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Research Papers: Evaporation, Boiling, and Condensation

Effect of Liquid Properties on Phase-Change Heat Transfer in Porous Wick Structures

[+] Author and Article Information
Steve Q. Cai

Teledyne Scientific & Image Company,
1049 Camino Dos Rios,
Thousand Oaks, CA 91360
e-mail: qingjun.cai@teledyne.com

Avijit Bhunia

Teledyne Scientific & Image Company,
1049 Camino Dos Rios,
Thousand Oaks, CA 91360

Contributed by the Heat Transfer Division of ASME for publication in the JOURNAL OF HEAT TRANSFER. Manuscript received December 15, 2014; final manuscript received October 21, 2015; published online November 24, 2015. Assoc. Editor: Amitabh Narain.

J. Heat Transfer 138(3), 031504 (Nov 24, 2015) (7 pages) Paper No: HT-14-1814; doi: 10.1115/1.4031929 History: Received December 15, 2014; Revised October 21, 2015

In a heat pipe, operating fluid saturates wick structures system and establishes a capillary-driven circulation loop for heat transfer. Thus, the thermophysical properties of the operating fluid inevitably impact the transitions of phase-change mode and the capability of heat transfer, which determine both the design and development of the associated heat pipe systems. This article investigates the effect of liquid properties on phase-change heat transfer. Two different copper wick structures, cubic and cylindrical in cross section, 340 μm in height and 150 μm in diameter or width, are fabricated using an electroplating technique. The phase-change phenomena inside these wick structures are observed at various heat fluxes. The corresponding heat transfer characteristics are measured for three different working liquids: water, ethanol, and Novec 7200. Three distinct modes of the phase-change process are identified: (1) evaporation on liquid–vapor interface, (2) nucleate boiling with interfacial evaporation, and (3) boiling enhanced interface evaporation. Transitions between the three modes depend on heat flux and liquid properties. In addition to the mode transition, liquid properties also dictate the maximum heat flux and the heat transfer coefficient. A quantitative characterization shows that the maximum heat flux scales with Merit number, a dimensionless number connecting liquid density, surface tension, latent heat of vaporization, and viscosity. The heat transfer coefficient, on the other hand, is dictated by the thermal conductivity of the liquid. A complex interaction between the mode transition and liquid properties is reflected in Novec 7200. In spite of having the lowest thermal conductivity among the three liquids, an early transition to the mode of the boiling enhanced interface evaporation leads to a higher heat transfer coefficient at low heat flux.

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Figures

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Fig. 5

Phase-change images on the cubic wick structure with ethanol as working fluid at: (a) 25 W/cm2, (b) 75 W/cm2, (c) 175 W/cm2, and (d) 250 W/cm2

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Fig. 6

Phase-change images on the cubic wick structure with 3M Novec 7200 working fluid at: (a) 25 W/cm2, (b) 50 W/cm2, (c) 75 W/cm2, and (d) 100 W/cm2

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Fig. 4

Phase-change images on the cubic wick structure withwater as working liquid at: (a) 25 W/cm2, (b) 75 W/cm2, (c) 175 W/cm2, and (d) 250 W/cm2

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Fig. 3

Schematic diagram of the wick sample setup and temperature measurement

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Fig. 2

Schematic diagram of the visualization and characterization system for phase-change heat transfer studies

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Fig. 1

Copper wick structures made through electroplating processes: (a) cylindrical wick pillars, 150 μm in diameter and (b) the cubic wick pillars with a square cross section of 150 μm × 150 μm

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Fig. 7

Schematic diagram of phase-change modes: (a) evaporation on liquid–vapor interface, (b) nucleate boiling and evaporation at the meniscus, and (c) boiling enhanced interface evaporation inside the wick structure

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Fig. 8

Heat flux versus superheat of the cylindrical pillar wick

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Fig. 9

Heat flux versus superheat of the cubic pillar wick

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Fig. 10

The total temperature drop equals to the sum of drops in the solid wick unit and across the operating fluid film

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Fig. 11

Heat transfer coefficient versus superheat of cylindrical pillar wick

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Fig. 12

Heat transfer coefficient versus superheat of cubic pillar wick

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