall thermal resistance, respectively. The performance of the vapor chamber is analyzed with varying two-layer wick geometric feature sizes.