• 대한기계학회논문집
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• 제18권4호
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• pp.997-1008
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• 1994

Heat transfer by laminar oscillating flow in a circular pipe has been studied analytically. The general solution with respect to the arbitrary wall boundary condition is obtained by superposing the fluid temperatures with the sinusoidal wall temperature distributions. The fulid temperature distributions are two dimensional, but uniform flow assumption is used to simplify the velocity distribution. The heat transfer characteristics in the thermally developing regions are analyzed by applying the general solution to the two cases of thermal boundary conditions in which the wall temperature and wall heat flux distributions have a square-wave form, respectively. The results show that the length of the thermally developing region becomes larger in proportion to the oscillation frequency at slow oscillation and eventually approaches to the value comparable to the swept distance as the frequency increases. The time and cross-section averaged Nusselt number in the developing region is inversely proportional to the square root of the distance from the position where the wall boundary condition is changed suddenly. In the developed region, Nusselt number is only determined by the oscillation frequency.

• 대한기계학회논문집B
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• 제20권12호
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• pp.4003-4012
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• 1996

Hydro dynamically developing and simultaneously (hydro dynamically and thermally) developing laminar flows of a Bingham plastic in a circular pipe have been investigated numerically. Solutions have been obtained by using a four-step fractional method combined with an equal order bilinear finite element method. For the hydro dynamically developing flow, shorter entrance length is required to reach fully developed velocity field for larger yield stress and non-monotonic pressure drop along the pipe centerline is observed when the yield stress exceeds a certain critical value. For the simultaneously developing flow, the heat transfer characteristics show the same trends as those predicted for the thermally developing flow (Graetz problem).

• 대한기계학회논문집B
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• 제20권12호
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• pp.3991-4002
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• 1996

Thermally fully developed and thermally developing laminar flows of a Bingham plastic in a circular pipe have been studied analytically. For thermally fully developed flow, the Nusselt numbers and temperature profiles are presented in terms of the yield stress and Peclet number, proposing a correlation formula between the Nusselt number and the Peclet number. The solution to the Graetz problem has been obtained by using the method of separation of variables, where the resulting eigenvalue problem is solved approximately by using the method of weighted residuals. The effects of the yield stress, Peclet and Brinkman numbers on the Nusselt number are discussed.

• Journal of Mechanical Science and Technology
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• 제20권12호
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• pp.2230-2241
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• 2006

This paper presents a sole application of boundary element method to the conjugate heat transfer problem of thermally developing laminar flow in a thick walled pipe when the fluid velocities are fully developed. Due to the coupled mechanism of heat conduction in the solid region and heat convection in the fluid region, two separate solutions in the solid and fluid regions are sought to match the solid-fluid interface continuity condition. In this method, the dual reciprocity boundary element method (DRBEM) with the axial direction marching scheme is used to solve the heat convection problem and the conventional boundary element method (BEM) of axisymmetric model is applied to solve the heat conduction problem. An iterative and numerically stable BEM solution algorithm is presented, which uses the coupled interface conditions explicitly instead of uncoupled conditions. Both the local convective heat transfer coefficient at solid-fluid interface and the local mean fluid temperature are initially guessed and updated as the unknown interface thermal conditions in the iterative solution procedure. Two examples imposing uniform temperature and heat flux boundary conditions are tested in thermally developing region and compared with analytic solutions where available. The benchmark test results are shown to be in good agreement with the analytic solutions for both examples with different boundary conditions.

• 태양에너지
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• 제15권3호
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• pp.91-103
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• 1995

튜브 내의 입구영역에서 대류와 비회복사(mon-gray radiation)가 동시에 일어날 때의 열전달 특성을 수치해석적으로 연구하였다. 작동유체는 이산화탄소, 수증기, 질소의 혼합가스라 하였고, 유동은 속도장과 온도장이 동시에 발달하는 층류 유동으로 가정하였다. 복사전달방정식을 풀기 편하게 하기 위해 P-1 근사법이 사용되었고 혼합가스의 비회흡수계수(non-gray sbsorption coefficient)는 지수광폭밴드모형(exponential wide band model)을 이용해서 구하였다. 열전달 특성에 대한 온도조건의 영향을 조사하기 위해 튜브의 축방향에 대한 평균온도와 뉴셀트수(Nusselt number)의 변화를 몇 가지 다른 온도조건에 대해 나타내었다. 속도장과 온도장이 동시에 발달하는 경우의 뉴셀트수를 속도장은 완전히 발달되어 있고 온도장만 발달하는 경우의 뉴셀트 수와 비교하였다. 또한, 가스의 성분조성이 대류와 비회복사 뉴셀트수에 미치는 영향을 조사하였다.

