• Journal of Power System Engineering
• /
• v.4 no.2
• /
• pp.17-24
• /
• 2000

직경비가 0.26, 0.39, 그리고 0.56인 경우 거칠기를 내벽면에, 외벽면에, 그리고 양벽면에 각각 설치한 동심환형관에서 완전히 발달된 난류유동에 대한 거칠기 효과를 실험과 이론으로 해석하였다. 그리고 거칠기 효과를 정량적으로 파악하기 위해 양벽면 모두 매끈한 환형관의 경우도 포함하였다. 실험에서 마찰계수를 구하는데 필요한 속도분포와 전단응력들은 피토튜브와 X-형 열선풍속계를 사용하였다. 이론 해석은 수정대수 난류모델을 사용하였고 그 결과를 이론값과 비교하여 아래의 결론을 얻었다. 1) 무디다이어그램 (Moody diagram)은 거칠기가 설치된 이중관에는 적용할 수 없음을 보였다. 2) 경우(c) 와 (d)에서는 반경비가 클수록 반드시 마찰계수가 증가하지 않았다. 3) 4개의 경우중 거칠기로 인해 마찰계수가 증가하는 크기의 순서는 경우(d)가 가장 컸고 경우(a)가 가장 작았다.

• Nuclear Engineering and Technology
• /
• v.27 no.6
• /
• pp.888-893
• /
• 1995

The turbulence structure for fully developed flow through the subchannels formed by the bare rod array depends on the pitch to rod diameter ratio. For fairly open spaced bare rod arrays, the distributions of the three components of the turbulent normal stresses are similar to those measured in circular pipe. However, for more closely spaced arrays, the turbulence structure, especially in the gap region, depart markedly from the pipe flow distribution. A linear relationship between turbulent normal stresses and turbulent kinetic energy for fully developed turbulent flow through regularly spaced bare rod arrays has been developed. This correlation can be used in connection with various theoretical analyses applied in turbulence research.

• Journal of Advanced Marine Engineering and Technology
• /
• v.18 no.1
• /
• pp.41-50
• /
• 1994

동심 이중관내에서 외관내벽의 사각돌출형 조도요소에 의한 비대칭 난류유동과 열전달 특성을, 열전달과 마찰계수에 미치는 조도의 합성효과를 조사하기 위해, 연구하였다. 이론해석에서는 수정 플란틀 혼합길이(mixing length)이론의 난류모델을 속도분포와 마찰계수를 구하는데 사용하였다. 최대 속도지점에서 안쪽과 바깥쪽의 두 속도 형상들은 힘의 평형에 의해 일치시켰다. 그리고나서, 온도분포와 열전달 계수를 계산하였다. 속도형상과 마찰계수들의 해석결과는 반경비 (${\alpha}$)= 0.13, 0.26, 0.4, 그리고 0.56 경우의 실험과 매우 잘 일치하였다. 마찰계수와 Nusselt number에 미치는 반경비, 조도비, 그리고 조도에 대한 피치비 등과 같은 여러 변수들의 효과들을 조사하였다. 본 연구는 일정 조도 요소들이 전체적 효율 측면에서 볼 때 열전달을 우리하게 향상시킨다는 것을 증명하였다.

• Journal of Advanced Marine Engineering and Technology
• /
• v.17 no.1
• /
• pp.45-54
• /
• 1993

동심 이중관내에서 사각돌출형 조도요소에 의한 비대칭 난류유동과 열 전달특성을, 열전달과 마찰계수에 미치는 조도의 합성효과를 조사하기 위해, 연구하였다. 이론해석에서는 한쪽면에 거칠기가 있는 평행평판의 유동에 대한 수정 플란틀 혼합길이(mixing length)이론의 난류모델을 속도분포와 마찰계수를 구하는데 사용하였다. 최대속도지점에서 안쪽과 바깥쪽의 두 속도형상들은 힘의 평형에 의해 매치(match) 시켰다. 그리고나서, 온도 분포와 열전달 계수를 계산하였다. 속도형상과 마찰계수들의 해석결과는 실험과 매우 잘 일치하였다. 마찰계수와 Nusselt number에 미치는 조도비, 조도에 대한 피치비, 그리고 반경비등과 같은 여러 변수들의 효과들을 조사하였다. 본 연구는 일정 임의의 조도 요소들이 전체적 효율 측면에서 볼 때 유리하게 열전달을 향상시킨다는 것을 증명하였다.

