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Research Papers: Evaporation, Boiling, and Condensation

Dropwise Condensation on Superhydrophobic Microporous Wick Structures

[+] Author and Article Information
Sean H. Hoenig

Advanced Cooling Technologies, Inc.,
1046 New Holland Avenue,
Lancaster, PA 17601
e-mail: Sean.Hoenig@1-ACT.com

Richard W. Bonner, III

Advanced Cooling Technologies, Inc.,
1046 New Holland Avenue,
Lancaster, PA 17601
e-mail: Richard.Bonner@1-ACT.com

Contributed by the Heat Transfer Division of ASME for publication in the JOURNAL OF HEAT TRANSFER. Manuscript received April 28, 2017; final manuscript received December 6, 2017; published online April 6, 2018. Assoc. Editor: Gennady Ziskind.

J. Heat Transfer 140(7), 071501 (Apr 06, 2018) (7 pages) Paper No: HT-17-1238; doi: 10.1115/1.4038854 History: Received April 28, 2017; Revised December 06, 2017

Previous research in dropwise condensation (DWC) on rough microtextured superhydrophobic surfaces has demonstrated evidence of high heat transfer enhancement compared to smooth hydrophobic surfaces. In this study, we experimentally investigate the use of microporous sintered copper powder on copper substrates coated with a thiol-based self-assembled monolayer to attain enhanced DWC for steam in a custom condensation chamber. Although microtextured superhydrophobic surfaces have shown advantageous droplet growth dynamics, precise heat transfer measurements are underdeveloped at high heat flux. Sintered copper powder diameters from 4 μm to 119 μm were used to investigate particle size effects on heat transfer. As powder diameter decreased, competing physical factors led to improved thermal performance. At consistent operating conditions, we experimentally demonstrated a 23% improvement in the local condensation heat transfer coefficient for a superhydrophobic 4 μm diameter microporous copper powder surface compared to a smooth hydrophobic copper surface. For the smallest powders observed, this improvement is primarily attributed to the reduction in contact angle hysteresis as evidenced by the decrease in departing droplet size. Interestingly, the contact angle hysteresis of sessile water droplets measured in air is in contradiction with the departing droplet size observations made during condensation of saturated steam. It is evident that the specific design of textured superhydrophobic surfaces has profound implications for enhanced condensation in high heat flux applications.

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Figures

Grahic Jump Location
Fig. 6

The local vapor-to-surface temperature difference plotted against the local condensation heat flux is used to analyze the heat transfer efficiency of the superhydrophobic microporous wick structures. Reference data are from Stylianou and Rose [27] for a comparable smooth hydrophobic copper surface.

Grahic Jump Location
Fig. 7

The heat flux averaged local heat transfer coefficient plotted against copper powder diameter to demonstrate the trend in thermal performance from smallest to largest powder size. The smallest superhydrophobic copper powder (4 μm) surface outperforms a smooth hydrophobic surface by 23%.

Grahic Jump Location
Fig. 8

Droplet growth and coalescence for a smooth, 4 μm, 21 μm, 43 μm, 61 μm, and 119 μm copper powder surface. Highly pinned amorphous droplets with high hysteresis contribute to decreased heat transfer efficiency for larger powder sizes. Small departing droplets with low hysteresis contribute to increased heat transfer efficiency for the smallest powder size.

Grahic Jump Location
Fig. 9

Maximum departing droplet radius plotted against copper powder diameter. Note that the smallest superhydrophobic copper powder (4 μm) surface has consistently smaller departing droplet radii compared to that on a smooth hydrophobic copper surface.

Grahic Jump Location
Fig. 4

Self-assembled monolayer coating deposited on a sintered microporous copper powder monolayer and copper substrate

Grahic Jump Location
Fig. 3

A smooth copper surface (a) is compared with a microporous copper powder wick sintered to a copper substrate (b). Image (c) is SEM for a 61 μm average diameter powder sample.

Grahic Jump Location
Fig. 2

Exploded view of the condensation chamber and its associated components in SolidWorks demonstrate its functionality

Grahic Jump Location
Fig. 1

The test setup demonstrated in SolidWorks in an isometric orientation. The actual orientation is vertical with the evaporator below the condensation chamber. The true orientation is represented in the inset image.

Grahic Jump Location
Fig. 5

Water droplets approximately 10.12 μL deposited on superhydrophobic microporous copper powder. The average powder diameter used for each sample is (a) none, (b) 4 μm, (c) 21 μm, (d) 43 μm, (e) 61 μm, and (f) 119 μm.

Grahic Jump Location
Fig. 10

The heat flux averaged local heat transfer coefficient plotted against maximum departing droplet radius for each surface. This evidence provides further validation to why the smallest superhydrophobic copper powder (4 μm) surface demonstrates better thermal performance than a smooth hydrophobic copper surface. The small departing droplet radii with low hysteresis are efficiently removed from the surface. A power–law relationship exists for the data with a value of −0.574. This is consistent with the findings from the Bonner correlation [34], which indicated a power–law relationship value between −1/2 and −2/3 for the condensation of steam. The trendline equation is: hl = 253rd−0.57.

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