• 대한기계학회논문집B
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• 제27권12호
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• pp.1724-1733
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• 2003

This paper assesses the prediction performance of explicit algebraic stress and heat-flux models for reduction of heat transfer coefficient in a strongly-heated vertical tube. Two explicit algebraic stress models and four explicit algebraic heat-flux models are selected for assessment. Eight combinations of explicit algebraic stress and heat-flux models are used in predicting the turbulent gas flows with intense heating, which yields the significant property-variation. The results showed that the two combinations of GS-AKN and WJ-mAKN predicted the Nusselt number and the axial wall temperature variations well and that the predictions of Nusselt number with WJ-combinations spread in a wider range than those with Gs-combinations. WJ is the explicit algebraic stress model of Wallin and Johansson and GS is the model of Gatski and Speziale and that AKN is the explicit heat-flux model of Abe, Kondoh and Nagano and mAKN is the modified AKN.

• 대한기계학회:학술대회논문집
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• 대한기계학회 2001년도 춘계학술대회논문집E
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• pp.45-50
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• 2001

This experimental study concerns the characteristics of a transitional flow in a concentric annulus with a diameter ratio of 0.52, whose outer cylinder is stationary and inner one rotating. The pressure losses and skin-friction coefficients have been measured for the fully developed flow of a 0.2 % aqueous solution of sodium carbomethyl cellulose (CMC) at a inner cylinder rotational speed of $0{\sim}600$ rpm. The transitional flow has been examined by the measurement of pressure losses, to reveal the relation of the Reynolds numbers with the skin-friction coefficients, in the laminar and transitional flow regimes. The occurrence of transition has been checked by the gradient change of pressure losses and skin-friction coefficient with respect to the Reynolds numbers. The increasing rate of skin-friction coefficient due to the rotation is uniform for laminar flow regime, whereas it is suddenly reduced for transitional flow regime and, then, it is gradually declined for turbulent flow regime. Consequently, the critical(axial-flow) Reynolds number decrease as the rotational speed increases. Thus, the rotation of inner cylinder promotes the early occurrence of transition due to the onset of taylor vortices.

• 한국결정성장학회지
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• 제7권4호
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• pp.536-547
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• 1997

축방향으로 균일한 자장이 가하여진 Czochralski 도가니에서 도가니와 성장결정이 다양한 각속도로 회전하는 경우에 대하여 유동장, 온도장 및 산소의 농도장이 수치적으로 연구되었다. 도가니 벽의 가열에 의한 부력의 효과와 자유표면에서 온도차이에 의한 열모세관효과로 인한 용융물질의 유동은 도가니와. 성장결정의 회전에 따른 원심력에 의하여 차등적으로 억제될 수 있다. 이 원심력에 가장 큰 영향을 주는 인자는 도가니의 회전속도이며 이것이 초크랄스키 결정성장과정에서 속도장, 온도장, 농도장에 큰 영향을 주고 있다. 도가니의 회전속도가 작을 때에는 성장결정의 회전이 원심력을 효과적으로 발생시킨다.

• 한국전산유체공학회지
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• 제19권4호
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• pp.68-74
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• 2014

This computational study features the transient compressible and inviscid flow analysis on a metallic plasma discharge from the opposing composite electrodes which is subjected to pulsed electric power. The computations have been performed using the flux corrected transport algorithm on the axisymmetric two-dimensional domain of electrode gap and outer space along with the calculation of plasma compositions and thermophysical properties such as plasma electrical conductivity. The mass ablation from aluminum electrode surfaces are modeled with radiative flux from plasma column experiencing intense Joule heating. The computational results shows the highly ionized and highly under-expanded supersonic plasma discharge with strong shock structure of Mach disk and blast wave propagation, which is very similar to muzzle blast or axial plasma jet flows. Also, the geometrical effects of composite electrodes are investigated to compare the amount of mass ablation and penetration depth of plasma discharge.

• 한국소성가공학회:학술대회논문집
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• 한국소성가공학회 2003년도 춘계학술대회논문집
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• pp.210-215
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• 2003

A terminal pin, which is a part of high-voltage capacitors, has a head section of plate-shaped geometry with 0.8 thickness. The current manufacturing process, in which the head section is welded on the body part, has given wide deviations of part qualities such as geometrical accuracy, mechanical strength and electrical stability. In this paper, a cold forging process sequence was designed in order to produce the terminal pin as one piece. The plate-shaped head section requires an upsetting in the lateral direction of a cylindrical billet, which is followed by a blanking process. The deformed geometry of the upsetting, however, could not be predicted precisely by intuition since metal flows of an axial and a lateral direction of the cylindrical billet would occur simultaneously. Therefore, the geometry of the initial billet was determined by three dimensional finite element analysis in order to avoid defects in blanking process and intermediate forging processes were designed by applying design rules and two dimensional FE analysis. In addition, cold forging tryouts were conducted by using the die sets which were manufactured based on the designed process sequence.

