A stepwise development and validation of a RANS based CFD modelling approach for the hydraulic and thermal-hydraulic analyses of liquid metal flow in a fuel assembly

Abstract

The demand of reliable and clean energy at affordable prices poses a formidable challenge to the world. The initiatives from various international organizations reserve an important role for liquid metal cooled reactor systems. Assessment of such reactors usually involves unconventional thermal-hydraulics. Consequently, Reynolds Averaged Navier Stokes (RANS) based Computational Fluid Dynamics (CFD) approaches are expected to play a vital role along with various ongoing experiments for the design and the safe operation of these nuclear reactors. One of the major issues is the heat transport in the fuel assembly by the liquid metal. The known difficulties in heat transfer experiments, especially with liquid metals, necessitate the application of RANS in computing details of flow and temperature distribution. Considering these aspects, a four step approach is described in the current paper. As a first step, the heat transfer in a liquid metal flow inside a heated tube is analyzed using a RANS approach and then compared with some of the empirical correlations. The computed Nusselt number reveals the required development length of the thermal boundary layer in liquid metal. Furthermore, these simulations reveal the need of further assessment of this approach and all the existing correlations, and the care that should be taken while applying one of these correlations. In the second step, numerical simulations of the flow of heavy liquid metal around a heated rod in an annular cavity confirm that a RANS strategy can be employed in liquid metal flows. Furthermore, a comparison between computed and experimental non-dimensional axial temperature at the heated rod surface shows that among the considered turbulence models the use of a Baseline-Reynolds Stress Model (BSL-RSM) with automatic wall treatment (AWT) can be preferred for complex geometries. This is also demonstrated in the third step by computing the flow distribution in a triangular arrangement of a fuel assembly and by comparing with an existing hydraulics experiment in rod-bundle. These analyses reveal that the use of the BSL-RSM turbulence model with AWT allows prediction of the cross-flow in this rod-bundle. As the forth and last step, the integral TEGENA (TEmperatur- und GEschwindigkeitsverteilungen in Stabbündel mit turbulenter NAtriumströmung) experiment has been selected for further assessment of RANS based CFD approach in computing both the flow and temperature field. A comparison with literature demonstrates that the use of symmetric boundary condition in such a tightly packed parallel rod-bundle leads to a distorted flow field. The experimentally and the computationally obtained temperature field at a plane in the outlet reveal its acceptable predictive capability. Furthermore, application of two different Reynolds Stress Models yields almost the same temperature distribution as a result of the use of simple first-order gradient model for the turbulent heat fluxes. Consequently, these four steps support the use of this modelling approach for investigating the heat transport in (heavy) liquid metals. Finally, the preferred RANS approach has been applied for thermal-hydraulic evaluation in the square arrangement of a bare rod-bundle as is to be employed in the European Lead-cooled reactor System (ELSY). Also, the influence of rod pitch-to-diameter ratio has been analyzed by numerically re-arranging these rods in the different square lattice. Moreover, arranging the bare rods in triangular lattice at the different rod pitch-to-diameter ratios shows the effect of the square and triangular lattice in thermal-hydraulics. Lastly, comparisons with some of the existing heat transfer correlations for the triangular and the rectangular lattice allows us to identify preferred correlations for these lattice arrangements of bare and liquid metal cooled rod-bundles.