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Research Papers: Evaporation, Boiling, and Condensation

Effect of Nanostructured Roughness on Evaporating Thin Films in Microchannels for Wenzel and Cassie–Baxter States

[+] Author and Article Information
Jun-Jie Zhao

Key Laboratory of Thermal Science and
Power Engineering of MOE,
Beijing Key Laboratory for CO2 Utilization and
Reduction Technology,
Tsinghua University,
Beijing 100084, China;
Guodian Power Development Company Limited,
China Guodian Corporation,Beijing 100101, China

Yuan-Yuan Duan

Key Laboratory of Thermal Science and
Power Engineering of MOE,
Beijing Key Laboratory for CO2 Utilization and
Reduction Technology,
Tsinghua University,
Beijing 100084, China
e-mail: yyduan@tsinghua.edu.cn

Xiao-Dong Wang

State Key Laboratory of Alternate Electrical Power
System With Renewable Energy Sources,
North China Electric Power University,
Beijing 102206, China
e-mail: wangxd99@gmail.com

Bu-Xuan Wang

Key Laboratory of Thermal Science and
Power Engineering of MOE,
Beijing Key Laboratory for CO2 Utilization and
Reduction Technology,
Tsinghua University,
Beijing 100084, China

1Corresponding author.

Contributed by the Heat Transfer Division of ASME for publication in the Journal of Heat Transfer. Manuscript received July 19, 2012; final manuscript received November 27, 2012; published online March 20, 2013. Assoc. Editor: Louis C. Chow.

J. Heat Transfer 135(4), 041502 (Mar 20, 2013) (9 pages) Paper No: HT-12-1380; doi: 10.1115/1.4023230 History: Received July 19, 2012; Revised November 27, 2012

A model based on the augmented Young–Laplace equation and kinetic theory was developed to describe the nanostructured roughness effects on an extended evaporating meniscus in a microchannel for Wenzel and Cassie–Baxter states. The roughness geometries were analytically related to the disjoining pressure, slip length and thermal resistance across the roughness layer. The results show that the equivalent Hamaker constant and adsorbed film thickness increase with nanopillar height for Wenzel state. Thus, the spreading and wetting properties of the evaporating thin film increase with roughness for Wenzel state, leading to an elongated thin film and enhanced heat transfer rate compared to a flat hydrophilic surface. The equivalent Hamaker constant and disjoining pressure effect decrease with increasing nanopillar height for Cassie–Baxter state. The system wettability, thin film length and heat transfer rate increase with increasing slip length and with decreasing roughness for Cassie–Baxter state. A smaller roughness coexisting with a larger slip length on rough surfaces for Cassie–Baxter state results in a much higher heat transfer rate relative to a flat surface.

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Figures

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Fig. 1

Extended evaporating meniscus and coordinate system

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Fig. 2

Wetting states of an evaporating thin liquid film on rough surfaces: (a) Wenzel state; (b) Cassie–Baxter state; (c) two typical elements in the Wenzel state

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Fig. 3

Thermal resistance across the periodic nanoscale roughness layer: (a) for a flat surface; (b) for Wenzel state; (c) for Cassie–Baxter state

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Fig. 4

Nanostructured roughness effect for 5 K and 10 K superheats for Wenzel state: (a) equivalent Hamaker constant; (b) adsorbed film thickness

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Fig. 5

Nanostructured roughness effect on the film profile for 5 K superheat for Wenzel state: (a) film thickness; (b) thickness gradient; (c) curvature

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Fig. 6

Nanostructured roughness effect on the pressure profile for 5 K superheat for Wenzel state: (a) liquid pressure difference; (b) vapor pressure difference

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Fig. 7

Nanostructured roughness effect on the thin film heat transfer for 5 K and 10 K superheats for Wenzel state: (a) local evaporative heat flux; (b) total heat transfer rate

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Fig. 8

Nanostructured roughness effect on the thin film evaporation for 5 K superheat for Cassie–Baxter state: (a) film thickness; (b) local evaporative heat flux; (c) total heat transfer rate

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