Optimal design of capillary-wick for high heat flux thermal management system

Abstract

A thermal management system for high heat flux is in great demand as the device sizes become smaller. A failure of the optimal thermal management leads to poor performance, large degradation, and/or complete system breakdown. The optimal thermal management system requires both a low thermal resistance [order of 0.01 K/(W/cm2)] and high heat flux cooling capability [order of 1 kW/cm2]. A kinetics of a phase change, i.e., liquid-vapor, provides a good potential to remove high heat flux in evaporators due to a significant latent heat. However, the challenges are the limited liquid supply to the high heat flux surface due to the viscous-capillary limit and a large thermal resistance due to low effective thermal conductivity of the evaporator wick. Here, the optimal evaporator wick for the enhanced liquid supply system is studied using the monolayer and liquid artery wick, to simultaneously reduce the thermal resistance and enhance the heat flux. An innovative liquid artery (thick) wick with monolayer (thin) wick has been demonstrated previously, qCHF ~ 600 W/cm2 with AhRk,e < 0.05 K/(W/cm2), where the monolayer wick limits the liquid supply to the heated surface and heat transport. Further improvements require the optimal designs of the monolayer wick for desired permeability, porosity, saturation, and capillary meniscus. Geometric optimizations are performed using both the thermal equilibrium and non-equilibrium models including developed close form solutions. For 60 μm particle with the distance between particles 1.12 times the particle diameter, qCHF is obtained around 87 to 90 degree of the angle to the liquid contact, leading to the qCHF ~ 1 kW/cm2. The capillary meniscus recess is also discussed as the liquid morphologies of the monolayer wick drastically change near the particle neck. The effects of the particle size and distance among the particles on the heat flux and thermal resistance are also discussed.