Numerical Simulation and Optimization of Thin-Film Evaporation in Variable Micropillar Wicks

Abstract Thin-film evaporation in micropillar wicking structures is a promising passive cooling strategy for high heat flux electronics. This study numerically investigates thin-film evaporation in well-defined silicon micropillar wicks, where water is transported passively via capillary wicking from the reservoir to the evaporator. A coupled force balance and conservation law framework is employed to determine the meniscus shape, capillary pressure, fluid velocity in the micropillar wicks, and associated heat transfer characteristics. The dry-out heat flux, defined as the maximum heat flux the evaporator can dissipate when the smallest contact angle equals the receding contact angle, is evaluated for different wick designs. For uniform wicks with fixed micropillar geometry, the maximum dry-out heat flux is ≈84 W/cm2. To enhance thermo-fluidic performance, variable wicks are designed with sparse micropillars near the water reservoir and dense micropillars near the evaporator center. By dividing the wick into multiple sections with optimized diameters, the dry-out heat flux reaches ≈147 W/cm2, a 75% improvement over uniform wicks. Further optimization of the variable wicks using a genetic algorithm (GA) increases the dry-out heat flux to ≈165 W/cm2, a 96% enhancement compared to uniform wicks. Unlike uniform wicks, where dry-out starts at the evaporator center, optimized variable wicks experience dry-out at an intermediate location due to increased capillary pressure near the center. These findings provide useful insights into the design and optimization of wicking structures for thin-film evaporation in advanced passive cooling of electronic devices.