Refined Eddy Viscosity Schemes and Large Eddy Simulations for Ascending Mixed Convection Flows

Volume 2: Fuel Cycle and High Level Waste Management; Computational Fluid Dynamics, Neutronics Methods and Coupled Codes; Stude

Cotton

Addad

et al. 2008

Self Cite

Coolant flows in the cores of current gas-cooled nuclear reactors consist of ascending vertical flows in a large number of parallel passages. Under post-trip conditions such heated turbulent flows may be significantly modified from the forced convection condition by the action of buoyancy, and the thermal-hydraulic regime is no longer one of pure forced convection. These modifications are primarily associated with changes to the turbulence structure, and indeed flow laminarization may occur. In the laminarization situation heat transfer rates may be as low as 40% of those in the corresponding forced convection case. The heat transfer performance of such 'mixed' convection flows is investigated here using a range of refined ReynoldsAveraged-Navier-Stokes (RANS) turbulence models. While all belong to the broad class of Eddy Viscosity Models (EVMs), the various RANS closures have different physical parameterizations and might therefore be expected to show different responses to externally-imposed conditions. Comparison is made against experimental and Direct Numerical Simulation (DNS) data. In addition, Large Eddy Simulation (LES) results have been generated as part of the study. Three different CFD codes have been employed in the work: 'CONVERT', 'STAR-CD', and 'Code_Saturne', which are respectively in-house, commercial, and industrial packages. It is found that the early EVM scheme of Launder and Sharma [1] is in the closest agreement with consistentlynormalized DNS results for the ratio of mixed-to-forced convection Nusselt number (Nu/Nu 0 ). However, in relation to DNS and experimental data for forced convection Nusselt number, other models perform better than the LaunderSharma scheme. The present investigation has revealed discrepancies between direct-simulation, experimental, and the current LES studies.

“…A more complete assessment of a turbulence model requires an examination of mean flow and turbulence profiles and this will be done in a companion work [22]. …”

Section: Discussionmentioning

confidence: 99%

“…More detailed descriptions of the three turbulence models may be found in the original papers and Keshmiri et al [22].…”

Section: Refined Eddy-viscosity Models Implemented Inmentioning

confidence: 99%

RANS and LES Investigations of Vertical Flows in the Fuel Passages of Gas-Cooled Nuclear Reactors

Volume 2: Fuel Cycle and High Level Waste Management; Computational Fluid Dynamics, Neutronics Methods and Coupled Codes; Stude

Cotton

Addad

et al. 2008

Self Cite

“…The commercial code 'STAR-CD' [11] is used to generate results using the low-Reynoldsnumber k-ε model of Lien, Chen and Leschziner [12] ('LRN k-ε model', below), the k-ω-SST model of Menter [13] ('LRN k-ω-SST model'), and a variant of Durbin's v 2 -f model due to Iaccarino [14] ('v 2 -f model'). In earlier works by the authors and their colleagues, the turbulence models in STAR-CD have been validated against in-house and industrial codes with the same turbulence models [15,16]. All fluid properties are assumed to be constant.…”

Section: Numerical Proceduresmentioning

confidence: 99%

Thermal-hydraulic analysis of flow in rib-roughened passages with application to gas-cooled nuclear reactors

Proceedings of the Sixth International Symposium on Turbulence, Heat and Mass Transfer

Cotton

Addad

et al. 2009

Self Cite

The present paper reports RANS simulations of the flow and heat transfer in a 2-dimensional ribroughened passage. Rib pitch-to-height ratios of 6, 9, and 12 are examined. The blockage ratio of the transversely-mounted rectangular ribs is 10%, making the case 'very rough'. The Reynolds number, based on the channel bulk velocity and hydraulic diameter, is 30,000. Three low-Reynolds-number linear EVMs, namely the Lien-Chen-Leschziner k-ε model, the Menter k-ω-SST model, and a variant of Durbin's v 2-f formulation are examined. Results for a square rib profile are compared to those for the rib profile used in the fuel pins of Advanced Gas-cooled Reactors. All computations are undertaken using the commercial CFD code 'STAR-CD'. In comparison with experimental data, it emerges that the v 2-f model generally returns more accurate results than the k-ε or k-ω-SST closures.

“…In the present study, two turbulence models of the Eddy-Viscosity type are employed. (A number of models of this general class have been used by the author and his colleagues in the computation of smooth surface buoyancy-affected, or 'mixed convection' flows [2][3][4] …”

Section: Introductionmentioning

confidence: 99%

Numerical sensitivity analysis of 3- and 2- dimensional rib-roughened channels

2012

Heat Mass Transfer

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Rough surfaces have been used as a tool to enhance heat transfer by increasing the level of turbulence mixing in the flow. In numerically simulating such flows, it is common to simulate a 3D rib-roughened channel with a 2D domain in order to reduce the computational time and power. The main purpose of the present work is to investigate the accuracy of the above approximation. In order to do so, initially a 3D channel is simulated using ReynoldsAveraged Navier-Stokes technique and comparison is made against 2D simulations as well as experimental data. In addition, the effects of rib thermal boundary condition and near-wall treatments are also investigated. All computations are undertaken using the commercial CFD code 'STAR-CD'. The Reynolds number, based on the channel bulk velocity and hydraulic diameter, is 30,000. Two lowReynolds-number linear Eddy-Viscosity Models, namely the Lien-Chen-Leschziner k -e model and a variant of Durbin's v 2 -f formulation are used. In the CFD simulations reported here, the focus is on the experimental data of Rau et al. (ASME J Turbomach 120:368-375, 1998). It was found that the present results for a 3D channel are in relatively good agreement with the data. It was also shown that a 2D channel can be used to represent the flow in the centre-line of a 3D channel with relatively good accuracy.