Performance modeling of micropost wicks for micro heat pipes at low heat fluxes

Well defined wick microstructures comprised of hexagonally packed cylindrical posts with varying solid fraction (.227 - .534) are analyzed for heat transfer performance for heat pipe applications. The equilibrium free fluid surface profile under the influence of surface tension is calculated for each wick structure wetted with water. The fluid geometry is analyzed using a numerical solver so that the thermal performance (defined as a heat transfer coefficient) of the wick evaporator can be determined. Conduction through both the solid wick and liquid are considered and resistance to evaporation at the liquidvapor interface is included. The analysis is compared to the results of a heat transfer experiment using water with a superhydrophilic micro-manufactured copper post array. The equilibrium meniscus assumption is shown to be valid for heat fluxes less than ∼ 30 W/cm². For low contact angles, ∼ 50% of the heat transfer is shown to occur within the region where fluid layer thickness is less than 2um. Heat transfer performance is shown to be a strong function of contact angle, especially for well wetting fluids. Solid fraction is shown to not be a good predictor of thermal performance. A non-dimensional normalized thin film area is presented and is a strong indicator of thermal performance. Evaporator heat transfer coefficients greater than 20 W/cm ²K are predicted for large values of normalized thin film area. The modeling methods presented can be used as a design and analysis tool for predicting the effects of microscale geometry and topology on the heat transfer performance of microstructured wicks operating at low heat fluxes.