Simulating fuel assemblies with low resolution CFD approaches

In addition to the traditional fuel assembly simulations using system codes, subchannel codes or porous medium approaches, as well as detailed CFD simulations to analyze single sub channels, a Low Resolution Geometry Resolving (LRGR) CFD approach and a Coarse-Grid-CFD (CGCFD) approach are taken. Both methods are based on a low resolution mesh that allows the capture of large and medium scale flow features such as recirculation zones, which are difficult to be reproduced by the system codes, subchannel codes and porous media approaches.The LRGR approach allows for instance fine-tuning the porous parameters which are important input for a porous medium approach. However, it should be noted that the prediction of detailed flow features such as secondary flows (small flows in the direction perpendicular to the main flow) is not feasible. Using this approach, the consequences of flow blockages for detection possibilities and cladding temperatures can be discussed.The goal of the CGCFD approach with SGM is that it can be applied to simulate complete fuel assemblies or even complete cores capturing the unique features of the complex flow induced by the fuel assembly geometry and its spacers. In such a case, grids with a very low grid resolution are employed. Within the CGCFD a subgrid model (SGM) accounts for sub grid volumetric forces which are derived from validated CFD simulations. The volumetric forces take account of the non resolved physics due to the coarse mesh. The current paper discusses and presents both, the CGCFD and the LRGR approaches.

► Justification for development of low resolution mesh approaches.
► Mathematical background of the approaches.
► Meshing considerations for different approaches are presented.
► Examples of applications are provided.