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Evaluation of Heat Transfer Fluids for Solar Thermal Applications
Youngsuk Nam(남영석),Andrej Lenert,Evelyn Wang 대한기계학회 2013 대한기계학회 춘추학술대회 Vol.2013 No.12
The choice of heat transfer fluids has significant effects on the performance, cost and reliability of solar thermal collector. We evaluate existing heat transfer fluids such as oils and molten salts based on new figure of merit capturing the combined effects of thermal storage capacity, convective heat transfer characteristics and hydraulic performance of the fluids. Other issues including thermal stability, freezing point and safety are also discussed. This work will help evaluate and engineer heat transfer fluids for solar thermal applications such as solar thermal power plants.
Youngsuk Nam(남영석),Hyunsik Kim(김현식),Seungwon Shin(신승원) 대한기계학회 2013 대한기계학회 춘추학술대회 Vol.2013 No.12
We report our energy and hydrodynamic analyses of coalescence-induced jumping on superhydrophobic surfaces. A full three-dimensional numerical model is developed using the level contour reconstruction method to investigate complex dynamics of contact lines and interfacial areas during the droplet coalescence on superhydrophobic surface (d ? 30μm and contact angle ? 160˚). The experimental characterization of coalescence-induced jumping is conducted with silanated CuO nanostructured surfaces to support the numerical study. The energy analysis shows that approximately half (40%-60%) of the excess amount of surface energy released during the coalescence is converted to kinetic energy on the superhydrophobic surface before the droplet detachment starts. The hydrodynamic analysis shows that the rapid increase in the kinetic energy at the beginning of the coalescence is initiated from low pressure associated with the high negative curvature of the liquid bridge. It also confirms that the asymmetric nature of the droplet evolution with the superhydrophobic wall generates high pressure at the bottom contact area that provides enough driving force to make the merged droplet jump from the wall. This work clarifies the dynamic mechanisms of selfpropelled jumping droplets and provides the necessary framework to develop the superhydrophobic condenser surfaces for various applications.
구리 기반 발수 나노표면의 상변화 응축 열전달 성능 향상
김현식(Hyunsik Kim),남영석(Youngsuk Nam) 대한기계학회 2014 대한기계학회 춘추학술대회 Vol.2014 No.11
We investigated the condensation mobility and resulting heat transfer performance on Cu-based water repellent surfaces including hydrophobic, oil-infused superhydrophobic and superhydrophobic surfaces. We observed the transient microscale condensation behaviors using a high-speed microscopy combined with a temperature and humidity controlled stage up to 5 hours with controlling the supersaturation level at 1.64. Unlike previous studies based on the nucleation density correlations, we experimentally characterized the nucleation density, droplet size distribution and growth rate, and then incorporated them into the developed condensation heat transfer model to compare the condensation heat transfer performance of each surface. Due to the spontaneous coalescence induced jumping, superhydrophobic surface can maintain a high heat transfer performance while other surfaces show a continuous decrease in heat transfer due to the increase in the conduction resistance across the droplet. We investigated the thermal resistance terms through the droplets to find out which factors influence on heat transfer performance. Then the conduction resistances through the droplet inside and nanostructures are significant for condensation heat transfer on superhydrophobic surfaces, while only conduction is significant on hydrophobic and oil surfaces.
다공성 금속 폼을 포함한 나노-구조물 증발 표면의 열 및 물질 전달 특성에 관한 연구
류승걸(Seunggeol Ryu),남영석(Youngsuk Nam) 대한기계학회 2015 대한기계학회 춘추학술대회 Vol.2015 No.11
Development of high-power density devices, such as microprocessor, power amplifiers, high electron mobility transistor and lasers, has increased the demand for highly-efficient thermal management solutions. In this work, we propose hybrid evaporator wicks composed of nanostructured micro-post combined with porous metal (Cu) liquid supply layers. The nanostructured micro-post was fabricated using the standard photo-lithography and electrochemical deposition followed by controlled chemical oxidation methods. Then porous metal layers were attached to the wicks to facilitate the liquid supply to the wicks by reducing the liquid phase pressure drop. Experiments were conducted within the environmental chamber with controlled pressure and saturation temperature. The heat transfer coefficient and critical heat flux were characterized by varying the geometric parameters of micro-post and porous metal structure to develop the strategy to achieve high heat transfer coefficient as well as high critical heat flux. The results show that the properly-designed porous metal liquid supply structures can significantly enhance the critical heat flux with maintaining high heat transfer coefficient. The suggested wick structures and experimental approaches will help develop compact heat spreaders for high heat flux applications.