• Transactions of the Korean Society of Mechanical Engineers
• /
• v.18 no.4
• /
• pp.1072-1080
• /
• 1994

The fully developed turbulent momentum and heat transfer induced by the square-ribed roughness elements on the outer wall surface in concentric annuli are studied analytically based on a modified turbulence model. The analytical results of the fluid flow are verified by experiment. The resulting momentum and heat transfer are discussed in terms of various parameters, such as the radius ratio, the relative roughness, the roughness density, Reynolds number, Nusselt number and Prandtl number. The study demonstrates that certain artificial roughness elements may be used to enhance heat transfer rates with advantages from the overall efficiency point of view.

• Transactions of the Korean Society of Mechanical Engineers
• /
• v.17 no.6
• /
• pp.1541-1546
• /
• 1993

To extend the widely used Gibson and Launder's second order closure model to the low-Reynolds-number region near a wall, modifications have been made for velocity pressure-gradient interaction and dissipation terms in the stress equations, and also for the dissipation rate equation. From the computation of fully developed plane channel flow, it is found that the results with present model agree well with the data of direct numerical simulation in the predictions of stress components. And, the computed mean velocity profile coincides with the universal velocity law.

• Journal of the Korean Society of Propulsion Engineers
• /
• v.3 no.2
• /
• pp.1-9
• /
• 1999

Applicability of the pressure gradient method which is formulated based on pressure gradient is verified against turbulent flow analysis. In the pressure gradient method, pressure gradient instead of pressure itself is obtained using continuity constraint. Since correct pressure gradient is found only when mass conservation is satisfied, pressure gradient method can reflect physics of flow field properly The pressure gradient method is formulated with semi-staggered grid system which locates each primitive variables on the same grid point but evaluates pressure gradient in-between. This grid system ensures easy programming and reflection of correct physics in analysis. For verifying applicability of this method, the pressure gradient method is applied to turbulent flow analysis with low Reynolds number $\kappa$-$\varepsilon$ model. Turbulent flows include fully developed channel flow, backward-facing step flow, and conical diffuser flow. Prediction results show that the pressure gradient method can be applied to turbulent flow analysis. However, the pressure gradient method requires somewhat long computation time. Proper way to find optimum under-relaxation factor, $\gamma$, is also need to be developed.

• Transactions of the Korean Society of Mechanical Engineers
• /
• v.14 no.5
• /
• pp.1282-1289
• /
• 1990

A numerical solution for heat transfer of pulsating turbulent pipe flow was presented under the condition of fully developed dynamic regime and uniform well heat flux. The k-.epsilon. turbulent model was adopted to describe turbulent characteristics. The results were given at following conditions ; Time-averaged Reynolds number equal to 10000 ; Strouhal number ranged from 0.0005 to 0.05 ; The peak velocity fluctuation varied from 20 to 80 percent of the mean velocity. It was found that the effect of pulsation on local heat transfer rate is greater at downstream than upstream and the heat transfer was increased or decreased according to the pulsating conditions.

• Journal of the Society of Naval Architects of Korea
• /
• v.40 no.3
• /
• pp.9-15
• /
• 2003

An experimental study has been carried out as a basic research for development of the friction drag reduction technology for ships by polymer injection. Experimental apparatus and procedures have been devised and prepared to measure the changes of the wall friction with injection of a polymer solution and basic experimental data on the friction drag reduction are obtained for a turbulent fiat-plate boundary layer and fully-developed channel flows. Variations of the friction drag reduction with some important parameters of polymer injection, such as the concentration of polymer solution, its injection flow rate and the measuring position downstream from the injection slot, are also investigated. Important experimental data and results obtained in the present study are presented. The amount of friction drag reduction up to 50% is observed.

• Journal of Advanced Marine Engineering and Technology
• /
• v.22 no.3
• /
• pp.303-310
• /
• 1998

We report on the analytical results of examination of fully developed asymmetric flow and heat transfer between parallel plane plates. The asymmetry was introduced by roughening one of the plane while the other was left smooth. The integral method together with a turbulence model based on modified Prandtl's mixing length theory for the rough was used to determine the velocity distribution and friction. The temperature distrtibution is then predicted and heat transfer coefficients are calculated. The present paper shows that the heat transfer increases more than the friction factor for a given roughness structure. Generally the results show the strong effect of asymmetry on engineering parameters. Furthermore it is the roughness structure which influences the nature of asymmetry and heat transfer.