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Research Papers

Flooded Two-Phase Flow Dynamics and Heat Transfer With Engineered Wettability on Microstructured Surfaces

[+] Author and Article Information
Bo Chen, Zhou Zhou, Junxiang Shi, Steven R. Schafer

Department of Mechanical and
Aerospace Engineering,
University of Missouri,
Columbia, MO 65211

Chung-Lung (C. L.) Chen

Department of Mechanical and
Aerospace Engineering,
University of Missouri,
Columbia, MO 65211
e-mail: chencl@missouri.edu

1Corresponding author.

Manuscript received May 2, 2014; final manuscript received September 3, 2014; published online May 14, 2015. Assoc. Editor: Yogesh Jaluria.

J. Heat Transfer 137(9), 091021 (Sep 01, 2015) (12 pages) Paper No: HT-14-1286; doi: 10.1115/1.4030237 History: Received May 02, 2014; Revised September 03, 2014; Online May 14, 2015

Due to excessive droplet feeding, a period of flooding occurs as part of a typical droplet based thermal management cycle. The conventional superhydrophilic surface, which is designed for thin film evaporation because of its highly wettable character, has a limited improvement on the thermal performance during the flooded condition. This paper investigates microstructures which combine micropillars and four engineered wettability patterns to improve the heat dissipation rate during flooding. Using the transient, 3D volume-of-fluid (VOF) model, the bubble behaviors of growth, coalescence, and departure are analyzed within different microstructures and the effects of pillar height and wettability patterns on the thermal performance are discussed. The wettability gradient patched on the pillar's side is demonstrated to promote the bubble's upward movement due to the contact angle difference between the upper and lower interfaces. However, insufficient pulling force results in large bubbles being pinned at the pillar tops, which forms a vapor blanket, and consequently decreases the heat transfer coefficient. When only a patch of hydrophobic material is present on the pillar top, effective pulling forces can be developed to help bubbles in the lower level depart from the pillar forest, since bubble merging between them generates most of the power required to pull the bubbles to the surface. The simulation results, including heat source temperatures and heat transfer coefficients, indicate that a patch of hydrophobic material on the pillar top works best out of all of the cases studied.

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Figures

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Fig. 1

Computational domain and flooding scenario

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Fig. 2

Configurations of series with different pillar heights

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Fig. 3

Heat transfer coefficients of different microstructures

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Fig. 4

Liquid–gas interface pins at the top edges of the pillars

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Fig. 5

Configurations of series with different wettability patterns

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Fig. 6

Bubble evolution on four kinds of microstructures

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Fig. 7

Comparison of bubble upward velocity between microstructures with wholly hydrophilic and wettability gradient patterns

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Fig. 8

Comparison of velocity field between microstructures with wholly hydrophilic and wettability gradient patterns at 0.058 ms

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Fig. 9

Sketch of surface tension force analysis

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Fig. 10

Imaginary movement of a bubble pinning between two pillars

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Fig. 11

Pulling effect of sit-on-top bubble

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Fig. 12

Evolution and comparison of velocity field between microstructures with wettability gradient patterns and hydrophobic patch on pillar tops. (a)–(c): with wettability gradient from contact angle of 5 deg–55 deg, flow time are 0.058 ms, 0.064 ms, and 0.070 ms, respectively; (d): top patched by hydrophobic material with contact angle of 110 deg.

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Fig. 13

Bottom surface temperatures for different wettability patterns

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Fig. 14

Heat transfer coefficients on surfaces with different wettability patterns

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