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

Abstract

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.

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

“…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.…”

“…The paths leading to the advancement of the Manchester f v 2 − model are described in some detail in a companion paper by the authors and their colleagues (Keshmiri et al [2008]). In terms of the chronology of the model type development, it is sufficient here to note that that the original f v 2 − scheme was proposed by Durbin [1991]; refinements were subsequently advanced by Lien and Durbin [1996], Hanjalić et al [2004] and Laurence et al [2004].…”

“…The turbulence models, which all embody different modifications of the parent form, are those of Launder and Sharma ['LS', 1974], Cotton and Ismael ['CI', 1998], Craft, Launder and Suga ['Suga', 1996] and the 'Manchester f v 2 − ' scheme (details of which are given in Keshmiri et al [2008]). The classical Smagorinsky/Lilly Sub-Grid-Scale model is adopted for the LES computations.…”

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

“…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.…”

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

“…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] …”