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Research Papers: Evaporation, Boiling, and Condensation

Dropwise Condensation on/in High Roughness Structures

[+] Author and Article Information
Steve Q. Cai

Teledyne Scientific Company,
1049 Camino Dos Rios,
Thousand Oaks, CA 91360
e-mail: qingjun.cai@teledyne.com

Avijit Bhunia

Teledyne Scientific Company,
1049 Camino Dos Rios,
Thousand Oaks, CA 91360

Contributed by the Heat Transfer Division of ASME for publication in the JOURNAL OF HEAT TRANSFER. Manuscript received December 22, 2015; final manuscript received November 17, 2016; published online January 18, 2017. Assoc. Editor: Jim A. Liburdy.

J. Heat Transfer 139(4), 041501 (Jan 18, 2017) (6 pages) Paper No: HT-15-1808; doi: 10.1115/1.4035354 History: Received December 22, 2015; Revised November 17, 2016

Water droplets on bio-mimicked hierarchical roughness exhibit superhydrophobic properties, such as large contact angles, minor dynamic hysteresis, and high mobility. Vapor condensation on such superhydrophobic surface enables rapid condensate removal and surface cleaning, thereby significantly enhancing the heat transfer coefficient. In this paper, research attention is given to dropwise condensation on/in specially designed one-tier and hierarchical roughness structures. Utilizing a normal optical tomographic system composed of a Sensi-Cam and a Nikon microscope, close-up visualization is conducted to characterize small condensate droplets, in size of a few micrometers, between structural units of roughness. Experimental snapshots show that, within the one-tier roughness, condensate droplets tend to stick to surrounding structures. Low mobility of these droplets extends their residence time, and therefore increases their average diameter. In comparison, surface energy of the hierarchical structure is significantly reduced. As a result, small condensate droplets behave nonsticky to their surroundings, which enable rapid drain of the droplets and accomplish self-cleaning of the structure. Because of high mobility, the droplet average diameter in the two-tier structure is smaller than those in the one-tire roughness. Condensation sites reach the maximum in the middle of the structure where dew point of moisture is reached. Less condensation droplets on both the top and bottom of the roughness are blamed to the unsaturated moisture and the reduced humidity, respectively.

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Figures

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Fig. 3

Roughness structures for dropwise condensation studies: (a) one-tier conical roughness structure, (b) zoom-in of tapered silicon pillars, (c) hierarchical roughness structure, and (d) fabric CNTs synthesized on tip of the silicon pillars

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Fig. 2

High-aspect ratio roughness and hierarchical structure decrease the critical contact angle to stabilize the Cassie state

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Fig. 1

Hydrophobicity states of droplets on surface roughness

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Fig. 4

Visualization of dropwise condensation on/in the roughness structures: (a) visualization system, (b) a roughness sample placed between the copper cold plate and the microscope, and (c) a schematic of the normal optical tomography

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Fig. 5

The Cassie state droplets over the roughness structures: (a) liquid bridges and spherical droplets co-exist in/on the one-tier roughness and (b) spherical droplets on the two-tier structure

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Fig. 6

Dropwise condensation views in the one-tier roughness structure: (a) at the tip of the structure, (b) in 1/3 depth, (c) in 2/3 depth, and (d) on the bottom of structure

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Fig. 8

Droplet density versus the structural depth of both the one-tier and hierarchical roughness

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Fig. 9

Average droplet diameter versus the structural depth of both the one-tier and hierarchical roughness

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Fig. 7

Droplet condensation in the hierarchical roughness structure: (a) at the tips of silicon pillars, (b) at 1/3 depth, (c) at 2/3 depth, and (d) at the bottom

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Fig. 10

Schematic diagram of dropwise condensation and precipitation on the one-tier and hierarchical roughness with conical tips: (a) in a one-tier structure, a distorted droplet between the silicon pillars creating capillary force difference for draining and (b) condensate droplets can be drained from the bottom of the hierarchical roughness

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