• Proceedings of the KSME Conference
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• 2000.04b
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• pp.625-630
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• 2000

Numerical and experimental investigations are peformed for the rarefied gas flows in pumping channels of a helical-type drag pump. Modern turbomolecular pumps include a drag stage in the discharge side, operating roughly in $10^{-2}{\sim}10Torr$. The flow occurring in the pumping channel develops from the molecular transition to slip flow traveling downstream. Two different numerical methods are used in this analysis: the first one is a continuum approach in solving the Navier-Stokes equations with slip boundary conditions, and the second one is a stochastic particle approach through the use of the direct simulation Monte Carlo(DSMC) method. The flow in a pumping channel is three-dimensional(3D), and the main difficulty in modeling a 3D case comes from the rotating frame of reference. Thus, trajectories of particles are no longer straight lines. In the Present DSMC method, trajectories of particles are calculated by integrating a system of differential equations including the Coriolis and centrifugal forces. Our study is the first instance to analyze the rarefied gas flows in rotating frame in the presence of noninertial effects.

• Journal of applied mathematics & informatics
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• v.29 no.5_6
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• pp.1583-1602
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• 2011

A special system of partial differential equations (PDEs) occur in a natural way while studying a class of irrotational inviscid fluid flow problems involving infinite channels. Certain aspects of solutions of such PDEs are analyzed in the context of flow problems involving multiple layers of fluids of different constant densities in a channel associated with arbitrary bottom topography. The whole analysis is divided into two parts-part A and part B. In part A the linearized theory is employed along with the standard Fourier analysis to understand such flow problems and physical quantities of interest are derived analytically. In part B, the same set of problems handled in part A are examined in the light of a weakly non-linear theory involving perturbation in terms of a small parameter and it is shown that the original problems can be cast into KdV type of nonlinear PDEs involving the bottom topography occurring in one of the coefficients of these equations. Special cases of bottom topography are worked out in detail and expressions for quantities of physical importance are derived.

• 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.7
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• pp.780-788
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• 2004

This paper reports the fluid flow and heat transfer around a module cooled by forced air flow generated by a piezoelectric(PZT) cooling fan. The fluids are locally accelerated by a flexible PZT fan which deflects inside a fluid transport system of comparatively simple structure mounted on a PCB in a parallel-plate channel(450${\times}$80${\times}$700㎣). Input voltages of 20-100V and a resonance frequency of 23㎐ were used to vibrate the cooling fan. Input power to the module was 4W. The fluid flow around the module was visualized by using PIV system. The temperature distributions around a heated module were visualized by using liquid crystal film(LCF). The cooling effect using a PZT fan was independent of the vent area ratios at the channel inlet and was similar to the forced convection cooling. We found that the flow type was Y-shape and the cooling effect was increased by the wake generated by a piezoelectric cooling fan.

• Korean Journal of Air-Conditioning and Refrigeration Engineering
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• v.19 no.4
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• pp.339-345
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• 2007

In the present study, a 10-cell PEMFC stack with straight type cathode flow channels is employed to investigate the effect of inlet air flow header configuration on the overall fuel cell performance. Four different types of inlet flow headers are considered and the flow patterns according to the air inlet flow header configuration are numerically obtained. The computed result for a modified header predicts about 8.5% improvement in the air flow distribution at 10-cell cathode channel inlets. Experiments are also carried out to confirm the numerical findings by measuring actual air flow distributions and the polarization curves of the PEMFC stack.

• Journal of Korean Society of Coastal and Ocean Engineers
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• v.30 no.6
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• pp.242-252
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• 2018

It is well known that an Oscillating Water Column Wave Energy Converter (OWC-WEC) is one of the most efficient wave absorber equipment. This device transforms the vertical motion of water column in the air chamber into the air flow velocity and produces electricity from the driving force of turbine as represented by the Wells turbine. Therefore, in order to obtain high electric energy, it is necessary to amplify the water surface vibration by inducing resonance of the piston mode in the water surface fluctuation in the air chamber. In this study, a new type of OWC-WEC with a seawater channel is used, and the wave deformation by the structure, water surface fluctuation in the air chamber, air outflow velocity from the nozzle and seawater flow velocity in the seawater channel are evaluated by numerical analysis in detail. The numerical analysis model uses open CFD code OLAFLOW model based on multi-phase analysis technique of Navier-Stokes solver. To validate model, numerical results and existing experimental results are compared and discussed. It is revealed within the scope of this study that the air flow velocity at nozzle increases as the Ursell number becomes larger, and the air velocity that flows out from the inside of the air chamber is larger than the velocity of incoming air into the air chamber.

