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Research Papers: Evaporation, Boiling, and Condensation

Subcooled Pool Film Boiling Heat Transfer From Spheres With Superhydrophobic Surfaces: An Experimental Study

[+] Author and Article Information
Li-Wu Fan

Mem. ASME
Institute of Thermal Science and Power Systems,
School of Energy Engineering,
Zhejiang University,
Hangzhou 310027, China;
State Key Laboratory of Clean Energy Utilization,
Zhejiang University,
Hangzhou 310027, China
e-mail: liwufan@zju.edu.cn

Jia-Qi Li, You-You Su, Huan-Li Wang, Ting Ji

Institute of Thermal Science and Power Systems,
School of Energy Engineering,
Zhejiang University,
Hangzhou 310027, China

Zi-Tao Yu

Institute of Thermal Science and Power Systems,
School of Energy Engineering,
Zhejiang University,
Hangzhou 310027, China;
State Key Laboratory of Clean Energy Utilization,
Zhejiang University,
Hangzhou 310027, China

1Corresponding author.

Contributed by the Heat Transfer Division of ASME for publication in the JOURNAL OF HEAT TRANSFER. Manuscript received October 30, 2014; final manuscript received August 4, 2015; published online September 2, 2015. Assoc. Editor: Sujoy Kumar Saha.

J. Heat Transfer 138(2), 021503 (Sep 02, 2015) (8 pages) Paper No: HT-14-1720; doi: 10.1115/1.4031303 History: Received October 30, 2014; Revised August 04, 2015

Pool film boiling was studied by visualized quenching experiments on stainless steel spheres in water at the atmospheric pressure. The surfaces of the spheres were coated to be superhydrophobic (SHB), having a static contact angle greater than 160 deg. Subcooled conditions were concerned parametrically with the subcooling degree being varied from 0 °C (saturated) to 70 °C. It was shown that film boiling is the overwhelming mode of heat transfer during the entire course of quenching as a result of the retention of stable vapor film surrounding the SHB spheres, even at very low wall superheat that normally corresponds to nucleate boiling. Pool boiling heat transfer is enhanced with increasing the subcooling degree, in agreement with the thinning trend of the vapor film thickness. The heat flux enhancement was found to be up to fivefold for the subcooling degree of 70 °C in comparison to the saturated case, at the wall superheat of 200 °C. A modified correlation in the ratio form was proposed to predict pool film boiling heat transfer from spheres as a function of the subcooling degree.

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Figures

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Fig. 4

Comparison of the boiling curves over the SHB spheres among the cases with various subcooling degrees from saturated to up to 70 °C

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Fig. 3

Comparison of the quenching curves over the SHB spheres among the cases with various subcooling degrees from saturated to up to 70 °C

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Fig. 2

Repeatability test results showing the comparison of (a) quenching and (b) boiling curves among three consecutive quenching runs on a single sphere sample, and the comparison of (c) quenching and (d) boiling curves among parallel runs on three individual spheres. All tests were performed at saturated condition.

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Fig. 1

Scanning electron microscope image showing the microscale morphology on an SHB surface in the presence of nanoparticle aggregates. The upper inset illustrates a ∼3 mL deionized water droplet deposited on the SHB surface at ambient condition, showing an apparent static contact angle of ∼165 deg. The lower inset is a representative roughness profile showing the arithmetic average roughness height (Ra) of 64 nm.

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Fig. 5

Snapshots on the dynamic vapor film evolution during quenching at the instants of various wall superheats for the subcooling degree of (a) 0 °C (saturated), (b) 5 °C, (c) 10 °C, (d) 20 °C, and (e) 70 °C

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Fig. 7

Comparison of the measured subcooled pool film boiling heat transfer data with two existing correlations in the addition and ratio forms, proposed by Dhir and Purohit [11] and Michiyoshi et al. [14], respectively, at the wall superheat of (a) 50 °C, (b) 100 °C, (c) 150 °C, and (d) 200 °C

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Fig. 8

The measured scaling factor and its correlation as a function of the subcooling degree

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Fig. 6

Comparison of the variations of surface-averaged Nusselt number over the SHB spheres among the cases with various subcooling degrees from saturated to up to 70 °C

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