Sub-channel CFD for nuclear fuel bundles

Abstract

This paper presents a novel Computational Fluid Dynamics (CFD)-based sub-channel framework for nuclear power plants, which combines the advantageous features of modern CFD and traditional 1-D subchannel codes. The new method is capable of producing CFD-level 3-D results with locally desirable refinement when coupled with embedded resolved models, but because a very coarse mesh is used over most part of the domain, the computing cost is typically significantly smaller than that of a conventional CFD simulation. In this new Sub-Channel CFD (SubChCFD), a dual-mesh system is used, comprising, (i) a filtering mesh which aligns with the mesh used in typical sub-channel codes, enabling the use of existing engineering correlations to account for integral wall friction and heat transfer effects, and (ii) a computing mesh, which provides a platform for the solution of the governing equations with turbulence modelled using a mixinglength-type model. The method has been implemented in an open-source finite volume CFD code Code_Saturne and validated initially using a 5×5 bare bundle case based on the OECD/NEA MATiS-H benchmarking experiment. It has been found that SubChCFD is able of satisfactorily predicting both the velocity and temperature fields. To further investigate the performance for complex flow conditions, SubChCFD was applied to two full 3-D cases. The first is a 5×5 rod bundle case with local blockage in one of the sub-channels, creating significant localised cross flows. The second is a two-parallel-assembly channel with different input mass flow rates at the inlet of each assembly, allowing strong inter-assembly mixing. For both cases, SubChCFD has produced results which agree well with experiments and simulations using resolved CFD. It has also been demonstrated that SubChCFD exhibits excellent flexibility in comparison with traditional sub-channel codes and that it has the potential to serve as a substitution to sub-channel codes.

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

“…As explained in the introduction, SubChCFD is a blend of CFD and sub-channel code methodology (Liu et al, 2019). A dual mesh approach is used, including, namely, (i) a filtering mesh which aligns with the mesh used in typical sub-channel codes, enabling the integral wall friction and heat transfer effects calculated using existing engineering correlations, and (ii) a computing mesh, on which the Reynolds Average Navier-Stokes (RANS) equations are solved with a near wall closure method based on calculations of step (i).…”

“…The advanced Computational Fluid Dynamics (CFD) method is in principle superior to the traditional tools, but it still suffers from a long turnaround in computation time, which limits its application in addressing realistic engineering problems (Hanna et al, 2020;Viellieber and Class, 2015). To fill the gap, a hybrid technique, a CFD-based sub-channel analysis tool, has been proposed in our previous work (Liu et al, 2019), which is referred to as Sub-Channel CFD (SubChCFD).…”

“…Similar to sub-channel codes, fully validated industry-standard correlations are employed for wall modelling closure to ensure a higher modelling consistency so as to reduce the uncertainty of numerical simulations for specific designs of reactors. A detailed description of the baseline SubChCFD can be found in Liu et al (2019).…”

Coupled porous media approaches in sub-channel CFD

“…However, it is still not feasible to use CFD to perform core-level design calculations and safety relevant analyses for industry due to the prohibitively high computing cost required in resolving the numerous length scales of the flow in a reactor core, despite the remarkable advancement achieved recently in high performance computing techniques (Hanna et al, 2020;Viellieber and Class, 2015). To facilitate the application of CFD in the industry, a hybrid technique, CFD-based sub-channel analysis tool (referred to as Sub-Channel CFD (SubChCFD)), has been proposed in our previous work (Liu et al, 2019), which has proven to be very promising and has a potential to replace or supplement the traditional sub-channel analysis tools.…”

“…In essence, SubChCFD is a coarse mesh CFD approach, in which a full set of 3-D conservation equations are solved on a very coarse mesh. The mesh used is usually body-fitted and resolves the main geometry of the flow passages in a fuel assembly (namely the sub-channels), which, for example, consists of only 20 to 40 elements per layer of mesh for a typical interior-type sub-channel in a square lattice Pressurised Water-cooled Reactor (PWR) rod bundle (Liu et al, 2019). This greatly reduces the computing cost compared with conventional CFD.…”