• 대한기계학회:학술대회논문집
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• 대한기계학회 2000년도 추계학술대회논문집B
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• pp.461-466
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• 2000

Direct numerical simulations (DNS) is carried out to study fully-developed turbulent concentric annular pipe flow with two radius ratios at $Re_{Dh}\;=\;8900$. In case of $R_1/R_2\;=\;0.5$, the present result for the mean flow is in good agreement with the previous experimental data. Because of the transverse curvature effects, the distributions of mean flow and turbulent intensities are asymmetric in contrast to those of other fully-developed flows (channel and pipe flow). From the distributions of skewness of radial velocity fluctuations, it co be identified that all of the characteristics of channel, pipe and turbulent flow on a cylinder in axial flow can be appeared in concentric annular pipe flow.

• 대한기계학회:학술대회논문집
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• 대한기계학회 2000년도 춘계학술대회논문집B
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• pp.373-378
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• 2000

An experimental investigation on the incipience of nucleate boiling in forced flow of water is performed as a verification and extension of previous analysis. The effects of the subcooling, Reynolds number and surface curvature on the onset of nucleate boiling(ONB) in a concentric annulus flow channel with smooth inner heating surface is investigated experimentaly. Through flow visualization, the boiling phenomenon was observed directly and the experimental results were examined to find ONB heat flux. The results show that the variation of heat flux at ONB is increased linearly as the Reynolds number and subcooling are increased. The effect of surface curvature is very great specially for a small radius when radius of the inner heating tube is increased, the heat flux at ONB is almost inversely increased for the range of this investigation. It is found that the effect of convex surface curvature on ONB heat flux is very significant for a small radius.

• 대한기계학회논문집B
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• 제38권12호
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• pp.1027-1034
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• 2014

회전원판 위 표면에 작용하는 유동 전단응력은 표면마모를 발생시키는 주원인이다. 유체는 원심력에 의해 가장자리까지 자유표면을 갖는 박막유동으로 전달된다. 표면마모는 받음각 또는 곡률에 따른 전단응력 정도에 비례하여 차이를 보이게 된다. 전산해석을 통한 속도분포 기울기로 받음각에 기준한 곡률의 전단응력비를 비교하였다. 곡률반경 변화에 따른 전단유동의 응력감소를 모델링하여 표면마모를 효율적으로 줄일 수 있는 최적구간을 결정하는 것이 본 연구의 주제이며, 이 연구결과는 회전무화나 박막코팅과 같은 박막유동을 활용하는 기구의 최적설계에 적용될 수 있다.

• 대한기계학회논문집B
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• 제22권11호
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• pp.1509-1520
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• 1998

An experiment was carried out for the fully developed turbulent flow of water in tube coils on the condition of uniform heat flux. The present work was conducted for various ranges for Dean number(1794~1321), Prandtl number (2.5~4.5), curvature ratio parameters (22~60). Heat transfer to steady viscous flow in coiled tubes of circular cross section was studied for fully developed velocity and temperature fields under the thermal boundary condition of uniform heat flux. The peripherally local Nusselt number correlated as a function of Dean and Prandtl numbers. We studied the flow in heat coiled tubes under the influence of both centrifugal and buoyancy forces in order to gain insight into the flow pattern. In the present study, we obtained three emperical formulas, $Nu_v=0.0231Re^{0.84}Pr^{0.4}(a/R)^{0.13}$ (vertical) $Nu_c=0.0241Re^{0.86}Pr^{0.4}(a/R)^{0.08}$ (corrugated) $Nu_h=0.0227Re^{0.84}Pr^{0.4}(a/R)^{0.09}$ (horizontal).

• 설비공학논문집
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• 제10권3호
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• pp.282-291
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• 1998

Heat transfer performance are studied for the turbulent flow of water in 3 smooth tube coils having ratios of coil to tube diameter of 16, 21 and 27, and a corrugated-coiled tube having a ratio of coil to tube diameter of 29, for Reynolds numbers from 8000 to 60000 and is also compared with the limited results available to data. The experiments are carried out for the fully developed turbulent flow of water in tube coils under the condition of uniform heat flux. This work is limited 0 tube coils of R/a between 10 and 30. The tube having a ratio of coil to tube diameter of 27 among the 3 smooth tube coils shows the best heat transfer performance. The performance of coiled tube best transfer performance. The performance of coiled tube with a similar curvature ratio is better for a corrugated-coiled tube(R/a=17) than for a smooth coiled tube(R/a=16). An empirical relation which correlates most of the data within $\pm$25% was also developed. Test result shows that the Nusselt number is found to be affected by a secondary flow due to curvature.

