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RESEARCH PAPERS

# Evaporation/Boiling in Thin Capillary Wicks (l)—Wick Thickness Effects

[+] Author and Article Information
Chen Li

Rensselaer Polytechnic Institute, Department of Mechanical, Aerospace and Nuclear Engineering, Troy, NY 12180lic4@rpi.edu

G. P. Peterson

Yaxiong Wang

Foxconn Thermal Technology Inc., Austin, TX 78758yaxiongwang@foxconn.com

J. Heat Transfer 128(12), 1312-1319 (Jan 11, 2006) (8 pages) doi:10.1115/1.2349507 History: Received August 28, 2005; Revised January 11, 2006

## Abstract

Presented here is the first of a two-part investigation designed to systematically identify and investigate the parameters affecting the evaporation/boiling and critical heat flux (CHF) from thin capillary wicking structures. The evaporation/boiling heat transfer coefficient, characteristics, and CHF were investigated under steady-state conditions for a variety of capillary structures with a range of wick thicknesses, volumetric porosities, and mesh sizes. In Part I of the investigation we describe the wicking fabrication process and experimental test facility and focus on the effects of the capillary wick thickness. In Part II we examine the effects of variations in the volumetric porosity and the mesh size as well as presenting detailed discussions of the evaporation/boiling phenomena from thin capillary wicking structures. An optimal sintering process was developed and employed to fabricate the test articles, which were fabricated using multiple, uniform layers of sintered isotropic copper mesh. This process minimized the interface thermal contact resistance between the heated wall and the capillary wick, as well as enhancing the contact conditions between the layers of copper mesh. Due to the effective reduction in the thermal contact resistance between the wall and capillary wick, both the evaporation/boiling heat transfer coefficient and the critical heat flux (CHF) demonstrated dramatic improvements, with heat transfer coefficients up to $245.5kW∕m2K$ and CHF values in excess of $367.9W∕cm2$, observed. The experimental results indicate that while the evaporation/boiling heat transfer coefficient, which increases with increasing heat flux, is only related to the exposed surface area and is not affected by the capillary wick thickness, the CHF for steady-state operation is strongly dependent on the capillary wick thickness and increases proportionally with increase in the wick thickness. In addition to these observations, the experimental tests and subsequent analysis have resulted in the development of a new evaporation/boiling curve for capillary wicking structures, which provides new physical insights into the unique nature of the evaporation/boiling process in these capillary wicking structures. Sample structures and fabrication processes, as well as the test procedures are described in detail and the experimental results and observations are systematically presented and analyzed.

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## Figures

Figure 2

Schematic of the test sample and thermocouple locations

Figure 5

Schematic of the test facility

Figure 6

A comparison of pool boiling data obtained in the current test facility from smooth flat copper surfaces with those obtained by Auracher (2003), and predictions from models of Zuber (1959), Moissis and Berenson (1962), and Lienhard and Dhir (1973)

Figure 7

(a) Heat flux as a function of super heat [TW-Tsat]; (b) Heat transfer coefficient as a function of heat flux

Figure 8

(a) Heat flux as a function of superheat [TW-Tsat]; (b) heat transfer coefficient as a function of heat flux

Figure 9

CHF as function of thickness of sintered isotropic copper mesh

Figure 1

(a) Top view of staggered sintered isotropic copper mesh; (b) top view of inline stacked sintered isotropic copper mesh; (c) side view of sintered isotropic copper mesh. SEM image of sintered isotropic copper mesh with 1509m−1(145in.−1), 56μm(0.0022in.) wire diameter, and fabricated at a sintering temperature of 1030°C with gas mixture protection (75% N2 and 25% H2) for two hours.

Figure 3

Illustration of a typical test article

Figure 4

A comparison of the thermal conductivities of two solid copper bars that were sintered together and a single solid copper bar

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