• Korea-Australia Rheology Journal
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• 제21권1호
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• pp.59-69
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• 2009

The prediction of pressure drop for a droplet flow in a confined micro channel is presented using FE-FTM (Finite Element - Front Tracking Method). A single droplet is passing through 5:1:5 contraction - straight narrow channel - expansion flow domain. The pressure drop is investigated especially when the droplet flows in the straight narrow channel. We explore the effects of droplet size, capillary number (Ca), viscosity ratio ($\chi$) between droplet and medium, and fluid elasticity represented by the Oldroyd-B constitutive model on the excess pressure drop (${\Delta}p^+$) against single phase flow. The tightly fitted droplets in the narrow channel are mainly considered in the range of $0.001{\leq}Ca{\leq}1$ and $0.01{\leq}{\chi}{\leq}100$. In Newtonian droplet/Newtonian medium, two characteristic features are observed. First, an approximate relation ${\Delta}p^+{\sim}{\chi}$ observed for ${\chi}{\geq}1$. The excess pressure drop necessary for droplet flow is roughly proportional to $\chi$. Second, ${\Delta}p^+$ seems inversely proportional to Ca, which is represented as ${\Delta}p^+{\sim}Ca^m$ with negative m irrespective of $\chi$. In addition, we observe that the film thickness (${\delta}_f$) between droplet interface and channel wall decreases with decreasing Ca, showing ${\delta}_f{\sim}Ca^n$ Can with positive n independent of $\chi$. Consequently, the excess pressure drop (${\Delta}p^+$) is strongly dependent on the film thickness (${\delta}_f$). The droplets larger than the channel width show enhancement of ${\Delta}p^+$, whereas the smaller droplets show no significant change in ${\Delta}p^+$. Also, the droplet deformation in the narrow channel is affected by the flow history of the contraction flow at the entrance region, but rather surprisingly ${\Delta}p^+$ is not affected by this flow history. Instead, ${\Delta}p^+$ is more dependent on ${\delta}_f$ irrespective of the droplet shape. As for the effect of fluid elasticity, an increase in ${\delta}_f$ induced by the normal stress difference in viscoelastic medium results in a drastic reduction of ${\Delta}p^+$.

• 대한기계학회논문집A
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• 제31권10호
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• pp.981-985
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• 2007

We present a MEMS-based portable Direct Methanol Fuel Cell (micro-DFMC), featured by a platinum sputtered microcolumn electrode and a built-in fuel chamber containing a limited amount of methanol fuel. Also presented is a micro-DMFC stack structure having a common electrolyte sandwiched by the microcolumn electrodes. The single cells with ME16 and PE16 electrodes show the maximum power densities of $31.04{\pm}0.29{\mu}W/cm^2$ and $9.75{\pm}0.29{\mu}W/cm^2$, respectively; thus indicating the microcolumn electrode (ME16) generates the power density (3.2 times) higher than the planar electrode (PE16). The single cell tests of ME16 and ME4 electrodes (Fig.8) show the maximum power of $31.04{\pm}0.29{\mu}W/cm^2$, and $25.23{\pm}2.7{\mu}W/cm^2$, respectively; thus demonstrating the increased window frame reduces the normalized standard power deviation (standard deviation over the average power). The normalized deviation of 0.11 in ME4 cell has been reduced to 0.01 in ME16 cell due to the increased window frames. The maximum power density of 4-cell stack is 15.7 times higher than that of the single cell. 4-cell stack produces the power capacity of 20.3mWh/g during 980min operation at the voltage of 450mV with the load resistance of $800{\Omega}$.

• 전기전자학회논문지
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• 제19권3호
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• pp.304-310
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• 2015

SCALDO(Supercapacitor Assisted LDO) 레귤레이터는 기존에 널리 사용되고 있는 SMPS(Switch Mode Power Supply)의 장점인 높은 효율과 LDO(Low Drop-out) 레귤레이터의 장점인 안정적인 출력 및 우수한 EMI(Electro Magnetic Interference)특성을 함께 가지는 레귤레이터로 현재 새롭게 연구되고 있는 전원회로이다. 하지만, 현재까지 연구된 SCALDO 레귤레이터의 경우 회로 내부의 스위치제어에 많은 전력이 소비되어 회로 전체의 효율이 감소되는 단점이 있다. 본 논문에서는 기존 SCALDO 레귤레이터의 단점을 극복하고 저전력으로 구동이 가능한 MOSFET를 SCALDO 레귤레이터에 적용함으로써 스위치제어 소비전력을 최소화하여 회로 전체의 효율을 향상시킨 새로운 SCALDO 레귤레이터를 구현 하였으며, 기존 SCALDO 대비 효율이 최대 9.5% 상승됨을 확인하였다. 또한 기존의 MCU(Micro-controller unit)를 이용한 펌웨어제어를 비교기 및 T-F/F(Flip Flop)을 이용한 하드웨어 제어로 대체함으로써 회로의 제작과정을 단순화 하였다.

