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류승걸(Seunggeol Ryu),남영석(Youngsuk Nam),이충엽(Choongyeop Lee) 대한기계학회 2016 대한기계학회 춘추학술대회 Vol.2016 No.12
When a water drop impacts a mesh having sub-millimeter pores, the part of the water penetrates through a mesh if the impact velocity is sufficiently large. Here we show that different surface wettability, i.e., hydrophobicity and superhydrophobicity, leads to different water penetration dynamics on a mesh during drop impact. We show, despite the water repellency of a superhydrophobic surface, that water can penetrate a superhydrophobic mesh more easily (i.e., at a lower impact velocity) over a hydrophobic mesh via a penetration mechanism unique to a superhydrophobic mesh. On a superhydrophobic mesh, the water penetration can occur during the drop recoil stage, which appears at a lower impact velocity than the critical impact velocity for water penetration right upon impact. We propose that this unique water penetration on a superhydrophobic mesh can be attributed to the combination of the hydrodynamic focusing and the momentum transfer from the water drop when it is about to bounce off the surface, at which point the water drop retrieves most of its kinetic energy due to the negligible friction on superhydrophobic surfaces.
Enhanced heat transfer using metal foam liquid supply layers for micro heat spreaders
Ryu, Seunggeol,Han, Jeonghoon,Kim, Jichul,Lee, Choongyeop,Nam, Youngsuk Pergamon Press 2017 International journal of heat and mass transfer Vol. No.
<P><B>Abstract</B></P> <P>We propose a nanostructured metal foam liquid supply layer that can efficiently provide operating fluid to evaporator hot spots and can be easily integrated within micro heat spreaders. The liquid supply layer is incorporated onto the micropost evaporator wicks to enhance the capillary performance by combining the high permeability of liquid supply layers and the high capillary pressure of micropost wicks. The coverage ratio (<I>CR</I>) between the liquid supply layer and the evaporator wicks was varied from 15% to 100% to find the proper <I>CR</I> for efficiently increasing the liquid supply performance with minimizing the parasitic thermal resistance. By incorporating the liquid supply layer of <I>CR</I> 33% onto the Cu micropost wicks of ∼0.4 solid fraction, the results show that a high (>6W/cm<SUP>2</SUP> K) and stable heat transfer coefficient can be achieve at a high heat flux range (>400W/cm<SUP>2</SUP>), which outweighs the performance of previously-reported evaporator wicks. The achieved maximum heat flux was over 150% higher than the same wicks without the liquid supply layer. Our work shows the importance of the efficient liquid supply to hot spots and provides the strategy to increase the heat transfer performance at high heat flux region. The suggested liquid supply layer will help develop micro heat spreaders for the thermal management of high power density microprocessors, IGBTs and thermophotovoltaic cells.</P> <P><B>Highlights</B></P> <P> <UL> <LI> Nanostructured metal-foam liquid supply layer is suggested for micro heat spreaders. </LI> <LI> The benefits of the suggested liquid supply layer are experimentally clarified. </LI> <LI> Proper coverage ratio of the liquid supply layer is determined. </LI> <LI> Over 400W/cm<SUP>2</SUP> maximum heat flux was achieved using the liquid supply layer. </LI> <LI> High (>6W/cm<SUP>2</SUP> K) heat transfer coefficient was achieved with high heat flux. </LI> </UL> </P>
Anisotropic drop spreading on superhydrophobic grates during drop impact
Han, Jeonghoon,Ryu, Seunggeol,Kim, Hyunsik,Sen, Prosenjit,Choi, Dukhyun,Nam, Youngsuk,Lee, Choongyeop The Royal Society of Chemistry 2018 Soft matter Vol.14 No.19
<P>We study the influence of geometric anisotropy of micro-grate structures on the spreading dynamics of water drops after impact. It is found that the maximal spreading diameter along the parallel direction to grates becomes larger than that along the transverse direction beyond a certain Weber number, while the extent of such an asymmetric spreading increases with the structural pitch of grates and Weber number. By employing grates covered with nanostructures, we exclude the possible influences coming from the Cassie-to-Wenzel transition and the circumferential contact angle variation on the spreading diameter. Then, based on a simplified energy balance model incorporating slip length, we propose that slip length selectively enhances the spreading diameter along the parallel direction, being responsible for the asymmetric drop spreading. We believe that our work will help better understand the role of microstructures in controlling the drop dynamics during impact, which has relevance to various engineering applications.</P>
Fermi Level Pinning at Electrical Metal Contacts of Monolayer Molybdenum Dichalcogenides
Kim, Changsik,Moon, Inyong,Lee, Daeyeong,Choi, Min Sup,Ahmed, Faisal,Nam, Seunggeol,Cho, Yeonchoo,Shin, Hyeon-Jin,Park, Seongjun,Yoo, Won Jong American Chemical Society 2017 ACS NANO Vol.11 No.2
<P>Electrical metal contacts to two-dimensional (2D) semiconducting transition metal dichalcogenides (TMDCs) are found to be the key bottleneck to the realization of high device performance due to strong Fermi level pinning and high contact resistances (R-c). Until now, Fermi level pinning of monolayer TMDCs has been reported only theoretically, although that of bulk TMDCs has been reported experimentally. Here, we report the experimental study on Fermi level pinning of monolayer MoS2 and MoTe2 by interpreting the thermionic emission results. We also quantitatively compared our results with the theoretical simulation results of the monolayer structure as well as the experimental results of the bulk structure. We measured the pinning factor S to be 0.11 and -0.07 for monolayer MoS2 and MoTe2, respectively, suggesting a much stronger Fermi level pinning effect, a Schottky barrier height (SBH) lower than that by theoretical prediction, and interestingly similar pinning energy levels between monolayer and bulk MoS2. Our results further imply that metal work functions have very little influence on contact properties of 2D-material-based devices. Moreover, we found that Re is exponentially proportional to SBH, and thee processing parameters can be controlled sensitively upon chemical doping into the 2D materials. These findings provide a practical guideline for depinning Fermi level at the 2D interfaces so that polarity control of TMDC-based semiconductors can be achieved efficiently.</P>
EV / HEV 에서 사용되는 고전력 IGBT 모듈을 위한 소형 액체 냉각 시스템
기석간(Seokkan Ki),이주영(Jooyoung Lee),류승걸(Seunggeol Ryu),남영석(Youngsuk Nam) 대한기계학회 2017 대한기계학회 춘추학술대회 Vol.2017 No.11
In this work, we suggest a compact liquid cooling system based on a copper foam heat sink for thermal management of high power IGBT modules. Coolant path was designed for decreasing the pressure drop and effectively transferring the heat from heat sources. Cooling performance of the system was numerically evaluated with a commercial CFD software. We assumed a high power IGBT module for the condition of heat dissipation. We show that our liquid cooling system has a lower junction to coolant thermal resistance than previous cooling systems. Especially, low mass flow rate and pressure drop of the cooling loop was measured due to minimized coolant path and high porosity of the Cu foam. Numerical data was experimentally demonstrated with a given mass flow rate range. This study will be applied to high powered electronic applications where thermal management of multiple heat sources was needed.