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.
Dropwise Condensation on/in High Roughness Structures
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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.
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Cai, S. Q., and Bhunia, A. (January 18, 2017). "Dropwise Condensation on/in High Roughness Structures." ASME. J. Heat Transfer. April 2017; 139(4): 041501. https://doi.org/10.1115/1.4035354
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