Evaporation, Boiling, and Condensation

Enhanced Heat Transfer in Biporous Wicks in the Thin Liquid Film Evaporation and Boiling Regimes

[+] Author and Article Information
Dušan Ćoso, Vinod Srinivasan

Department of Mechanical Engineering,  University of California, Berkeley, CA 94720-1740

Ming-Chang Lu

Department of Mechanical Engineering,  National Chiao Tung University, Hsinchu, Taiwan 30010

Je-Young Chang

Intel Corporation, 5000 W. Chandler Boulevard, CH5-157, Chandler, AZ 85226

Arun Majumdar

Advanced Research Projects Agency-Energy (ARPA-E),  US Department of Energy, Washington, D.C. 20585Arun.Majumdar@hq.doe.gov

The overall heat loss due to spreading estimated by Eq. 3 and supported by the finite element models are accounted for in all presented data. In this way, we are able to isolate the total heat flux that is dissipated by evaporation/boiling within some uncertainty. Therefore, even though we cannot determine quantitatively the total area over which the bulk of the evaporation/boiling occurs, we can estimate the temperature Twall at the pin fin array base using Eq. 5. This way of defining the heat transfer coefficient includes the lateral heat spreading in the samples dissipated by evaporation/boiling.

J. Heat Transfer 134(10), 101501 (Aug 07, 2012) (11 pages) doi:10.1115/1.4006106 History: Received March 13, 2011; Revised December 29, 2011; Published August 06, 2012; Online August 07, 2012

Biporous media consisting of microscale pin fins separated by microchannels are examined as candidate structures for the evaporator wick of a vapor chamber heat pipe. The structures are fabricated out of silicon using standard lithography and etching techniques. Pores which separate microscale pin fins are used to generate high capillary suction, while larger microchannels are used to reduce overall flow resistance. The heat transfer coefficient is found to depend on the area coverage of a liquid film with thickness on the order of a few microns near the meniscus of the triple phase contact line. We manipulate the area coverage and film thickness by varying the surface area-to-volume ratio through the use of microstructuring. Experiments are conducted for a heater area of 1 cm2 with the wick in a vertical orientation. Results are presented for structures with approximately same porosities, fixed microchannel widths w ≈ 30 μm and w ≈ 60 μm, and pin fin diameters ranging from d = 3–29 μm. The competing effects of increase in surface area due to microstructuring and the suppression of evaporation due to reduction in pore scale are explored. In some samples, a transition from evaporative heat transfer to nucleate boiling is observed. While it is difficult to identify when the transition occurs, one can identify regimes where evaporation dominates over nucleate boiling and vice versa. Heat transfer coefficients of 20.7 (±2.4) W/cm2 -K are attained at heat fluxes of 119.6 (±4.2) W/cm2 until the wick dries out in the evaporation dominated regime. In the nucleate boiling dominated regime, heat fluxes of 277.0 (±9.7) W/cm2 can be dissipated by wicks with heaters of area 1 cm2 , while heat fluxes up to 733.1 (±103.4) W/cm2 can be dissipated by wicks with smaller heaters intended to simulate local hot-spots.

Copyright © 2012 by American Society of Mechanical Engineers
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Figure 1

Thermal resistance network for a typical sintered copper particle porous wick structure; Q is the applied power, Twall is the temperature at the base of the particle matrix, Tsubstrate is the temperature at the base of the wick substrate, Rsubstrate the thermal resistance of the substrate, Rmatrix is the overall thermal resistance of the particle/liquid matrix, Rfilm is the resistance of the liquid thin film that forms at the meniscus, Rint is the thermal resistance of the liquid/vapor interface, and Tvap is the vapor temperature

