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전단흐름 하에 이온교환막 위에서 발생하는 전기수력학적 와류
곽노균(Rhokyun Kwak) 한국가시화정보학회 2018 한국가시화정보학회지 Vol.16 No.1
Ion exchange membrane can transfer only cation or anion in electrically conductive fluids. Recent studies have revealed that such selective ion transport can initiate electroconvective instability, resulting vortical fluid motions on the membrane. This so-called electroconvective vortex (a.k.a. electroconvection (EC)) has been in the spotlight for enhancing an ion flux in electrochemical systems. However, EC under shear flow has not been investigated yet, although most related systems operate under pressure-driven flows. In this study, we present the direct visualization platform of EC under shear flow. On the transparent silicone rubber, microscale channels were fabricated between ion exchange membranes, while allowing microscopic visualization of fluid flow and ion concentration changes on the membranes. By using this platform, not only we visualize the existence of EC under shear flow, its unique characteristics are also identified: i) unidirectional vortex pattern, ii) its advection along the shear flow, and iii) shear-sheltering of EC vortices.
UV 나노임프린팅을 이용한 단일(單一) 나노 브리지 구조의 제작
곽노균(Rhokyun Kwak),정훈의(Hoon Eui Jeong),서갑양(kahp Y. Suh) 대한기계학회 2008 대한기계학회 춘추학술대회 Vol.2008 No.5
In this paper, a simple vacuum-assisted UV nanoimprint lithography (NIL) method is presented for fabricating well-defined monolithic nanobridges. A droplet of UV-curable resin of polyurethane acrylate (PUA) is dispensed and imprinted to form microscale structures with PDMS mold and subsequent curing for several seconds. Under proper conditions, the top layer is partially cured by the trapped air and provides further nanofabrication upon application of a nanoscale PUA mold at vacuum. It is noted that a pressure gradient is generated between a contacting and a non-contacting parts with the mold, resulting in a flow of the partial cured pre-polymer into the void regions. Finally, monolithic nanobridges is fabricated with high aspect ratio in large area. These nanobridges are potentially useful for smart optical and electronic devices.
이온농도분극에 의한 와류현상에서 관찰된 유동전단응력-이동장벽 효과
곽노균(Rhokyun Kwak),Van Sang Pham,강지윤,한종윤(Jongyoon Han) 대한기계학회 2015 대한기계학회 춘추학술대회 Vol.2015 No.11
Rotating shear flow is a common tool to suppress turbulence and control transport in plasma physics. Despite the desires to apply this phenomenon to other fields, it has rarely been observed in non-ionized fluids. Here, we visualize this effect on a generic neutral-fluid situation in electro-membrane systems: electroconvective vortex (EC) under Hagen-Poiseuille (HP) flow, which initiated by ion concentration polarization (ICP) on ion exchange membranes. This is the first demonstration of “flow-shear-induced transport barrier” in electrochemical systems, which was found only in magnetized flow previously. Combining scaling analysis and experiment, we capture the effect by pinpointing the threshold for shear suppression. Selected by balancing velocity fluctuation (with EC vortices) and flow shear (with noslip walls), the threshold is scaled as the dimensionless EC height dec/w=0.618. Stable EC occurs under the threshold, while chaotic EC occurs over the threshold by velocity fluctuation overwhelming flow shear. The threshold we identify here has wide applications to prevent chaotic flows in various electro-membrane systems.
Three Mathematical Programming techniques for Solving Transshipment Problems : a Wilcoxon Test
Kwak, N. K.,Hemaida, Ramadan S. 한국경영과학회 1994 韓國經營科學會誌 Vol.19 No.3
This paper presents three mathematical programming approaches to solving transshipment problems with interval supply and demand requirements. A linear goal programming model was developed based on the data obtained from a nationwide retail firm. Three mathematical programming model results were compared and analyzed, and three separate hypotheses were examined by using the Wilcoxon signed-ranked test for the model applicability. The test results were analyzed and interpreted for decision making.
온도 프로파일 가시화를 통한 프랙탈 구조 마이크로채널 히트싱크의 열수력학적 특성 최적화
이한솔,곽노균 한국가시화정보학회 2024 한국가시화정보학회지 Vol.22 No.1
As microchips’ degree of integration is getting higher, its cooling problem becomes important more than ever. One of the promising methods is using fractal microchannel heat sink by mimicking nature’s Murray networks. However, most of the related works have been progressed only by numerical analysis. Perhaps such lack of direct experimental studies is due to the technical difficulty of the temperature and heat flux measurement in complex geometric channels. Here, we demonstrate the direct visualization of in situ temperature profile in a fractal microchannel heat sink. By using the temperature-sensitive fluorescent dye and a transparent Polydimethylsiloxane window, we can map temperature profiles in silicon-based fractal heat sinks with various fractal scale factors (a=1.5–3.5). Then, heat transfer rates and pressure drops under a fixed flow rate were estimated to optimize hydrodynamic and thermal characteristics. Through this experiment, we found out that the optimal factor is a=1.75, given that the differences in heat transfer among the devices are marginal when compared to the variances in pumping power. This work is expected to contribute to the development of high-performance, high-efficiency thermal management systems required in various industrial fields.