Micromembrane-enhanced capillary evaporation
Introduction
Capillary evaporation [1] is one of the most efficient heat transfer modes and has been widely used in heat exchangers [2] and heat pipes [3], [4], [5]. Evaporators with high heat transfer coefficient (HTC) and critical heat flux (CHF) are highly desirable for compact heat exchangers for high heat flux applications [6], [7]. Most of porous coatings used in enhancing capillary evaporation are usually mono-porous structures [8]. For example, sintered particles and powders were developed to substantially enhance thin film evaporation HTC [8], [9]. The effects of porosity, wick thickness and other factors on the optimal design of the wicking structures were also examined [8], [9]. Copper woven mesh laminates [10], [11], [12], [13] were extensively studied to enhance the capillary evaporation HTC due to the augmented surface areas and increased capillary forces. However, the flow resistances in these microscale mono-porous structures remain high, resulting in low CHF due to the liquid supply crisis. Micro-grooves [14], [15] or channels [16] were superior for liquid supply because of the attributed low flow resistance, but the capillary forces induced by the disjoining pressure differences in grooves [15] were still too low to reach high CHF. This brief review shows that both the microscale mono-porous structures (such as sintered meshes or particles/powders) and micro-grooves or channels cannot meet the needs of high heat flux applications. To solve this dilemma, various types of bi-porous surfaces were proposed and developed [5], [17], [18], [19], [20], [21]. Semenic et al. [18], [20] found that biporous surfaces of sintered powders performed better than the mono-porous copper wicks because the working fluid can be supplied to hot spots through micropores inside the clusters even though the voids were filled with vapor. Cao et al. [22] reported that when a mono-dispersed wick was replaced by a bi-dispersed wick with the same small pore diameter, both HTC and CHF were increased significantly. Cai et al. [6] studied the heat transfer performance on the carbon nanotube (CNT) based bi-porous structures, which consisted of CNT array separated by microchannels. The nanoscale pores in the CNT bi-porous structure provided ultrahigh capillary pressure and augmented surface areas, which significantly reduced the menisci radii and increased thin-film evaporation area and evaporation efficiency. Ćoso et al. [23] examined a type of bi-porous media consisting of microscale pin fins periodically separated by microchannels to simultaneously increase the heat dissipation capacity as well as the HTC of the evaporator wick. Some of the bi-porous wicks have also been integrated in heat pipes [19], [24], [25], [26] to decrease the thermal resistance and increase working heat fluxes. Heat pipe performance was found to be greatly enhanced by applying modulated wick because of enhanced axial capillary liquid flows and additional evaporation surface area resulting from the cross-sectional area [5]. In these reported bi-porous structures, the main fluid passages were still through the micro or nanoscale mono-porous structures (such as microscale powders or CNTs). As a result, the overall liquid flow resistances still remain high.
On the other hand, the oscillating flow significantly increases HTC and CHF in closed mini/micro-channels as can be found in oscillating heat pipes [16], [27]. However, the oscillating capillary evaporation in unconfined or open microchannels was not reported.
The objective of this study is to develop a new type of micromembrane-enhanced evaporating surfaces that are capable of both generating high capillary pressure and managing flow resistances. The effects on capillary evaporation were systematically examined. These effects include the separation of liquid supply and capillarity generation as well as the induced oscillating flows in unconfined micromembrane-enhanced evaporating surfaces.
Section snippets
Design of micromembrane-enhanced capillary evaporating surfaces
During the capillary evaporation, the counter interactions of flow resistance and capillary force determine the overall liquid supply and thus, the CHF. Fine copper woven meshes with microscale pores can generate high capillary pressure, but the associated flow resistance through the in-plane direction was significantly high. Micro-grooves [14], [15] or channels [16] were superior for liquid supply because of the low flow resistance, but with limited capillarity [15]. The combination of the
Experimental apparatus and data reduction
Fine sintered copper woven meshes were employed as the primary evaporating membranes because of their superior thermal conductivity, high permeability and large surface areas [29], [32], [33]. Copper woven meshes with mesh number of 1509 m−1 (145 inch−1) and wire diameter of 56 μm (made by Belleville Wire Cloth, as shown in Fig. 2(a)–(c)) were attached on the microchannels by a diffusion bonding technique [32], [34] to minimize the contact thermal resistance [29]. Samples as shown in Fig. 2(a) and
Results and discussion
In this study, four types of evaporating surfaces with identical heating areas (1 × 1 cm2) were experimentally investigated. These include copper microchannels (channel height and width: 250 μm, channel wall width: 250 μm), single layer copper mesh screen (thickness: 80 μm), four-layer sintered copper mesh screens (thickness: 320 μm) and the micromembrane-enhanced evaporating surfaces (total thickness of sintered microchannels and mesh: 320 μm). Dimensions of all evaporating surfaces are specified in
Conclusions
In summary, significantly high HTC and CHF were achieved using the micromembrane-enhanced evaporating surfaces that were developed in this study. The separation of capillary pressure generation and water transport processes enabled by the micromembrane should be favorable to supply liquid. CHF on the developed micromembrane-enhanced evaporating surfaces was substantially enhanced to 152.2 W/cm2. Compared to microchannels, four-layer mesh screens, and single-layer mesh screen, CHF was enhanced
Acknowledgments
This work is supported by the Defense Advanced Research Projects Agency (DARPA) Thermal Ground Plane under Grant No. N66001-08-C-2006 issued by the Space and Naval Warfare Systems Center Pacific (SPAWAR) and by the Office of Naval Research (Program Officer Dr. Mark Spector) under Grant No. N000141210724. The authors also greatly appreciate Fazle Rabbi’s help for taking the Micro-XCT images.
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