A coupling approach between resolved and coarse-grid sub-channel CFD

As a follow-up development of the Computational Fluid Dynamics (CFD)-based sub-channel analysis tool, i.e. coarse-grid Sub-Channel CFD (SubChCFD), this paper aims at developing a coupling between SubChCFD and resolved CFD, thereby enhancing the performance and application flexibility of the coarse-grid model. A time-explicit domain-overlapping method is used to achieve the coupling, which ensures good flexibility and reasonable numerical stability. In such a coupling framework, embedded resolved sub-models are to be placed arbitrarily into a SubChCFD baseline model in regions selected for refinement. Two coupling modes are available: the one-way coupling mode, where the SubChCFD model provides the boundary conditions for the resolved sub-models, but no feedback from the resolved sub-model to the SubChCFD model is carried out; the two-way coupling mode, where feedback is enabled from the resolved sub-model back to the SubChCFD model to improve the solution of the latter.

The coupling methodology has been first tested using 2-D flow cases, including an internal flow at a T-junction and an external flow passing a square cylinder. It has then been applied to 3-D cases of nuclear rod bundles with complex conditions. One is a 7 × 7 rod bundle with locally ‘ballooned’ fuel rods where complex flow phenomena occur due to the blockage effect caused by area reduction in flow passages. The other is a 5 × 5 rod bundle with inward jet flow at one corner of the housing walls resulting in a strong cross flow. In all of the test cases, the results of the coarse-grid SubChCFD model with the two-way coupling approach are consistently improved compared with those of the uncoupled SubChCFD simulations.