• Proceedings of the KSME Conference
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• 2007.05b
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• pp.1888-1893
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• 2007

In this study, the microchannel plated heat exchanger were numerically studied for the enhancement of heat transfer in the channel configuration. Unit cold and hot fluid region with the microchannel were modeled and periodic boundary condition at the side wall was applied to continuously repeating geometry. The material of micro-structured plate is STS304 and working fluid is water. Triangular obstacles were placed in micro channel to enhance heat transfer. The performance of microchannel plated heat exchangers were numerically investigated with various obstacle configuration and Reynolds number under the parallel and counter flows. Heat transfer rate has increased about 18% compared with straight channel, but pressure drop also increased about 3.5 times. The main factor of increasing of pressure drop and heat transfer rate is considered that the momentum was lost to collide against obstacles, generation of secondary flow and boundary layer separation, wake and vortex forming phenomena.

• Proceedings of the KSME Conference
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• 2008.11a
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• pp.1902-1907
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• 2008

The current work presents a design and fabrication technique for a microchannel system to measure the local temperature distribution inside microchannel. This micro channel system fabricated by MEMS technique is integrated with a heater and an array of temperature sensors so that detailed heat transfer phenomena inside micro-scale channel can be studied. Materials widely used in semiconductor process were selected to fabricate a heater and temperature sensors on a silicon wafer. On these heater and sensors a channel wall was fabricated with SU-8. The friction constant and the local Nusselt number distribution measured for the deionized water flow in the microchannel is presented.

• Transactions of the Korean Society of Mechanical Engineers A
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• v.28 no.1
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• pp.63-69
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• 2004

It is of great interest to enhance mixing performance in a microchannel in which the flow is usually characterized as a low Reynolds number (Re) so that good mixing is quite difficult to be achieved in this laminar flow regime. In this regard, we present a new chaotic passive micromixer, named Barrier Embedded Micromixer (BEM), of which the mixing mechanism is based on chaotic flows. In BEM, chaotic flow is induced by periodic perturbation of the velocity field due to periodically inserted barriers along the channel wall while a helical type of flow is obtained by slanted grooves on the bottom surface of the channel in the pressure driven flow. To experimentally compare the mixing performance, a T-microchannel and a microchannel with only slanted grooves were also fabricated. All microchannels were made of PDMS (Polydimethylsiloxane) from SU-8 masters that were fabricated by conventional photolithography. Mixing performance was experimentally characterized with respect to an average mixing intensity by means of color change of phenolphthalein as pH indicator. It was found that mixing efficiency decreases as Re increases for all three micromixers. Experimental results obviously indicate that BEM has better mixing performance than the other two. Chaotic mixing mechanism, suggested in this study, can be easily applied to integrated microfluidic systems , such as Micro-Total-Analysis-System, Lab-on-a-chip and so on.

• Transactions of the Korean Society of Mechanical Engineers B
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• v.28 no.6
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• pp.682-687
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• 2004

Y-junction microchannels are widely used as a flew mixer. Fluids are entered from two branch channels and merged together at a combined channel. In this study, we suggest a simple method to create the fluid digitization using flow instability phenomena. Two immiscible liquids (water/oil) are infused continuously to each Y-junction inlets. Because of the differences in fluid and flow properties at the interface, oil droplet is formed automatically followed by flow instability. In order to clarify the hydrodynamic aspects involved in oil droplet formation, a quantitative flow visualization study has performed. Highly resolved velocity vector fields are obtained by a micro-PIV technique, so that detail flow structures around the droplet are illustrated. In this study, fluorescent particles were mixed with water only for visualization of oil droplet and velocity field measurement in water flow.

• Transactions of the Korean Society of Mechanical Engineers B
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• v.29 no.6 s.237
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• pp.747-754
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• 2005

Because the liquid-vapor interfacial shear stress affects seriously the liquid flow and the maximum heat transport rate of the grooved wick heat pipe, an accurate modeling for the pressure drop characteristics of the liquid flow is required. A novel method for calculating the liquid pressure drop and the velocity profile of an open channel flow in a microchannel with an arbitrary cross-section is suggested and validated by experiments. An experimental apparatus for the Poiseuille number of the liquid flow in open rectangular microchannels with the hydraulic diameters of 0.40mm, 0.43mm, 0.48mm is used in order to reproduce real situations in the grooved wick heat pipe. Analytic results from the suggested method are compared with the experimental data and they are in a close agreement with each other.

• Transactions of the Korean Society of Mechanical Engineers B
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• v.38 no.2
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• pp.139-146
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• 2014

Sodium borohydride ($NaBH_4$) is considered as a secure metal hydride for hydrogen storage and supply. In this study, the interfacial friction of two-phase flow in the dehydrogenation of aqueous $NaBH_4$ solution in a microchannel with a hydraulic diameter of $461{\mu}m$ is investigated for designing a dehydrogenation chemical reactor flow passage. Because hydrogen gas is generated by the hydrolysis of $NaBH_4$ in the presence of a ruthenium catalyst, two different flow phases (aqueous $NaBH_4$ solution and hydrogen gas) exist in the channel. For experimental studies, a microchannel was fabricated on a silicon wafer substrate, and 100-nm ruthenium catalyst was deposited on three sides of the channel surface. A bubbly flow pattern was observed. The experimental results indicate that the two-phase multiplier increases linearly with the void fraction, which depends on the initial concentration, reaction rate, and flow residence time.

• Proceedings of the KSME Conference
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• 2005.11a
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• pp.51.1-51.1
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• 2005

• Journal of the Korean Society of Visualization
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• v.15 no.2
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• pp.41-47
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• 2017

Microring/nanoring structure has high applicability for nano-antenna and biosensor thanks to its superior optical characteristics. Although coiling nanowires manufactured using immiscible emulsion droplets have an advantage in mass production, this process also forms nanowire bundles. In this study, we solved the nanowire bundle problem by size-selective sorting of the emulsion droplets in a pinched flow fractionation microchannel. Utilizing silver nanowires and immiscible emsulsion droplets, we investigated the correlation between the size of ring droplets and bundle droplet. We visualized the sorting process for glass particles and microring-containing emulsion droplets. Droplets were sorted based on their size, and the ratio of bundle droplets in solution decreased. This droplet-sorting strategy has potential to help the printing and coating process for manufacturing of ring structure patterns and developing of functional materials.

• Transactions of the Korean Society of Mechanical Engineers B
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• v.34 no.1
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• pp.53-59
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• 2010

In this paper, we propose a novel chaotic micromixer of which mixing mechanism is based upon magnetohydrodynamic (MHD) multi-vortical flow generation in a simple straight microchannel. In the microchannel of the micromixer has electrodes patterned on two side walls and bottom wall. Lorentz forces are variously induced by changing applied voltages at the patterned electrodes in order to pump and mix conductive fluids in the microchannel. Three-dimensional computational fluid dynamics simulations were conduced to characterize mixing behaviors inside the MHD micromixer. The mixing efficiencies were also evaluated for the various flow conditions.

• Journal of the Korean Society of Visualization
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• v.8 no.4
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• pp.43-47
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• 2010

We demonstrate the droplet bistability in a microchannel which has two symmetric necks that operate as capillary valves. It is shown that there are certain flow conditions, determined by droplet velocity and droplet size, to achieve bistability. Droplet bistabililty allows simple but precise control of droplet at a bifurcation channel. Therefore, by an appropriate channel design to induce droplet bistability, we can distribute droplets at a junction passively in the manner of perfect alternation and perfect switching in the choice of the outlets.