A multiscale model of a rod bundle using subchannel CFD

This paper deals with a coarse-mesh Computational Fluid Dynamics (CFD) approach, referred to as Subchannel CFD (SubChCFD), which combines features of the traditional 1-D subchannel analysis tools used in nuclear thermal hydraulic analyses and modern CFD. The SubChCFD, which has been previously developed for flow simulations, has been extended to perform thermal simulations and is then applied to a complex nuclear fuel bundle to explore and demonstrate the capacity of this new tool. SubChCFD allows resolved CFD sub-models to be nested into the regions that are of interest to locally improve the prediction, and can be coupled with the porous media approach to deal with any sub-scale fine structures that are difficult to be handled using a coarse mesh.

In the present case studied, a SubChCFD baseline model is created, covering the entire heated length of the test geometry, to capture the axial developments of the flow and heat transfer in the rod bundle. Spacer grids that are used to keep the rods in place are modelled as embedded porous media to account for the associated blockage effect and pressure losses. In addition, a resolved CFD sub-model is created and coupled with the baseline model to improve the prediction for the region where high fidelity experimental measurements were performed. Through the present test, SubChCFD shows good predictability, flexibility and scalability in modelling large nuclear reactor components with complex internal structures. With the advanced coupling functionality, it is able to produce comparable simulation results to that of conventional CFD methods for regions of interest, with greatly reduced computational cost.