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Research Papers: Micro/Nanoscale Heat Transfer

Tuning Superhydrophilic Nanostructured Surfaces to Maximize Water Droplet Evaporation Heat Transfer Performance

[+] Author and Article Information
Claire K. Wemp

Mechanical Engineering Department,
University of California,
Berkeley, CA 94720-1740
e-mail: ckunkle@berkeley.edu

Van P. Carey

Fellow ASME
Mechanical Engineering Department,
University of California,
6123 Etcheverry Hall,
Berkeley, CA 94720-1740
e-mail: vpcarey@berkeley.edu

1Corresponding author.

Contributed by the Heat Transfer Division of ASME for publication in the JOURNAL OF HEAT TRANSFER. Manuscript received August 3, 2017; final manuscript received April 20, 2018; published online May 25, 2018. Assoc. Editor: Yuwen Zhang.

J. Heat Transfer 140(10), 102401 (May 25, 2018) (10 pages) Paper No: HT-17-1447; doi: 10.1115/1.4040142 History: Received August 03, 2017; Revised April 20, 2018

Spraying water droplets on air fin surfaces is often used to augment the performance of air-cooled Rankine power plant condensers and wet cooling tower heat exchangers for building air-conditioning systems. To get the best performance in such processes, the water droplets delivered to the surface should spread rapidly into an extensive, thin film and evaporate with no liquid leaving the surface due to recoil or splashing. This paper presents predictions of theoretical/computational modeling and results of experimental studies of droplet spreading on thin-layer, nanostructured, superhydrophilic surfaces that exhibit very high wicking rates (wickability) in the porous layer. Analysis of the experimental data in the model framework illuminates the key aspects of the physics of the droplet-spreading process and evaporation heat transfer. This analysis also predicts the dependence of droplet-spreading characteristics on the nanoporous surface morphology and other system parameters. The combined results of this investigation indicate specific key strategies for design and fabrication of surface coatings that will maximize the heat transfer performance for droplet evaporation on heat exchanger surfaces. The implications regarding wickability effects on pool boiling processes are also discussed.

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Figures

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Fig. 1

Droplet-spreading process

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Fig. 2

Synchronous spreading regions

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Fig. 3

Dimensionless mass flux variation at the droplet/layer boundary

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Fig. 4

Droplet feeding hemi-spreading

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Fig. 5

Tightly packed pillar structure on copper surface enhanced with ZnO nanostructures (6 nm seeding particles, grown in solution for 8 h) imaged with a scanning electron microscope

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Fig. 6

Comparison of droplet spread data with the model predicted variation of R̂ = R/Rs with t̂ = t/ts for surface prep 1: (a) linearplot and (b) log–log plot

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Fig. 7

Comparison of droplet spread data with the model predicted variation of R̂ = R/Rs with t̂ = t/ts for surface prep 2 with 2 μl droplet and surface prep 2 with 3 μl droplet

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Fig. 8

High-speed video frames for a 2 μl water droplet spreading on copper surface with a ZnO nanostructured layer

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Fig. 9

Spreading of a 2 μl water droplet on copper surface with a ZnO nanostructured layer: (a) initial spreading at t = 0.014 s and (b) hemi-spreading at t = 4 s

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Fig. 10

Experimentally determined variation of liquid front radius with time as a 2 μl droplet spreads on a nanostructured ZnO surface

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Fig. 11

Wicked flow from a capillary tube on a nanoporous layer

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