• Transactions of the Korean Society of Mechanical Engineers
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• 제17권12호
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• pp.3196-3207
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• 1993

A second moment turbulent closure for the turbulent heat flux near a wall is developed by modification of model constants in pressure interaction term as the variables of the turbulent Reynolds number using the universal properties of turbulent heat flux near the wall. The present model shows that model constant for the wall reflection term in pressure interaction is most important in modelling of the near wall heat flux. Fully developed pipe flows with constant wall heat flux are tested to validate the proposed model. In most of calculation region, the predicted turbulent properties agree better with the experimetal data than the results from standard algebraic heat flux model which use the uniform model constants.

• Korean Journal of Air-Conditioning and Refrigeration Engineering
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• 제6권4호
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• pp.325-336
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• 1994

A numerical simulation of velocity and temperature fields and Nusselt number distributions is performed by using the algebraic stress model (ASM) for the velocity profiles and low Reynolds number ${\kappa}-{\varepsilon}$ model and the algebraic heat flux model(AHFM) for turbulent heat transfer in a $180^{\circ}$ bend with a constant wall heat flux. In the low Reynolds number ${\kappa}-{\varepsilon}$ model, turbulent Prandtl number is modified by considering the streamline curvature effect and the non-equilibrium effect between turbulent kinetic energy production and dissipation rate. Every heat flux term presented in the transport equation of turbulent heat flux is reduced to algebraic expressions in a way similar to algebraic stress model. Also. in the wall region, low Reynods number algebraic heat flux model(AHFM) is applied.

• Nuclear Engineering and Technology
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• 제54권11호
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• pp.4359-4372
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• 2022

Turbulent mixing in buoyant flows is an essential mechanism involved in many scenarios related to nuclear safety in nuclear power plants. Comprehensive understanding and accurate predictions of turbulent buoyant flows in the reactor are of crucial importance, due to the function of mitigating the potential detrimental consequences during postulated accidents. The present study uses URANS methodology to investigate the buoyancy-influenced flows in the reactor pressure vessel under the main steam line break accident scenarios. With a particular focus on the influence of turbulent heat flux closure models, various combinations of two turbulence models and three turbulent heat flux models are utilized for the numerical simulations of three ROCOM tests which have different characteristic features in terms of the flow rate and fluid density difference between loops. The simulation results are compared with experimental measurements of the so-called mixing scalar in the downcomer and at the core inlet. The study shows that the anisotropic turbulent heat flux models are able to improve the accuracy of the predictions under conditions of strong buoyancy whilst in the weak buoyancy case, a major role is played by the selected turbulence models with essentially a negligible influence of the turbulent heat flux closure models.

• 한국연소학회:학술대회논문집
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• 한국연소학회 2000년도 제20회 KOSCO SYMPOSIUM 논문집
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• pp.122-132
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• 2000

Computational fluid dynamic simulation is performed for the floating turbulent premixed flames stabilized in stagnation flows of Cho et al. [1] and Cheng and Shepherd [2]. They are both in the wrinkled flamelet regime far from the extinction limit with $u'/S^{0}_{L}$ less than unity. The turbulent flux is given in the first moment closure as a sum of the classical gradient flux due to turbulent motions and the countergradient flux due to thermal expansion. The parameter $N_{B}'s$ are greater than unity with the countergradient flux dominant over the gradient flux. The countergradient flux is assumed to be zero in $\bar{c}<0.05$. The flame surface density is modeled as a symmetric parabolic function with respect to $\bar{c}$. The product of the maximum flame surface density and the mean stretch factor is considered as a tuning constant to match the flame location. Good agreement is achieved with the measured $\tilde{w}$ and $\bar{c}$ profiles along the axis in both flames.

• 한국전산유체공학회:학술대회논문집
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• 한국전산유체공학회 2007년도 춘계 학술대회논문집
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• pp.171-176
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• 2007

A comparative study on the treatment of the turbulent heat flux with the elliptic mlending second moment closure for a natural convection is performed. Four cases of different treating the turbulent heat flux are considered. Those are the generalized gradient diffusion hypothesis (GGDH) the algebraic flux model (AFM) and the differential heat flux model (DFM). These models are implemented in the computer code specially designed for evaluation of turbulent models. Calculations are performed for a turbulent natural convection in the 1:5 rectangular cavity and the calculated results are compared with the experimental data. The results show that three models produce nearly the same accuracy of solutions.

• Journal of computational fluids engineering
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• 제12권2호
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• pp.26-31
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• 2007

A comparative study on the treatment of the turbulent heat flux with the elliptic blending second-moment closure for a natural convection flow is performed. Three cases of different treating the turbulent heat flux are considered. Those are the generalized gradient diffusion hypothesis (GGDH), the algebraic flux model (AFM) and the differential flux model (DFM). The constants in the models are adjusted with a primary emphasis placed on the accuracy of predicting the local Nusselt number. These models are implemented in a computer code specially designed for evaluation of turbulent models. Calculations are performed for a turbulent natural convection in the 1:5 rectangular cavity and the calculated results are compared with the available experimental data. The results show that the three models produce nearly the same accuracy of solutions. These results show that the GGDH, AFM and DFM models for treating the turbulent heat flux are sufficient for this simple shear flow where the shear production is dominant. It is observed that, in the weakly stratified region at the center zone of the cavity, the vertical velocity fluctuation is nearly zero in the GGDH solutions, which shows that the GGDH model may not be suitable for the strongly stratified flow. Thus, further study on the strongly stratified flow should be followed.

