Design of micropillar wicks for thin-film evaporation

Abstract

The generation of concentrated heat loads in advanced microprocessors, GaN electronics, and solar cells present significant thermal management challenges in 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, thin-film evaporation has received increased interest owing to advances in micro/nanofabrication and the potential to dissipate high heat fluxes by increasing the evaporative meniscus area. Yet, predictive tools to design various wicking structures are limited due to the complexity of the thermal-fluidic transport. In this work, we performed systematic experiments to characterize capillary-limited thin-film evaporation from silicon micropillar wicks in the absence of nucleate boiling. The insights gained from experiments were used to model the capillary pressure, permeability, and thermal resistance. Accordingly, we developed a semi-analytical model to determine the capillary-limited dryout heat flux and wall temperature with ±20% accuracy, compared to our experiments. The model provides a versatile platform to design and optimize micropillar wicks for next generation thermal management devices.International Journal of Heat and Mass Transfer 56 Nomenclature Area (m 2 ) Correction factor/height ratio (-)

Nanoporous membrane device for ultra high heat flux thermal management

“…The heat fluxes dissipated using our nanoporous membranes are 13.5 × higher than the highest previously reported heat fluxes in a pure evaporation regime for any fluid, dielectric or conducting, when normalizing to the evaporation area 14,16 . Previous work has demonstrated heat fluxes of 160-5 800 W/cm 2 normalized to the heater area with an evaporator-to-heater area ratio of 18-250, respectively 14,29 .…”

“…Since the capillary and viscous pressures are coupled to pore size, the highest critical heat flux (CHF) are typically for sintered wicks with particle sizes of 250-355 µm using water. This CHF is~500 W/cm 2 with nucleate boiling in the wicking structure and 50-80 W/cm 2 with pure evaporation [13][14][15][16] , that is, without nucleate boiling. Similarly, the overall heat transfer coefficient (h = q″/ΔT) in traditional capillary-driven evaporation structures is dominated by heat conduction in the wicking structure as well as through the evaporating liquid film.…”

A hierarchical manifold microchannel heat sink array for high-heat-flux two-phase cooling of electronics

“…A variety of heat sink designs have been employed to dissipate larger heat fluxes by delaying CHF or reducing the pressure drop in two-phase operation compared to a conventional design with straight, parallel channels fed by a single header. These designs have implemented one or more of features such as vapor venting [10], pin-fins and interrupted channels of various shapes and configurations [10][11][12], wick structures to aid in thin film evaporation [13][14][15], microchannels with reentrant cavities and/or inlet restrictors [16], microgaps [17], arrays of jets [18][19][20][21], diverging channels [22,23], microchannels fed with tapered manifolds [24], and stacked heat sinks [25]. Heat fluxes as high as 1127 W/cm² have been dissipated with dielectric fluids [26] using a 10 mm × 20 mm copper heat sink that incorporated both flow boiling in microchannels and jet impingement.…”

Novel Receiver-Enhanced Solar Vapor Generation: Review and Perspectives

“…Vaporization of water is a common phenomenon that contributes significantly to the utilization and production of fresh water in living organisms and industry [11,12]. In industries, the steam is generated by two means.…”