Numerical Simulation of the Flow in a Tight Lattice SFR Rod-Bundle with Grid Spacers
NURETH-14 - 2011 September 25-30

Presented at:
2011 September 25-30
Toronto, Canada
Session Title:
A13-1 Coherent Large Scale Structures in the Gaps of Rod-Bundles

Elia Merzari (Argonne National Laboratory)
Paul Fischer (Argonne National Laboratory)
William D. Pointer (Argonne National Laboratory)
Hisashi Ninokata (Tokyo Institute of Technology)


A series of fully transient simulations of turbulence have been performed for an infinite tight triangular lattice (typical of current SFR designs) with and without a grid spacer. The simulations have been performed using Large Eddy Simulation (LES) with the spectral element code Nek5000 and unsteady Reynolds Averaged Navier-Stokes (URANS) for P/D=1.08. Several structure recognition techniques and statistical methods have been applied in order to investigate the flow field and the three-dimensional pattern of the coherent structures. The results prove that, for the configuration studied, coherent structures are indeed present and contribute significantly to the flow dynamics.

The accurate prediction of the flow in rod bundles is crucial for both the design and the safe operation of nuclear reactor systems. However the geometry complexity and the non-uniformity of the flow introduce peculiar features that can be reproduced only with three-dimensional CFD. In fact, even in bare rod bundles, i.e., bundles that do not contain any spacing device, the structure of the flow is inherently complex, due to the presence of a large-scale instability.

Moreover, experimental analysis has clearly shown that when reducing the pitch-to-diameter ratio (P/D) the turbulence field in rod bundles deviates significantly from that in a circular tube. For extremely tight configurations the existence of large-scale coherent structures has been shown, which is responsible for the high inter-sub-channel heat and momentum exchange. While there is a fairly extensive literature on the presence of these structures in bare rod-bundles and simplified geometries, there are no available studies of their presence in geometry containing grid spacers or wires.

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