• 대한기계학회논문집
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• 제9권6호
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• pp.767-776
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• 1985

본 연구에서는 초기 조건(입구 R$_{i}$수)의 변화에 따른 속도 분포, 온도 분 포, 확산율, 계면의 변화등을 연구하며, 난류 혼합과 계면의 불안정에 기인한 속도장 과 온도장의 변화과정을 가시화 사진과 비교 분석한다.다.

• 설비공학논문집
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• 제20권12호
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• pp.795-801
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• 2008

An experimental study has been performed on the single phase flow and convective heat transfer in trapezoidal microchannels. The microchannel was about $270{\mu}m$ wide, $800{\mu}m$ deep. and 7 mm long, which might ensure hydrodynamically fully-developed laminar flow at a low Reynolds number. The experiments were conducted with R1l3 and water, with the Reynolds number ranging from approximately 30 to 5000 for friction factor and 30 to 700 for the Nusselt number. Friction factors in laminar are found to be in good agreement with the predictions of existing correlation suggesting that a conventional analysis approach can be employed in predicting flow friction behavior in microchannels. However turbulent friction factors are hardly predictable by the existing correlations. The experimental results show that the Nusselt number is not a constant but increases almost linearly with the Reynolds number even the flow is fully developed (Re < 100). The dependence of the Nusslet number on the Reynolds number is contradictory to the conventional theory. At a Reynolds number greater than 100, the Nusselt number increases slowly with the Reynolds number, where thennally developing flow is responsible for the increase of the Nusselt number with the Reynolds number.

• 대한기계학회논문집B
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• 제24권4호
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• pp.495-503
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• 2000

The present numerical study investigates flow characteristics and heat transfer enhancement of the viscoelastic non-Newtonian fluid in a 2:1 rectangular duct. The combined effect of temperature-dependent viscosity, buoyancy and secondary flow caused by second normal stress difference are all considered. The Reiner-Rivlin model is used as a viscoelastic fluid model to simulate the secondary flow and temperature-dependent viscosity model is adopted. Three types of thermal boundary conditions involving different combinations of heated walls and adiabatic walls are considered in this study. Calculated Nusselt numbers are in good agreement with experimental results in both the thermal developing and thermally developed regions. The heat transfer enhancement can be explained by the combined viscoelasticity-driven secondary flow, buoyancy-induced secondary flow and temperature-dependent viscosity.

• 대한기계학회논문집B
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• 제28권9호
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• pp.1124-1132
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• 2004

The inverse problem of determining unknown inlet temperature in thermally developing, hydrodynamically developed two phase laminar flow in a parallel plate duct is considered. The inlet temperature profile is determined by measuring temperature in the flow field. No prior information is needed for the functional form of the inlet temperature profile. The inverse convection problem is solved by minimizing the objective function with regularization method. The conjugate gradient method as iterative method and the Tikhonov regularization method are employed. The effects of the functional form of inlet temperature, the number of measurement points and the measurement errors are investigated. The accuracy and efficiency of these two methods are compared and discussed.

• 대한설비공학회지:설비저널
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• 제17권4호
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• pp.382-394
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• 1988

Dynamic characteristics of thermally-forced stratification process in a square enclosure with a linear temperature profile at the side walls have been investigated through flow visualization experiment and numerical analysis. The experiment was performed on air with the Rayleigh numbers of order $10^5$. A particle tracer method is used for the flow visualization and to obtain a sudden linear temperature profile at the side walls copper blocks which already have a linear temperature profile are come into contact with the thin copper plates of the test section. Immediately a meridional circulation is developed and heat transfer takes place from the wall to the interior region by circulation of fluid and finally a thermal stratification is achieved. In the numerical study, QUICK scheme for convective terms, SIMPLE algorithm for pressure correction, and the implicit method for the time marching are adopted for the integration of conservation equations. Comparison of flow visualization and numerical results shows that the developing flow patterns are very similar in dynamic nature even though there is a time lag due to the inevitable time delay in setting up a linear temperature profile. For high Rayleigh numbers, the oscillatory motion is likely to take place and stratified region is extended. However, initial temperature adjustment process is much slower than that for low Rayleigh numbers.