Research Papers

Effect of Meniscus Recession on the Effective Pore Radius and Capillary Pumping of Copper Metal Foams

[+] Author and Article Information
Mahmood R. S. Shirazy, Luc G. Fréchette

Department of Mechanical Engineering,
Institut Interdisciplinaire d'Innovation
Université de Sherbrooke,
Sherbrooke, PQ J1K2R1, Canada

Contributed by the Electronic and Photonic Packaging Division of ASME for publication in the JOURNAL OF ELECTRONIC PACKAGING. Manuscript received August 30, 2013; final manuscript received December 17, 2013; published online September 19, 2014. Assoc. Editor: Satish Chaparala.

J. Electron. Packag 136(4), 041003 (Sep 19, 2014) (8 pages) Paper No: EP-13-1098; doi: 10.1115/1.4026353 History: Received August 30, 2013; Revised December 17, 2013

An experimental study is performed to characterize the effect of meniscus recession on the effective pore radius and capillary pumping of copper metal foams which are to be used as wicks in heat pipes for electronic cooling. Knowledge of the effective pore radius is critical in defining the capillary pumping of a wicking material but is rarely measured under operating conditions. It is known that the meniscus of a liquid recedes when evaporating from a porous media, which could impact the effective pore radius and therefore the capillary pumping capabilities of the foam. To elucidate this impact, the evaporation rate is measured from foam strips wicking ethanol from a reservoir while applying heat fluxes to the foam. Using thermocouple and IR camera measurements, the measured evaporation rates are corrected to account for different thermal losses, including natural convection, direct thermal conduction to the liquid, and evaporation from the container. An analytical model is then developed to relate the evaporated mass to the maximum capillary pressure (minimum effective pore radius) provided by the foam. It is shown for the first time, that just before the onset of dryout, the recessed meniscus will lead to 15%, 28%, and 52% decrease in effective pore radius for samples with 68%, 75%, and 82% porosities, respectively. The capillary pumping therefore increases during evaporation. This can have significant impact on the prediction of the capillary limits in two phase capillary driven devices.

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Fig. 2

Schematic drawing of (a) the heater block and thermocouple locations (dimensions in mm) and (b) measurement setup with IR camera

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Fig. 3

Comparison of input power and individual contribution of each heat transfer mode to the measured heat flow of the 68% porosity foam

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Fig. 4

Axial temperature distribution in metal foams of (a) 68% porosity, (b) 75% porosity, and (c) 82% porosity. x = 0 is in the liquid container.

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Fig. 5

Effect of (a) wall temperature and (b) heat flux on the evaporation rates of the different foam porosities

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Fig. 6

IR profile of the 75% porosity foam (a) the whole surface is wet (wetted length, x = 35 mm), (b) liquid front decreased to x = 23 mm from liquid surface, and (c) liquid front in x = 13 mm from liquid surface

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Fig. 7

(a) Values of calculated effective pore radius for different heat fluxes and (b) values of K/reff for different heat fluxes

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Fig. 1

Morphology of the 75% porosity copper metal foams: (a) a macroscopic view of the copper metal foam; (b) capillary paths made by clusters of the spherical particles; and (c) small scale porosities formed between the spherical particles




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