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Seo, Donghyun,Oh, Seungtae,Shin, Seungwon,Nam, Youngsuk Pergamon Press 2017 International journal of heat and mass transfer Vol. No.
<P><B>Abstract</B></P> <P>We present the dynamic heat transfer analysis of condensed droplets growing and coalescing on hydrophobic (HPo) and superhydrophobic (SHPo) surfaces using a full 3D numerical simulation. In the model, two water droplets surrounded by fully-saturated water vapor grow on a horizontal surface through condensation until they coalesce together. The dynamic changes in the interfacial areas, temperature distributions and heat flux through each interface were analyzed. The effects of vapor phase temperature distribution, parasitic thermal resistance and surface flooding on the heat transfer rate are also quantified. The results show that a relatively high heat transfer rate through solid–vapor interface on SHPo partially compensates the low heat transfer rate through solid–liquid interface. The parasitic thermal resistance of the suggested SHPo may reduce the heat transfer performance over 30%. When the flooding occurs on HPo, the heat transfer rate rapidly decreases below a half of the value obtained at the beginning of coalescence. This work shows the importance of the heat transfer analysis considering dynamic changes in the interfacial area and resulting 3D temperature distributions, and will help develop the optimal condensation heat transfer surfaces.</P> <P><B>Highlights</B></P> <P> <UL> <LI> 3D dynamic thermal analysis of growing and merging condensates was performed. </LI> <LI> Dynamic changes in interfacial areas, temperature distribution were analyzed. </LI> <LI> The importance of vapor phase temperature on heat transfer rate was reported. </LI> <LI> The effects of parasitic thermal resistances and flooding on heat transfer were quantified. </LI> <LI> The framework for accurately predicting the condensation heat transfer was suggested. </LI> </UL> </P>
Seo, Donghyun,Lee, Choongyeop,Nam, Youngsuk American Chemical Society 2014 Langmuir Vol.30 No.51
<P>On superhydrophobic (SHPo) surfaces, either of two wetting statesthe Cassie state (i.e., nonwetted state) and the Wenzel state (i.e., wetted state)can be observed depending on the thermodynamic energy of each state and external conditions. Each wetting state leads to quite a distinctive dynamic characteristic of the water drop on SHPo surfaces, and it has been of primary interest to understand or induce the desirable wetting state for relevant thermofluid engineering applications. In this study, we investigate how the wetting state of microstructured SHPo surfaces influences the water-harvesting performance via dewing by testing two different patterns, including posts and grates with varying structural parameters. On grates, the observed Cassie wetting state during condensation is well described by the thermodynamic energy criteria, and small condensates can be efficiently detached from the surfaces because of the small contact line pinning force of Cassie droplets. Meanwhile, on posts, the observed wetting state is dominantly the Wenzel state regardless of the thermodynamic energy of each state, and the condensates are shed only after they grow to a sufficiently large size to overcome the much larger pinning force of the Wenzel state. On the basis of the mechanical force balance model and energy barrier consideration, we attribute the difference in the droplet shedding characteristics to the different dynamic pathway from the Wenzel state to the Cassie state between posts and grates. Overall, the faster droplet shedding helps to enhance the water-harvesting performance of the SHPo surfaces by facilitating condensation on the droplet-free area, as evidenced by the best water-harvesting performance of grates on the Cassie state among the tested surfaces.</P><P><B>Graphic Abstract</B> <IMG SRC='http://pubs.acs.org/appl/literatum/publisher/achs/journals/content/langd5/2014/langd5.2014.30.issue-51/la5041486/production/images/medium/la-2014-041486_0013.gif'></P>
Seo, Donghyun,Oh, Seungtae,Moon, Byungyun,Kim, Hyunsik,Kim, Juhyok,Lee, Choongyeop,Nam, Youngsuk Elsevier 2019 INTERNATIONAL JOURNAL OF HEAT AND MASS TRANSFER - Vol.128 No.-
<P><B>Abstract</B></P> <P>Condensation frosting causes serious economic and safety problems in many industrial applications. Recently, lubricant-impregnated surfaces (LIS) have been attracting much interest with their excellent anti-frosting ability. The facilitated removal of drops due to the low contact angle hysteresis of LIS has been suggested as the frosting suppression mechanism. Here, we demonstrate a hitherto-unexplored microscale frosting suppression mechanism on LIS by investigating microscopic condensation and freezing dynamics on LIS by varying the viscosities of the lubricants. Based on the ice propagation model, we show that the frosting propagation is suppressed on LIS with a low viscosity oil where the coalescence of droplets is promoted by the presence of oil. On the contrary, the coalescence between droplets is interrupted on LIS with a high viscosity oil, which facilitates the frost propagation. The criteria for the delay of condensation frosting were explained based on the competition between the lubricant drainage time and the drop growth time scale. Finally, we verify that microscopic frosting suppression mechanism of LIS persists up to macroscopic level by demonstrating that LIS is effective in suppressing condensation frosting on heat exchangers.</P> <P><B>Highlights</B></P> <P> <UL> <LI> Frosting delay on lubricant impregnated surfaces was investigated. </LI> <LI> Overlapped oil meniscus induced an active drop coalescence and frosting delay. </LI> <LI> Frost was quickly propagated on high viscosity lubricant due to drop packing phenomena. </LI> <LI> The packing phenomena was analyzed by considering oil drainage time and drop growth time. </LI> <LI> Suggested frosting delay mechanism was applied to macroscopic heat exchangers. </LI> </UL> </P>
Design of a novel compact antenna for a bluetooth LTCC module
Seo, Donghyun,Jeon, Seunggil,Kang, Namkee,Ryu, Join,Choi, Jae-Hoon Wiley Subscription Services, Inc., A Wiley Company 2008 MICROWAVE AND OPTICAL TECHNOLOGY LETTERS Vol.50 No.1
<P>A novel design concept for implementing an antenna on a compact LTCC module is proposed for a Bluetooth application. The integrated antenna consists of an LTCC module and a two-layered radiating element, which occupies a volume of 5 mm × 6.3 mm × 0.8 mm. The antenna has a 7.5 dB return loss bandwidth of 100 MHz, from 2.39 to 2.49 GHz and has nearly omni-directional radiation patterns with good antenna gain in spite of its small size. © 2007 Wiley Periodicals, Inc. Microwave Opt Technol Lett 50: 180–183, 2008; Published online in Wiley InterScience (www.interscience.wiley.com). DOI 10.1002/mop.23051</P>