Coupled porous media approaches in sub-channel CFD

In this paper, the porous media method is introduced into a recently developed Computational Fluid Dynamics (CFD)-based sub-channel analysis tool, Sub-Channel CFD (SubChCFD) through direct coupling to improve its capability in modelling complex structures, such as spacers or distorted fuel rod bundles. Two coupling methods have been developed, namely, embedding and interfacing coupling. The former is suitable for rod bundles with attached fine structures which do not change the basic geometry of the sub-channels, so a SubChCFD baseline model can be created based on the bare bundle configuration. Porous media sub-models can then be embedded in this model at locations where fine structures are installed to account for their influence on the flow and pressure drop. The interfacing method is more general and flexible, and used in a wider range of applications. In this method, separate meshes are used for the sub-domains where the porous media method is applied. These sub-domains are then interfaced with the domain covered by SubChCFD using a meshing joining technique and they are simulated together using a single set of governing equations. The methods developed are tested and validated with simulations of two rod bundle configurations, a 7 × 7 rod bundle with locally ballooned fuel rods and a 5 × 5 rod bundle equipped with simple support grids. It has been shown that a suitably selected method of the coupling can greatly simplify the modelling of complex structures using SubChCFD, thus providing additional flexibility and functionality to this newly developed CFD-based sub-channel framework.