• 설비공학논문집
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• 제29권8호
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• pp.408-418
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• 2017

Supercritical carbon dioxide ($sCO_2$) power system is emerging technology because of its high cycle efficiency and compactness. Meanwhile, PCHE (Printed Circuit Heat Exchanger) is gaining attention in $sCO_2$ power system technology because PCHE with high pressure-resistance and larger heat transfer surface per unit volume is fundamentally needed. Thermo-fluidic characteristics of $sCO_2$ in the micro channel of PCHE should be investigated. In this study, heat transfer characteristics of $sCO_2$ of various inlet conditions and cross-sectional shapes of single micro channel were investigated experimentally. Experiment was conducted at supercritical state of higher than critical temperature and pressure. Test sections were made of copper and hydraulic diameter was 1 mm. Convective heat transfer coefficients were measured according to each interval of the channel and pressure drop was also measured. Convective heat transfer coefficients from experimental data were compared with existing correlation. In this study, using measured data, a new empirical correlation to predict near critical region heat transfer coefficient is developed and suggested. Test results of single channel will be used for design of PCHE.

• 대한치과보철학회지
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• 제46권3호
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• pp.307-319
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• 2008

STATEMENT OF PROBLEM: It is known that an anodic oxidation technique, one of the methods for the implant surface treatment, remarkably increased surface area, enhanced wettability and accelerated the initial bone healing. Purpose: This study was performed to evaluate the wettability of anodized titanium surface which has a nanotubular structure, to assess osseointegration after the placement of implant with nano-size tubes on tibia of rats and to analyze quantitatively transferable rhBMP-2 on each surface. MATERIAL AND METHOD: Four different kinds of surface-treated titanium discs (polished (machined surface) group, micro (blasting surface) group, nano (anodizedmachined surface) group, and nano-micro (anodized-blasting surface) group) were fabricated (n=10). Three different media were chosen to measure the surface contact angles; distilled water, plasma and rhBMP-2 solution. After a single drop (0.025 $m{\ell}$) of solution, the picture was taken with the image camera, and contact angle was measured by using image analysis system. For the test of osseointegration, 2 kinds of anodized surface (anodized-machined surface, anodized-blasting surface) implants having 2.0 mm in diameter and 5.0 mm in length inserted into the tibia of Wistar rats. After 3 weeks, tibia were harvested and the specimens were stained with hematoxylin and eosin for histological analysis. To test the possibility of drug delivery, after soaking sample groups in the concentration of 250 ng/$m{\ell}$l of rhBMP-2 for 48 hours, the excess solution of rhBMP-2 were removed. After that, they were lyophilized for 24 hours, and then the rhBMP-2 on the surface of titanium was resolved for 72 hours in PBS. All the extracted solution was analyzed by ELISA. One-way analysis of variance (ANOVA) was performed on the data. RESULTS: The wettability is improved by anodic oxidation. The best wettability was shown on the nano-micro group, and it was followed by nano group, micro group, and polished group. In the histological findings, all implants showed good healing and the new bone formation were observed along the implant surface. After 3 days, nano-micro group delivered the most amount of rhBMP-2, followed by nano group, micro group, and polished group. CONCLUSION: It indicated that anodic oxidation on blasting surface produce functionally graded nano-micro porous structure and enhance hydrophilicity of the surface and osseointegration. The findings suggest that the nano-micro porous structure could be a useful carrier of osteogenic molecules like rhBMP-2.

• 에너지공학
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• 제15권4호
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• pp.243-249
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• 2006

본 실험적 연구는 슬릿휜-관열교환기의 냉매측 압력강하에 대하여 수행하였다. 응축기의 설계조건에서 미세휜관내 냉매 R22와 R134a의 압력강하에 대한 실험데이터와 앞서 제안한 상관관계식과 상호 비교하였다. 실험은 냉매 R22와 Rl34a의 응축기 입구온도 $60^{\circ}C$, 질량유속 $150{\sim}250\;kg/m^{2}s$ 범위에서 수행하였다. 공기의 유입조건은 건구온도 $35^{\circ}C$, 상대습도 40%이며, 공기유속의 범위는 $0.68{\sim}1.43\;m/s$이다. 실험결과 응축기의 과냉도 $5^{\circ}C$ 조건에서 R134a의 압력강하는 R22보다 $22{\sim}22.6%$ 높게 나타났으며, 냉매의 질량유속 $200{\sim}250\;kg/m^{2}s$의 범위에서 실험으로부터 측정한 R22와 Rl34a의 압력강하는 예측결과와 ${\pm}20%$내에 일치하였다.

