Micro-post evaporator wicks with improved phase change heat transfer performance

Abstract

We report a modeling and experimental study of micro-post wicks with improved phase change heat transfer performance for evaporators in micro-heat pipes and vapor chambers. A surface energy minimization algorithm is used to predict the shapes of liquid menisci around micro-posts whose geometries have been tailored to increase the fraction of thin-film regions with small thermal resistance. The effects of the apparent contact angle, solid thermal conductivity, and accommodation coefficient are studied. A circular post of uniform cross-section is used as a baseline to evaluate the performance of two alternative post geometries with the same virtual diameter and array spacing. Enhancement in the heat transfer coefficient due to increased fractions of thin-film regions is largest when the relative effects of the liquid conduction or evaporation resistance are large, that is, when the apparent contact angle is large or when the solid thermal conductivity is high. Deep reactive ion etching or electroplating was used to fabricate wick structures out of silicon or copper with aspect ratios as high as 14. The heat transfer performance of the wicks are experimentally tested in a controlled ambient and the effective heat transfer coefficient is determined using a 3D heat transfer model to account for heat spreading. At low and moderate heat fluxes (<20W/cm2), the experimentally measured performance is consistent with our modeling results.