• 대한기계학회논문집
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• 제17권8호
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• pp.2069-2078
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• 1993

The buoyancy-driven turbulent thermal convection is predicted using an anisotropic hybrid turbulence model, which is incorporated with a low Reynolds k-.epsilon. turbulence model and an anisotropic buoyant part of algebraic stress model(ASM). The numerical predictions are compared with the Davidson's model,(1) the full ASM and the experimental results of Cheesewright et al.(2) All the models are shown to predict good agreements with the experiments for the averaged turbulence quantities. It is found that the effect of an anisotropic part on the Reynolds stress and the turbulent heat fluxes is substantial. In this study, the present hybrid model gives a fairly reasonable prediction in terms of the computational accuracy, convergence and stability. The contribution of an anisotropic buoyant part to turbulent heat fluxes are also scrutinized over the range of Rayleigh numbers $(4.79{\times}10^{10}{\le}Ra{\le}7.46{\times}10^{10}).$

• 한국전산유체공학회지
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• 제16권4호
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• pp.39-46
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• 2011

A numerical study of a turbulent natural convection in an enclosure with the lattice Boltzmann method (LBM) is presented. The primary emphasis of the present study is placed on investigation of accuracy and numerical stability of the LBM for the turbulent natural convection flow. A HYBRID method in which the thermal equation is solved by the conventional Reynolds averaged Navier-Stokes equation method while the conservation of mass and momentum equations are resolved by the LBM is employed in the present study. The elliptic-relaxation model is employed for the turbulence model and the turbulent heat fluxes are treated by the algebraic flux model. All the governing equations are discretized on a cell-centered, non-uniform grid using the finite-volume method. The convection terms are treated by a second-order central-difference scheme with the deferred correction way to ensure accuracy and stability of solutions. The present LBM is applied to the prediction of a turbulent natural convection in a rectangular cavity and the computed results are compared with the experimental data commonly used for the validation of turbulence models and those by the conventional finite-volume method. It is shown that the LBM with the present HYBRID thermal model predicts the mean velocity components and turbulent quantities which are as good as those by the conventional finite-volume method. It is also found that the accuracy and stability of the solution is significantly affected by the treatment of the convection term, especially near the wall.

• 한국전산유체공학회지
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• 제12권3호
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• pp.55-61
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• 2007

A computational study on a strongly stratified natural convection is performed with the elliptic blending second-moment closure. The turbulent heat flux is treated by both the algebraic flux model (AFM) and the differential flux model (DFM). Calculations are performed for a turbulent natural convection in a square cavity with conducting top and bottom walls and the calculated results are compared with the available experimental data. The results show that both the AFM and DFM models produce very accurate solutions with the elliptic-blending second-moment closure without invoking any numerical stability problems. These results show that the AFM and DFM models for treating the turbulent heat flux are sufficient for this strongly stratified flow. However, a slight difference between two models is observed for some variables.

• Nuclear Engineering and Technology
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• 제30권6호
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• pp.581-591
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• 1998

Numerical analysis was peformed for the two-dimensional turbulent natural convection for a long horizontal line with different end temperatures. The turbulent model has been applied a standard k-$\varepsilon$ two equation model of turbulence similar to that the proposed by the Launder and Spalding. The dimensionless governing equations are solved by using SIMPLE (Semi-Implicit Method for Pressure Linked Equations) algorithm which is developed using control volumes and staggered grids. The numerical results are verified by comparison with the operating PWR test data. The analysis focuses on the effects of variation of the heat transfer rates at the pipe surface, the thermal conductivities of the pipe material and the thickness of the pipe wall on the thermal stratification. The results show that the heat transfer rate at the pipe surface is the controlling parameter for mitigating of thermal stratification in the long horizontal pipe. A significant reduction and disappearance of the thermal stratification phenomenon is observed at the Biot number of 4.82$\times$10$^{-1}$ . The results also show that the increment of the thermal conductivity and thickness of the wall weakens a little the thermal stratification and somewhat reduces temperature gradient of y-direction in the pipe wall. These effects are however minor, when compared with those due to the variation of the heat transfer rates at the surface of the pipe wall.