• Transactions of the Korean Society of Mechanical Engineers B
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• v.27 no.5
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• pp.635-643
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• 2003

Parametric study for the analysis of performance characteristics of a planar -type solid oxide fuel cell(SOFC) using computational flow analysis is conducted. A planar -type SOFC, which is composed by two gas channels (fuel and ai.) and one set of anode-electrolyte-cathode assembly, is modeled as a two -dimensional isothermal case. Results of computational analysis of flow field including distributions of mass fractions in gas channels are used to the performance analysis of the fuel cell. Flow analysis makes it possible to consider current density distributions along the length of the cell in the process of performance analysis of the SOFC. As results of parametric study, it is found that the mole fraction of fuel at the inlet of fuel channel, operating pressure and temperature are closely related to the performance characteristics of SOFC.

• Corrosion Science and Technology
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• v.15 no.5
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• pp.245-252
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• 2016

The pipelines and equipments are degraded by flow-accelerated corrosion (FAC), and a large-scale test facility was constructed for simulate the FAC phenomena in secondary coolant environment of PWR type nuclear power plants. Using this facility, FAC test was performed on weld pipe (carbon steel and low alloy steel) at the conditions of high velocity flow (> 10 m/s). Wall thickness was measured by high temperature ultrasonic monitoring systems (four-channel buffer rod type and waveguide type) during test period and room temperature manual ultrasonic method before and after test period. This work deals with the complex effects of flow velocity on the wall thinning in weld pipe and the test results showed that the higher flow velocity induced different increasement of wall thinning rate for the carbon steel and low alloy steel pipe.

• Journal of the Korean Society of Fisheries and Ocean Technology
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• v.42 no.2
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• pp.86-96
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• 2006

This research aims at establishing the fundamental characteristics of the kite through the analysis of the flow field around various types of kites. The approach of this study were adopted for the analysis; visualization by PIV(particle image velocimetry). Also, the lift and drag tests of kites had been performed in our previous finding(Bae et al., 2004a; Bae et al., 2004b). For this situation, models of canvas kite were deployed in the circulating water channel for the PIV test using the same conditions as in the lift and drag tests. The results obtained from the above approach are summarized as follows: Given the rectangular and triangular kites when attack angle is $20^{\circ}$, vortex by the boundary layer separation was seen in the leading edge and the flow towards the trailing edge was more turbulent. But, the inverted triangular type kite was seen to be stable without any boundary layer separation or turbulence. The increase of the attack angle resulted in the eddy in order of the rectangular, triangular and inverted triangular type. The magnitude of the eddy followed the same order. The effect of edge-eddy was biggest in the triangular type followed by the rectangular and then the inverted triangular type. The kite as the buoyancy device or the opening device will be very useful when the appropriate applications and the stability are met.

• KSCE Journal of Civil and Environmental Engineering Research
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• v.3 no.4
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• pp.21-31
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• 1983

Circulation phenamena in open channel with abrupt variation in flow section are studied by solving numerically two dimensional Navier-Stokes equations integrated over depth. Galerkin type finite element method is used as numerical scheme. Numerical results by both implicit and explicit schemes tested in one-demensional rectangular channel agree closely with the known solution. The numerical experiments carded out in the open channel with a pool indicate the expected flow pattern and the center of the circulation coincides with the geometrical center, but the vectors of velocity appear father small, and it remains to be further investigated. Numerically simulated flow profiles along the channel with constrictions such as bridge piers and abutments are shown to be close to hydraulic experimental results. Thus further refined numerical technique is expected to be able to serve as a tool to evaluate the effect of bridge backwater.