• Korean Journal of Air-Conditioning and Refrigeration Engineering
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• v.16 no.9
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• pp.828-837
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• 2004

The present study investigated two-phase flow distribution, phase separation and pressure drop in multi-microchannel tubes under adiabatic condition. The test section consisted of inlet and outlet headers with the inner diameter of 19.4㎜ and 15 parallel microchannel tubes. Each microchannel tube brazed to the inlet and outlet headers and had 8 rectangular ports with the hydraulic diameter of 1.32㎜. The key experimental parameters were orientation of header (horizontal and vertical), flow direction of refrigerant into the inlet header (in-line, parallel and cross flow) and inlet quality (0.1, 0.2 and 0.3). It was found that the orientation of the header had relatively large effect on the flow distribution and phase separation, while the inlet quality didn't affect much on them. The horizontal header showed the better flow distribution and phase separation characteristics than the vertical one. The parallel flow condition with the horizontal header showed the best performance for the flow distribution and phase separation characteristics under the test conditions. Two-phase pressure drops through the microchannel tubes with the horizontal header were higher than those of the microchennel tubes with the vertical header due to gravitational effect.

• 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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• 2004.11a
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• pp.1534-1539
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• 2004

This experiment has been carried out to measure the process of droplet formation between water phase fluid(PVA 3%) and organic phase fluid(oil) and vector fields measured by a Dynamic Micro-PIV method in the inside of a droplet while generated. Droplet length controlled by changing flow rate conditions in microchannel. Water-in-oil(W/O) droplets successfully generated at a Y junction and cross microchannel. But oil-in-water(O/W) droplets could not be formed at a Y junction microchannel. That is, PVA 3% flow could not be detached from the PDMS surface and ran parallel with oil flow. When PVA 3% flow rate was constant, droplet length and time period decreased as oil flow rate increased, but droplet frequency increased. When PVA 3% and oil flow rate ratio was constant, droplet length and time period decreased as flow rate increased, but droplet frequency increased. All that case, Standard deviation of droplet formation have less than 5% at averaged droplet length and regular-sized droplets were reproducibly formed.

• Transactions of the Korean Society of Mechanical Engineers B
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• v.27 no.5
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• pp.644-654
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• 2003

An analytical solution for a vertically stratified viscous flow in a microchannel with a rectangular cross-section is constructed, assuming fully developed laminar flow where the interfaces between the fluid layers are flat. Although the solution is for n-layer flow, restricted results to symmetrical three-layer flow are presented to investigate the effects of the viscosity and thickness ratios of the fluid layers and the aspect ratio of the microchannel on the flow field. Relations between the flow rate and thickness ratios of the fluid layers with varying viscosity distributions are found, considering the cross -sectional velocity profiles which vary noticeably with the three parameters and differ significantly from the velocity profiles of the flow between infinite parallel plates. Interfacial instability induced by the viscosity stratification in the microchannel is discussed referring to previous studies on the instability analysis for plane multilayer flow. Exact solution derived in the present study can be used for examining a diffusion process and three -dimensional stability analysis. More works are needed to formulate the equations including the effects of interfacial' tension between immiscible liquids and surface wettability which are important in microscale transport phenomena.

• Transactions of the Korean Society of Mechanical Engineers B
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• v.32 no.12
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• pp.915-923
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• 2008

The performance experiments for a microchannel printed circuit heat exchanger (PCHE) of high-performance and high-efficiency on the two technologies of micro photo-etching and diffusion bonding were performed in this study. The microchannel PCHE were experimentally investigated for Reynolds number in ranges of 100 $\sim$ 700 under various flow conditions in the hot side and the cold side. The inlet temperatures of the hot side were conducted in range of $40^{\circ}C\;{\sim}\;50^{\circ}C$ while that of the cold-side were fixed at $20^{\circ}C$. In the flow pattern, the counter flow was provided 6.8% and 10 $\sim$ 15% higher average heat transfer rate and heat transfer performance than the parallel flow, respectively. The average heat transfer rate, heat transfer performance and pressure drop increases with increasing Reynolds number in all the experiment. The increasing of inlet temperature in the experiment range has not an effect on the heat transfer performance while the pressure drop decrease slightly with that of inlet temperature. The experimental correlations to the heat transfer coefficient and pressure drop factor as a function of the Reynolds number have been suggested for the microchannel PCHE.

