Characterization of Microstructures for Heat Transfer Performance in Passive Cooling Devices

Abstract

The topology and geometry of microstructures play a crucial role in determining heat transfer performance in passive cooling devices such as heat pipes. It is therefore important to characterize microstructures based on their wicking performance, the thermal conduction resistance of the liquid filling the microstructure, and the thin-film characteristics of the liquid meniscus. In the present study, the free-surface shapes of the static liquid meniscus in common microstructures have been modeled using the program, Surface Evolver, for zero Bond number. Four well-defined topologies, viz., surfaces with parallel rectangular ribs, horizontal parallel cylinders, vertically aligned cylinders, and spheres (the latter two in both square and hexagonal packing arrangements), have been modeled. Non-dimensional capillary pressure, average distance of the free liquid surface from solid walls (a measure of the conduction resistance of the liquid), total exposed area and thin-film percentage of surface area of the liquid meniscus have been computed. These parameters are presented as functions of the non-dimensional geometrical parameters of the microstructures, volume of the liquid filling the structure, as well as the contact angle between the liquid and solid. Based on these non-dimensional performance parameters, the microstructure, contact angle and non-dimensional liquid volume for the best performance are identified.

A microscale model for thin-film evaporation in capillary wick structures

“…This is also consistent with previous predictions by the authors based on a thermal resistance network model and static liquid meniscus shapes in the wick microstructure [1,2]. The thin-film area of the meniscus was reported to be highest for the case of packed spheres on a surface, based on staticequilibrium liquid meniscus shapes.…”

“…The solid-liquid contact angle has been demonstrated to have a significant effect on the thin-film area of a liquid meniscus [1,2]. Although copper-water is the solid-liquid combination considered in this work (with corresponding contact angles of 84° and 33° at the advancing and receding liquid fronts, respectively [24]), the contact angle is now varied to study its effect on evaporation.…”

“…The porosity, defined as the ratio of void volume to the total volume, is taken to be same in all four cases. As defined in [2], the unit cells for the four surfaces are shown in Figure 1 and described below. In Figure 1 (a), h is the initial height of the filling liquid measured from the bottom, and r is the radius of the wire or sphere.…”

“…Surface Evolver computes the total surface energy of a given system and the equilibrium minimum energy configuration is obtained by a gradient-descent method. Details of this method can be found in [1,2]. The static liquid meniscus shapes are exported from Surface Evolver to GAMBIT [19] and the domain is discretized for finite volume computations in FLUENT [19].…”

“…With the increasing power density of electronic components, there is a need to optimize heat pipe microstructures to maximize the heat transfer rate and to minimize thermal resistance. The wicking and thin-film evaporation characteristics of diverse wick microstructures were recently investigated by the authors [1,2]. Four different wick microstructures were modeled using Surface Evolver [3] and static liquid-meniscus shapes were obtained for different wick geometries and solid-liquid contact angles.…”