• Journal of Mechanical Science and Technology
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• 제18권10호
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• pp.1782-1798
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• 2004

A numerical study of a natural convection in a rectangular cavity with the low-Reynolds-number differential stress and flux model is presented. The primary emphasis of the study is placed on the investigation of the accuracy and numerical stability of the low-Reynolds-number differential stress and flux model for a natural convection problem. The turbulence model considered in the study is that developed by Peeters and Henkes (1992) and further refined by Dol and Hanjalic (2001), and this model is applied to the prediction of a natural convection in a rectangular cavity together with the two-layer model, the shear stress transport model and the time-scale bound ν$^2$- f model, all with an algebraic heat flux model. The computed results are compared with the experimental data commonly used for the validation of the turbulence models. It is shown that the low-Reynolds-number differential stress and flux model predicts well the mean velocity and temperature, the vertical velocity fluctuation, the Reynolds shear stress, the horizontal turbulent heat flux, the local Nusselt number and the wall shear stress, but slightly under-predicts the vertical turbulent heat flux. The performance of the ν$^2$- f model is comparable to that of the low-Reynolds-number differential stress and flux model except for the over-prediction of the horizontal turbulent heat flux. The two-layer model predicts poorly the mean vertical velocity component and under-predicts the wall shear stress and the local Nusselt number. The shear stress transport model predicts well the mean velocity, but the general performance of the shear stress transport model is nearly the same as that of the two-layer model, under-predicting the local Nusselt number and the turbulent quantities.

• Journal of computational fluids engineering
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• 제21권4호
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• pp.102-111
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• 2016

In this paper a computation of turbulent natural convection in enclosures with the elliptic-blending based differential and algebraic flux models is presented. The primary emphasis of the study is placed on an investigation of accuracy of the treatment of turbulent heat fluxes with the elliptic-blending second-moment closure for the turbulent natural convection flows. The turbulent heat fluxes in this study are treated by the elliptic-blending based algebraic and differential flux models. The previous turbulence model constants are adjusted to produce accurate solutions. The proposed models are applied to the prediction of turbulent natural convections in a 1:5 rectangular cavity and in a square cavity with conducting top and bottom walls, which are commonly used for validation of the turbulence models. The relative performance between the algebraic and differential flux model is examined through comparing with experimental data. It is shown that both the elliptic-blending based models predict well the mean velocity and temperature, thereby the wall shear stress and Nusselt number. It is also shown that the elliptic-blending based algebraic flux model produces solutions which are as accurate as those by the differential flux model.

• Transactions of the Korean Society of Mechanical Engineers B
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• 제22권4호
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• pp.417-427
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• 1998

The effect of secondary flow on the heat transfer of a turbulent wake generated by a flat plate was experimentally investigated. The secondary flow was induced in a curved duct in which the flat plate wake generator was installed. All three components of turbulent heat flux were measured in the plane containing the mean radius of curvature of the curved duct. The results showed that mean temperature profiles deviate from the similarity of the straight wake because of the cold fluid transported from the free-stream. The half-width of the mean temperature profile increased rapidly by upwash motion of the secondary flow. The changes to turbulence structure caused by the secondary flow show more pronounced effect on heat transport than on momentum transport. This is because the response to the variation of flow conditions is delayed in temperature field. Negative production of the turbulent heat flux is observed in the inner wake region. From the conditional averaging, it has been found that the negative production of the turbulent heat flux is generated due to a mixing process between the hot and low momentum eddies occupied in the inner wake region and the cold and high momentum eddies in the potential region.

• Transactions of the Korean Society of Mechanical Engineers B
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• 제29권11호
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• pp.1265-1276
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• 2005

The elliptic conceptual second moment models for turbulent heat fluxes, which are proposed on the basis of elliptic-blending and elliptic-relaxation equations, are applied to calculate the combined forced and natural turbulent convection in a vertical plane channel. The models satisfy the near-wall balance between viscous diffusion, viscous dissipation and temperature-pressure gradient correlation, and also have the characteristics of approaching its respective conventional high Reynolds number model far away from the wall. Also the models are closely linked to the elliptic blending model which is used for the prediction of Reynolds stress. In order to calibrate the heat flux models, firstly, the distributions of mean temperature and scala flux in fully developed channel flow with constant wall difference temperature are solved by the present models. The buoyancy effect on the turbulent characteristics including the mean velocity and temperature, the Reynolds stress tensor, and the turbulent heat flux vector are examined. In the opposing flow, the turbulent transport is greatly enhanced with both the Reynolds stresses and the turbulent heat fluxes being remarkably increased; whereas, in the aiding flow, the opposite change is observed. The results of prediction are directly compared to the DNS to assess the performance of the model predictions and show that the behaviors of the turbulent heat transfer in the whole flow region are well captured by the present models.