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Figure 2

(a) Cross section and plan views of the evaporator geometry showing arrays of pin fins with periodic microchannels; here d is the side length of a pin of square cross section, p is the pin spacing, H is the depth of the wick, w is the channel width, and D is the width of the pin fin array; (b) heat flow path and the corresponding thermal resistance network for the silicon wick structures investigated here: Q is the applied power, Twall is the temperature at the base of the pin fin array, Tsubstrate is the measured substrate temperature, Rsubstrate is the thermal resistance of the substrate from which the pins are protruding, Rpins is the thermal resistance of the pin array, RL is the bulk liquid resistance which fills the pores, Rfilm is the resistance of the liquid thin film that forms at the meniscus, Rint is the thermal resistance of the liquid/vapor interface, and Tvap is the vapor temperature

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Figure 3

Schematic of fabrication process used to generate the evaporator wick geometries: (a) Photoresist (PR) is deposited. (b) Lithography is used to define and develop the pattern. (c) Deep silicon etching is used to obtain a wick of desired depth. (d) After cleaning, the sample is oxidized to improve wettability and an ITO heater is deposited subsequently. (e) Copper electrodes are deposited and the sample is cut to a desired size.

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Figure 4

Experimental apparatus used to measure the heat transfer performance of the evaporator wicks

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Figure 5

Sketch of resistances to heat flow generated at the back surface of the wick and the placement of the thermocouples for the wall temperature measurement

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Figure 6

Temperature contours (in °C) from finite element analysis for an imposed heat flux of 100 W/cm2 and evaporation coefficient of 12 W/cm2 -K. Here one end of wafer is assumed to be at the saturation temperature of water Tsat  = 100 °C. Black dashed line indicates relative size of the 1 × 1 cm2 heater centered on a 2 × 2 cm2 wick. These contours represent the measured substrate temperature field Tsubstrate .

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Figure 7

Sketch of the wick and liquid level based on experimental observations at low to moderate and high heat fluxes

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Figure 8

(a) Heat transfer data for pin fin arrays of variable depth H, for constant pin fin size d = 13.5 μm, pore size p = 16.5 μm, pin array width w = 144 μm, and microchannel width w = 31 μm. (b) Heat transfer coefficient variation with wick depth H. Only error bars at dryout heat fluxes are shown for each data set.

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Figure 9

Images of nucleate boiling at various heat fluxes in a wick sample for d = 13.5 μm, p = 16.5 μm, H = 207 μm, and w = 30 μm

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Figure 10

Burst sequence at an imposed heat flux of ∼230 W/cm2 : (a) Bubble bursting and violent ejection of liquid droplets. (b) Formation of dry area around nucleation site. (c) and (d) Advancing wetting front. (e) Liquid almost fully rewetting the wick. (f) Start of next burst cycle.

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Figure 11

Sketch of arrangement of wick and heater surfaces with respect to the liquid reservoir: (a) Awick  = 3.8 cm2 , Aheater  = 1 cm2 (b) Awick  = 1.5 cm2 , Aheater  = 1 cm2 and (c) Awick  = 1.1 cm2 , Aheater  = 0.0625 cm2

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Figure 12

Effect of the ratio of wick surface area to heater area, for d = 7.1 μm, p = 9.0 μm, w = 30 μm, and H = 149 μm. Only the error bar at dryout heat flux is shown for each data set, where the two sets with lower attained heat fluxes have error bars that are smaller than the marker size.

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Figure 13

(a) Heat transfer data for various pin fin and pore size with channel size w = 30 μm, nominal array width D = 150 μm, and nominal wick depth H = 145 μm. (b) Heat transfer data for varied pin fin size with channel size w = 60 μm, nominal array width D = 260 μm, and nominal depth H = 150 μm. Only error bars at dryout heat fluxes are shown for each data set.

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Figure 14

Sketch of a thin liquid film and meniscus around a micro pin fin. Here Lfilm is the liquid film length along the pin that forms at the meniscus, p is the pore size spanning the pins, d is the side length of a square cross section pin, and δ is the thickness of the thin liquid film.

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Figure 15

Variation of the thermal conductance of the thin liquid film as a function of pore size at moderate evaporative heat fluxes of ∼60 W/cm2



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