Many thermal systems rely on passive (capillary) or active (pumping) liquid supply to evaporation surfaces. Adjacent to evaporation surface, liquid supply and vapor removal have to compete (e.g., pool and flow boiling) unless they are separated with capillary bodies holding the liquid (wetting phase) and vapor. Efficient heat transfer under large local heat flux continues to demand innovative and transformative surface enhancement designs, and one venue has been use of capillary bodies to control distribution of phases adjacent to the evaporation surface. While the capillarity provides the suction for the liquid supply to the surface, there are various liquid and vapor choking limits to consider, so this is a fundamental thermal-hydraulic problem with important applications. A new three-dimensional, multilength scale, distributed evaporation wick is proposed for heat sink capable to removing 1000 W/cm2 with saturated liquid, and this will be a record. It is proposed to create and make available on Internet, illustrative thermal-hydraulics of the capillary bodies, such as the canopy wick (CW). The results of this project on theoretical optimization of CW will advance the thermal management of high-flux sources and will be used in a collaboration with industry which will fabricate and test a heat-sink prototype at high heat fluxes representative of those found in high-power laser applications. The industry collaboration allows for industrial interactions for the University of Michigan students.
In saturated, flow boiling the local thermal resistance depends on the local phase distributions, gravity direction, and two-phase flow instabilities. A unique wick structure is proposed to control the liquid delivery, vapor removal, and heat transfer making this resistance independent of the location (distance from the leading edge, or local vapor quality) and gravity, and reducing it to less than 0.05 K/(W/cm2) (i.e., heat transfer coefficient lager than 2x105 W/m2-K), and increase the dryout limit [critical heat flux (CHF)] to larger than 1000 W/cm2. This multidimensional capillary structure, called the canopy wick (CW) aims at separating and controlling the liquid and vapor flow paths based on the integrated evaporation and vapor-escape structures. The CW divides the liquid delivery and liquid spreading-evaporation functions and is an evolution of the modulated wicks previously developed by PI and Advanced Cooling Technologies (ACT) Inc. for passive systems (pool boiling and vapor chambers). The CW provides liquid to a thin evaporation wick (sintered particle monolayer, where the receding meniscus and local thermal nonequilibrium allow for low thermal resistance) covering the heated surface, delays surface dryout (increasing CHF) through high-permeability liquid-directing multiple artery (posts) wick, and creates vapor space and venting with perforated screenlayer roof. The screen-roof perforation opening and pitch are selected to create vapor flow inertial or delayed formation of vapor blanket in the otherwise liquid boundary-layer flow over it. In the proposed experiment, the CW will be constructed from sintered micrometer copper particles (larger particles for posts compared to monolayer) and perforated copper screenlayer (few layers of wire screen), and tested under water flow boiling with one-side heating (large cross-section area channel). The cascading capillary pressure (liquid pressure) in the three porous bodies is carefully matched by pore-size selection to ensure uninterrupted liquid flow. The post height and pitch are of the order of millimeter, the vapor preformation pitch may be larger than the posts to control the exiting vapor momentum, and the channel will have a hydraulic diameter of the order of centimeter.
