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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.

The present work is concerned with the modelling of ascending turbulent ('mixed convection') flow in a vertical heated pipe. All fluid properties are assumed to be constant and buoyancy is accounted for within the Boussinesq approximation. Four Eddy Viscosity Models (EVMs) are examined against experimental and numerical (direct simulation) data. The EVMs embody distinct physical refinements with respect to the parent high-Reynolds-number k-ε model. New Large Eddy Simulations (LES) are also presented. Three different CFD codes have been employed in the study: 'CONVERT', 'Code_Saturne', and 'STAR-CD', which are respectively inhouse, industrial, and commercial packages. In general, forced convection flows are best resolved by the LES computations and Cotton-Ismael turbulence model (Cotton and Ismael [1998]). In mixed convection flows the picture changes and the Launder-Sharma closure [Launder and Sharma, 1974] and 'Manchester f v 2 − ' model [Keshmiri et al., 2008] are in closest agreement with the direct simulation heat transfer data. Under conditions of maximum heat transfer impairment, the mean flow and turbulence profiles are best captured by the Large Eddy Simulations and LS and f v 2 − models. However, no single scheme could be said to be in excellent agreement with the data examined.

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