• 한국연소학회:학술대회논문집
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• 한국연소학회 2005년도 제31회 KOSCO SYMPOSIUM 논문집
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• pp.181-188
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• 2005

Large eddy simulation(LES) methodology used to model the isothermal swirling flows in a dump combustor and the turbulent premixed flame in a model gas turbine combustor. The LES solver was implemented on parallel computer consisting 16 processors. In isothermal flow simulation, the results was compared with that of ${\kappa}-{\varepsilon}$ model as well as experimental data, in order to verify the capability of LES code. To model the turbulent premixed flame in a gas turbine, the G-equation flamelet model was used. The results showd that LES and RANS well predicted the mean velocity field of a non-swirling flow. However, in swirling flow, LES showed a better performance in predicting the mean axial and azimuthal velocities, and the central recirculation zone than those of RANS. In a model gas turbine combustor, the operation condition of high pressure and temperature induced the different phenomena, such as flame length and flow-field information, comparing with the condition of ambient pressure and temperature. Finally, it was identified that the flame and heat release oscillations are related to the vortex shedding generated by swirl flow and pressure wave propagation.

• 한국결정성장학회지
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• 제4권3호
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• pp.210-222
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• 1994

축방향의 균일한 자장이 Czochralski 도가니에 가하여졌을 때의 유동장, 온도장 및 산소의 농도장이 수치적으로 연구되었다.Czochralski 유동장과 농도장에 작용하는 부력, thermocapillarity, 원심력, 자성력, 산소의 확산계수, 산소의 segregation coefficient, SiO형태의 evaporation, 도가니벽의 ablation 등이 고려되었다. 회전방향으로의 대칭성으로부터 자오면에서의 속도성분과 회전방향의 속도성분, 온도, 전류의 흐름 등이 먼저 정상상태에 도달하였다고 가정하고 초기에 일정한 산소의 농도가 주어진 상황에서 비정상 상태의 산소의 농도장이 해석되었다. Czochralski 유동장에서의 대류와 확산에 의한 산소의 전달현상이 파악되었으며 결정성장 표면으로 흡수되는 산소의 농도가 연구되었다.

• 한국전산유체공학회지
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• 제9권4호
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• pp.55-63
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• 2004

The three-dimensional Stokes flow within a staggered screw channel is obtained by using a finite volume method. The geometry is intended to mimic the single screw extruder having staggered arrangement of flights. The flow solution is then subjected to the analysis of the stirring performance. In the analysis of the stirring performance, the stretching-mapping method developed by the author is employed for calculating the materials' stretching exponents, which are to be used in quantification of the mixing effect. The numerical results Indicate that the staggered geometry gives indeed far much better stirring-performance than the standard (nonstaggered) flight geometry. It was also shown that care must be given to the selection of the basis planes for evaluating the local stretching rate, and it turns out that the best method (H-method) has its basis plane just on the half way between the past and future evolution of fluid particles subjected to the defromation. In evaluating the stretching exponent, the expansion ratio must be considered which is one of the characteristic differences of the actual three-dimensional flows from the two-dimensionmal counterparts. The larger axial pressure-difference causes in general the smaller stirring performance while the flow rate is increased. The smaller channel length also increases the stirring performance.

• Journal of Mechanical Science and Technology
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• 제15권7호
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• pp.906-920
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• 2001

This paper presents three-dimensional mean velocities, turbulent intensities and Reynolds shear stresses measured in the Y-Z plane of the gas swirl burner with a cone type baffle plate by using an X-type hot-wire probe. This experiments is carried out at the flow rate of 450ℓ/min which is equivalent to the combustion air flow rate necessary to heat release 15,000 kcal/hr in a gas furnace. Mean velocities and turbulent intensities etc. show that their maximum values exist around the narrow slits situated radially on the edge of and in front of a burner. According to downstream regions, they have a peculiar shape like a starfish because the flows going out of the narrow slits and the swirl vanes of an inclined baffle plate diffuse and develop into inward and outward of a burner. The rotational flow due to the inclined flow velocity going out of swirl vanes of a cone type baffle plate seems to decrease the magnitudes of mean velocities V and W respectively by about 30% smaller than those of mean velocity U. The turbulent intensities have large values of 50%∼210% within the range of 0.5

• 대한기계학회:학술대회논문집
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• 대한기계학회 2002년도 학술대회지
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• pp.337-340
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• 2002

The present experimental and numerical investigations are performed for the characteristics of transitional flow in a concentric annulus with a diameter ratio of 0.52, whose outer cylinder is stationary and inner one rotating. The flow field of an annulus has been numerically solved using a finite volume method. The pressure losses and Skin-friction coefficients have been measured for the fully developed flow of water and $0.2{\%}$ aqueous solution of sodium carboximethy1 cellulose (CMC), respectively at inner cylinder rotational speed of $0{\~}600rpm$. The transitional flow has been examined by the measurement of pressure losses to reveal the relation of the Reynolds and Rossby numbers with the skin-friction coefficients. The occurrence of transition has been checked by the gradient changes of pressure losses and skin-friction coefficients with respect to the Reynolds numbers. Consequently the critical(axial-flow) Reynolds number decreases as the rotational speed increases. Thus, the rotation of inner cylinder promotes the early occurrence of transition due to the onset of Taylor vortices.