• 한국자동차공학회논문집
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• 제6권6호
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• pp.155-165
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• 1998

The resistance coefficient and heat transfer performance are studied for the turbulent water flow in a smooth coiled tube having variable curvature ratios and a corrugated-coiled tube having a ratio of coil to tube diameter of 22. Experiments are carried out for the fully developed turbulent flow of water in tube coils on the uniform wall temperature condition. This work is limited to tube coils of R/a between 22 and 60 and Reynolds numbers from 13000 to 53000. The tube having a ratio of coil to tube diameter of 27 among the 3 smooth tube coils shows the best heat transfer performance. A corrugated-coiled tube(R/a=60) shows more excellent performance than a smooth coiled tub (R/a=60) at a similar curvature ratio. The friction factor f is sensitive to changes in the velocity profile caused by a temperature gradient. Allowance was made for the pressure loss in the short inlet and outlet lengths and due to the presence of the thermocouple inlet and outlet as a result of separate experimental on a straight tube. It is to be expected that the allowance at the exit will be somewhat too low because of secondary flow effects carried over from the coil.

• 한국전산유체공학회:학술대회논문집
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• 한국전산유체공학회 2000년도 춘계 학술대회논문집
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• pp.120-126
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• 2000

An S-duct flow is subjected to an entrance flow of Mach 0.6. The duct turns $30^{\circ}$ and reverses its turn by $30^{\circ}$ followed by a straight section. Such an internal flow induces a secondary flow due to curvature effect. Goal of this paper is to show the sensitivity of outflow boundary conditions on the quality of numerical solutions as well as to show curvature effect on the flow field. The often-used Baldwin-Lomax turbulence model is shown to be less functional on the concave region when the secondary flow has its strong Influence.

• Journal of Advanced Marine Engineering and Technology
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• 제38권10호
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• pp.1269-1274
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• 2014

Due to the presence of Dean flow in curved ducts, helical channels have drawn attention recently because of the practical industrial applications. The manipulation of fluids through microfluidic devices is widely used in many scientific and industrial areas. In the present study, numerical simulations were performed on a helical microchannel to predict the impact of different design parameters that affect Dean flow. Important geometric parameters such as the channel cross section, pitch, radius of curvature, and number of turns were considered for the analysis. The study also incorporates the effect of varying flow rate on Dean flows. It was found from the simulation results that microchannel cross section and pitch have a significant impact on maintaining the Dean flow, compared to the radius of curvature, number of turns, and flow rate.

• 대한기계학회논문집B
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• 제26권6호
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• pp.850-858
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• 2002

The present study investigates in detail the combined effects of the Coriolis force and centrifugal force on the development of turbulent flows in a square-sectioned U-bend rotating about an axis parallel to the center of bend curvature. When a viscous fluid flows through a curved region of U-bend, two types of secondary flow occur. One is caused by the Coriolis force due to the rotation of U-bend and the other by the centrifugal force due to the curvature of U-bend. For positive rotation, where the rotation is in the same direction as that of the main flow, both the Coriolis force and the centrifugal force act radially outwards. Therefore, the flow structure is qualitatively similar to that observed in a stationary curved duct. On the other hand, under negative rotation, where these two forces act in opposite direction, more complex flow fields can be observed depending on the relative magnitudes of the forces. Under the condition that the value of Rossby number and curvature ratio is large, the flow field in a rotating U-bend can be represented by two dimensionless parameters : $K_{TC}$ =Re $\sfrac{1}{4}$√λand a body force ratio F=λ/Ro. Here, $K_{TC}$ has the same dynamical meaning as $K_{TC}$ =Re√λ for laminar flow.

• 대한기계학회논문집B
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• 제26권1호
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• pp.159-169
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• 2002

The present study investigates in detail the combined effects of the Coriolis and centrifugal farce on the development of laminar flows in a square-sectioned U-bend rotating about an axis parallel to the center of bend curvature. When a viscous fluid flows through a rotating curved region, two types of secondary flow occur. One is caused by the Coriolis force due to the rotation of U-bend and the other by the centrifugal farce due to the curvature of U-bend. When the values of Rossby number and curvature ratio are large, the flow field in a rotating U-bend can be represented by two dimensionless parameters ; the Dean number K$\_$LC/=Re/√λ and a body ratio F=λ/Po. For positive rotation, where the rotation is in the same direction as that of the main flow, both the Coriolis force and the centrifugal force act radially outwards, the directions of the two secondary flows are the same. Therefore, the flow structure is qualitatively similar to that observed in a stationary curved duct with a larger f7c. On the other hand, in case of negative rotation, where two farces act in opposite direction, more complex flow fields can be observed depending on the relative magnitudes of the forces.