Thin-film evaporation from well-defined silicon micropillar wicks for high-heat-flux thermal management

Abstract

The generation of concentrated heat loads in advanced microprocessors, power amplifiers, and concentrated photovoltaics present significant thermal management challenge for defense, space and commercial applications. Liquid to vapor phase-change strategies are promising due to the high latent heat of vaporization of the working fluid. In particular, capillary pumped thin-film evaporation from micropillar wicks has received significant attention owing to advances in micro/nano-fabrication and the potential to dissipate high heat fluxes by increasing the evaporative area. Yet, predictive tools to design various wicking structures are not available due to limited understanding of the thermal-fluidic transport. This thesis reports experimental characterization and modeling of capillary-limited thin-film evaporation from micropillar wicks. We fabricated test devices and experimentally characterized the thermal performance of well-defined silicon micropillar wicks. The experiments were designed to investigate the capillary-limited dryout heat flux by ensuring pure thin-film evaporation in the absence of nucleate boiling. The tests were performed in a temperature controlled saturated vapor environment to accurately control the operating conditions. We also developed a unified semi-analytical thermal-fluidic model that incorporates the capillary pressure, permeability, and thermal resistance to help explain the experimental results. We then extended this work to study capillary-limited thin-film evaporation for dissipating extreme heat fluxes. We experimentally dissipated =6 kW/cm2 from a 640x620 [mu]m2 footprint, the largest heat flux reported to date when compared to past thin-film evaporation studies with similar size hotspots. We also demonstrated the potential of our devices to cool concurrent hotspots as well as when moderate uniform background heat flux was superposed with a hotspot. Our thermal management strategy is self-regulating and provides on-demand cooling unlike existing thermal management solutions. To gain insight into the fundamental physics of fluidic and thermal transport within the micropillar wick and explain the ultra-high heat fluxes demonstrated in our experiments, we developed a semi-analytical thermal-fluidic model that can predict the capillary-limited dryout heat flux via thin-film evaporation. The model compares well with our experiments. The results of this investigation will assist to better understand the fluidic and thermal transport of thin liquid films in microstructured wicks during thin-film evaporation. These studies suggest that capillary-pumped thin-film evaporation is a promising thermal management strategy for the next generation of high performance electronics. The insights gained from this thesis can be used as guidelines to improve the design and optimize the heat transfer performance of wicking structures which are commonly used in phase-change based thermal management devices such as heat pipes, vapor chambers, and other closed-loop configurations.