• 대한기계학회논문집A
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• 제30권4호
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• pp.373-378
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• 2006

We present the design, fabrication, and characterization of a multi-chip microelectrofluidic bench, achieving both electrical and fluidic interconnections with a simple, low-loss and low-temperature electrofluidic interconnection method. We design 4-chip microelectrofluidic bench, having three electrical pads and two fluidic I/O ports. Each device chip, having three electrical interconnections and a pair of two fluidic I/O interconnections, can be assembled to the microelectofluidic bench with electrical and fluidic interconnections. In the fluidic and electrical characterization, we measure the average pressure drop of $13.6{\sim}125.4$ Pa/mm with the nonlinearity of 3.1 % for the flow-rates of $10{\sim}100{\mu}l/min$ in the fluidic line. The pressure drop per fluidic interconnection is measured as 0.19kPa. Experimentally, there are no significant differences in pressure drops between straight channels and elbow channels. The measured average electrical resistance is $0.26{\Omega}/mm$ in the electrical line. The electrical resistance per each electrical interconnection is measured as $0.64{\Omega}$. Mechanically, the maximum pressure, where the microelectrofluidic bench endures, reaches up to $115{\pm}11kPa$.

• 대한기계학회논문집B
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• 제24권11호
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• pp.1470-1477
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• 2000

Due to the ozone depletion and global warming potentials, some refrigerants(CFx and HCFCs) have been rapidly substituted. R410A is considered as the alternative refrigerant of R22 for the air-conditioners used a home and in industry. Experiments on the condensation heat transfer characteristics inside a smooth or a micro-fin tube with R410A are performed in this study. The test tubes 7/9.52 mm in outer diameters and 3 m in length are used. Varying the mass flux of the refrigerant and the condensation temperatures, the average heat transfer coefficients and pressure drop are investigated. It is shown that the heat transfer is enhanced and the amount of pressure drops are larger in the microfin tube than the smooth tube. From the heat transfer enhancement coefficient and the pressure penalty factor, it is found that the high heat transfer enhancement coefficients are obtained in the range of small mass flux while the penalty factors are almost equal.

• 대한기계학회논문집B
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• 제28권12호
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• pp.1549-1556
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• 2004

We have deigned, fabricated and compared four different types of static chaos microfluid mixers, including the mixers using straight channel flow, microblock-induced alternating whirl flow, microchannel-induced lamination flow, and combined alternating whirl-lamination flow. Among them, the alternating whirl-lamination (AWL-type) mixer, composed of 3-D rotationally arranged microblocks and dividing microchannels fabricated by conventional planar lithography process, is effective to reduce the mixing length over wide flow rate ranges. We characterize the performance of the fabricated mixers, through the flow visualization technique using phenolphthalein solution. We verify that the AWL-type microfluid mixer shows the shortest fluid mixing length of 2.8mm∼5.8mm for the flow rate range of Re=0.26∼26 with the pressure drop lower than 5kPa. Compared to the previous mixers, requiring the mixing lengths of 7∼17mm, the AWL-type microfluid mixer results in the 60% reduction of the mixing lengths. Due to the reduced mixing lengths within reasonable pressure drop ranges, the present micromixers have potentials for use in the miniaturized Micro-Total-Analysis-Systems($\mu$TAS).

• 대한의용생체공학회:학술대회논문집
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• 대한의용생체공학회 1997년도 추계학술대회
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• pp.380-384
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• 1997

This study was performed to understand the exogenous-water-drop induced thermomechanical effect on the tooth in the free-running Er:YAG laser mode for the proper use of water as a laser energy absorber and coolant in dentistry. The ree-running Er:YAG laser was used in the dental hard tissue ablation study. A Microjet system was employed to dispense precise water drops. Ablation rate, recoil momentum, and temperature rise in the pulp cavity were measured with and without an exogenous water drop on the tooth surface. Exogenous water enhanced ablation rate in the thick tooth in which the ablation rate on the dry surface does not increase linearly but shows plateau. Optimal exogenous water volume was shifted from 2 nl to 4 nl as the laser energy was increased from 48 mJ to 145 mJ. The magnitude of the recoil momentum was increased as the volume of exogenous water increased. The results of this study suggest that we must pay attention to the recoil momentum or recoil pressure study or the optimal and safe usage of water in the dental treatment because these mechanical effects depend on the volume of exogenous water on the tooth surface.