• 설비공학논문집
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• 제7권4호
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• pp.556-565
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• 1995

The physical model of interest is based upon the concentric cylinder, where the outside cylinder is filled with optically thick and high temperature phase change material(PCM). The fluid is flowing through the inside cylinder to transfer the appropriate energy. The fluid is flowing through the inside cylinder to transfer the appropriate energy. The governing equations for the phase change material including internal thermal radiation and for the turbulent transfer fluid have been employed and numerically solved. The optically thick phase change justifies the P-l spherical harmonics approximation, which is believed to be appropriate choice particularly for the much coupled problem like in this study. The solid/liquid interface, temperature distribution within the PCM and the heat flux from the PCM to the transfer fluid have been obtained and compared with those of laminar transfer fluid. The numerical results show that the turbulent transfer fluid accelerates the solid/liquid interface and results in the increase of heat transfer rate from the PCM. The internal thermal radiation within the PCM, however, does not always playa role to increase the heat transfer rate throughout the inside cylinder. It is believed that the combined heat flux has been picked up more in the inflowing area than in the pure conductive phase change material.

• 설비공학논문집
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• 제11권2호
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• pp.157-166
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• 1999

Numerical calculations of turbulent natural convection in an enclosure of the 20 kYA oil-immersed transformer model are presented. The transformer is modelled as two concentric cylinders with different heights and diameters. The thermal boundary layers are well represented in the temperature distributions along the wall of the transformer model. The flow stratification between the hot and cold walls can not be seen in the transformer model. The turbulence eddy viscosity has its maximum at the center of the core and its maximum values at the top of the core are larger than those at the bottom of the core.

• 대한기계학회논문집
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• 제12권5호
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• pp.1138-1149
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• 1988

본 연구에서는 논란의 여지가 많은 문제점을 구간별 특성 척도 분석으로 보다 명확히 하고, 매질이 물과 공기인 경우 실험을 통하여 난류 열대류 구조를 분석하여 척도분석 및 Kraichnan등의 분석을 통해 확인된 바 있는 지수법칙층의 존재를 확인하 고자 한다.

• 한국전산유체공학회:학술대회논문집
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• 한국전산유체공학회 2006년도 추계 학술대회논문집
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• pp.77-82
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• 2006

Evaluation of the elliptic blending turbulence model (EBM) together with the two-layer model, shear stress transport (SST) model and elliptic relaxation model (V2-F) is performed for a better prediction of natural convection and thermal stratification. For a natural convection problem the models are applied to the prediction of a natural convection in a rectangular cavity and the computed results are compared with the experimental data. It is shown that the elliptic blending model predicts as good as or better than the existing second moment differential stress and flux model for the mean velocity and turbulent quantities. For thermal stratification problem the models are applied to the thermal stratification in the upper plenum of liquid metal reactor. In this analysis there exist much differences between the turbulence models in predicting the temporal variation of temperature. The V2-F model and EBM better predict the steep gradient of temperature at the interface of thermal stratification, and the V2-F model and EBM predict properly the oscillation of temperature. The two-layer model and SST model fail to predict the temporal oscillation of temperature.

• 대한기계학회논문집
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• 제12권1호
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• pp.137-151
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• 1988

본 논문에서는 아래 평판은 높은 온도 $T_{h}$로 유지되고, 위의 평판은 낮은 온도 $T_{i}$로 유지되어 있을 때 온도차에 의하여 발생하는 난류 열대류 문제를 난류모델 방정식을 사용하여 수치적으로 해석하고자 한다.다.다.

• 한국윤활학회:학술대회논문집
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• 한국윤활학회 2000년도 제31회 춘계학술대회
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• pp.93-98
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• 2000

Turbulence on the journal bearing operation is examined. And the thermal variability is studied for isothermal, convective and adiabatic conditions on the walls within some degree of journal misalignment. An efficient algorithm for the solution of the coupled turbulent Reynolds and energy equations is used to examine the effects of the various factors. The calculation data of turbulent analysis are compared with the ones of laminar analysis. Heat convection is found to play but a small role in determining friction and load. The friction distribution patterns through inside a journal bearing now appear different with high values at the front part of the bearing due to the high speed and low temperature, and a sudden decrease past the pressure maximum.