• Transactions of the Korean Society of Mechanical Engineers B
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• v.36 no.9
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• pp.885-894
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• 2012

A numerical study of a microchannel steam methane reforming reactor has been performed to understand the characteristics of heat and mass transfer. The integration of Rh-catalyzed steam methane reforming and Pt-catalyzed methane combustion has been simulated. The reaction rates for chemical reactions have been incorporated into the simulation. This study investigated the effect of contact time, flow pattern (parallel or counter), and channel size on the reforming performance and temperature distribution. The parallel and counter flow have opposite temperature distribution, and they show a different type of reaction rate and species mole fraction. As the contact time decreases and channel size increases, mass transfer between the catalyst layer and the flow is limited, and the reforming performance is decreased.

• Proceedings of the KSME Conference
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• 2007.05b
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• pp.2108-2112
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• 2007

The air and water flow distribution are experimentally studied for a round header-ten microchannel tube configuration. Three different inlet orientations (parallel, side, normal) were investigated. Tests were conducted with downward flow configuration for the mass flux from 70 to 130 kg/$m^2s$, quality from 0.2 to 0.6, non-dimensional protrusion depth (h/D) from 0.0 to 0.5. It is shown that, for almost all the test conditions, normal inlet yielded the best flow distribution, followed by side and parallel inlet. Possible reasoning is provided using flow visualization results.

• Transactions of the Korean Society of Mechanical Engineers B
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• v.32 no.2
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• pp.125-132
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• 2008

The bubble dynamics and heat transfer associated with nucleate boiling in parallel microchannels is studied numerically by solving the equations governing conservation of mass, momentum and energy in the liquid and vapor phases. The liquid-vapor interface is tracked by a level set method which is modified to include the effects of phase change at the interface and contact angle at the wall. Also, the reversible flow observed during flow boiling in parallel microchannels has been investigated. Based on the numerical results, the effects of contact angle, wall superheat and the number of channels on the bubble growth and reversible flow are quantified.

• 한국전산유체공학회:학술대회논문집
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• 2008.03b
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• pp.472-473
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• 2008

Flow boiling in parallel microchannels has received attention as an effective heat sink mechanism for power-densities encountered in microelectronic equipment. the bubble dynamics coupled with boiling heat transfer in microchannels is still not well understood due to the technological difficulties in obtaining detailed measurements of microscale two-phase flows. In this study, complete numerical simulation is performed to further clarify the dynamics of flow boiling in microchannels. The level set method for tracking the liquid-vapor interface is modified to include the effects of phase change and contact angle. The method is further extended to treat the no-slip and contact angle conditions on the immersed solid. Also, the reverse flow observed during flow boiling in parallel microchannels has been investigated. Based on the numerical results, the effects of channel shape and inlet area restriction on the bubble growth, reverse flow and heat transfer are quantified.

• Journal of Advanced Marine Engineering and Technology
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• v.31 no.3
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• pp.272-281
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• 2007

A numerical simulation on nitrogen gas flow in a long parallel plate microchannel was performed to obtain the effect of compressibility and rarefaction on gaseous flow in microchannels. The simulation was based on steady. two dimensional compressible Navier-Stokes and energy equations with noslip and first order slip boundary conditions. The channel was $1.2{\mu}m$ deep and $3000{\mu}m$ long. The Reynolds numbers were in the range of order from $10^{-2}$ to $10^{-1}$. So the flow was assumed to be laminar. The computations were performed on various pressure ratios. The outlet pressure was fixed to atmospheric pressure. The outlet Knudsen number was 0.0585, consequently the flow was in the slip flow regime. The computations were performed with the assumption of isothermal channel walls. The results were compared with the experimental data. The agreement was good.