Observation of water condensate on hydrophobic micro textured surfaces
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ORIGINAL
Observation of water condensate on hydrophobic micro textured surfaces Ki Wook Kim • Sang Cheol Do • Jong Soo Ko Ji Hwan Jeong
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Received: 26 June 2012 / Accepted: 11 March 2013 / Published online: 22 March 2013 Ó Springer-Verlag Berlin Heidelberg 2013
Abstract We visually observed that a dropwise condensation occurred initially and later changed into a filmwise condensation on hydrophobic textured surface at atmosphere pressure condition. It was observed that the condensate nucleated on the pillar side walls of the micro structure and the bottom wall adhered to the walls and would not be lifted to form a spherical water droplet using environmental scanning electron microscope.
1 Introduction Many energy systems, such as power plants and heat pumps, use condensers to reject exhaust heat to the heat sinks. Vapor-phase working fluid condenses on the heat transfer surface of the condensers and changes to a liquid phase. The condensation can be classified by three modes depending on the condensate flow behavior: filmwise condensation (FWC), dropwise condensation (DWC), and conventional condensation. In the DWC condensation mode, the condensate forms liquid droplets on the cooling surfaces and the liquid droplets roll off the inclined surface easily. The condensation heat transfer performance of the DWC is superior to other modes since the condensate does not wet the surface so that thermal resistance is minimized. That is why the DWC has been of interest to heat transfer researchers since Schmidt et al. [1] first recognized it. The progress in dropwise condensation research from 1930 to the end of twentieth century was reviewed and well addressed by Rose [2]. K. W. Kim S. C. Do J. S. Ko J. H. Jeong (&) School of Mechanical Engineering, Pusan National University, Busan 609-735, Korea e-mail: [email protected]
The condensation mode may change depending on working fluid, surface structure, and the surface energy of the heat transfer surface. The surface energy can be estimated by measuring the contact angle of a condensate droplet. On a metallic surface, filmwise condensation usually occurs due to the large surface energy of the metallic surface. Previous researchers were keen to find robust ways to reduce the surface energy. A conventional approach to realize DWC on a solid subcooled surface was to make use of polymer coating. Vemuri et al. [3] used selfassembled monolayer (SAM) coatings of n-octadecyl mercaptan and Ma et al. [4] coated the polytetrafluoroethylene (PTFE) film on the metal substrates to bring about dropwise condensation. This method, however, was not very successful because the hydrophobic characteristic of the polymer coat did not last long enough but degraded. In the meantime, carbon based hard amorphous coating techniques were also investigated. Koch et al. [5] coated hydrogenated amorphous carbon film onto metallic surfaces and successfully promoted DWC of stream at atmospheric pressure. They also investigated the performance of diamond-like carbon (DLC) coating on metallic surface
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