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Sodium fast reactor designs often implement a hexagonal array of fuel rods with wire-wrappers to encourage the exchange of coolant between subchannels. The ability to accurately predict inter-subchannel mixing can be used as a metric for turbulence model performance in capturing wire-wrapped fuel rod bundle flow behavior. In this study inter-subchannel mixing predictions by Large Eddy Simulation (LES) and Reynolds Averaged Navier Stokes (RANS) models are compared. The results indicate that the lower order RANS approach is capable of predicting inter-subchannel mixing inside a 19 rod bundle with acceptable accuracy. The RANS model was extended to 37, 61, and 91 rod bundles to observe the effects of bundle size on inter-subchannel exchange for the center-most subchannels. Transverse velocity magnitude and mass exchange were observed to increase with larger bundle sizes. Inter-subchannel mixing is observed to be a strong function of bundle size for bundles up to 91 rods. The results indicate that the inner subchannels of larger bundles may converge upon a characteristic flow pattern. The 91 rod bundle is not large enough to isolate the inner subchannels from shroud effect, and larger bundles will need to be investigated.

Fluid-structure interactions are complex, multi-physics phenomena of consequence for many fluid-flow domains. Modern multi-physics codes are becoming capable of simulating with great accuracy the interaction between fluid and structure dynamics. While fluid-structure interactions can occur in many forms, flow-induced vibrations are of particular interest. Such vibrations can result in the fatigue and even failure of a vibrating geometry. The prediction and minimization of flow induced vibrations are of particular importance for heat exchangers, which commonly contain bundles of tubes experiencing high-velocity crossflow. The present study simulates the fluid-structure interaction for flexibly mounted tube bundles undergoing crossflow and compares the results with experiment. The simulation code consists of a spectral-element fluid solver directly coupled with a finite-element solid mechanics solver. The fluid solver locally adapts the fluid mesh to accommodate the moving solids. In order to minimize computational expense, low Reynolds number flows are considered, allowing for a thin, pseudo 2-D domain. The flow remains laminar for the majority of the domain, with local areas of turbulence. The pins are connected to springs that supply a restorative force equivalent to the flexible mounts of the corresponding experiment. Fluid-only simulations are performed for flow spanning low to moderate velocities and compared visually with experimental flow visualizations. Coupled fluid-structure interactions are simulated with low velocity and vibration amplitudes. The measured vibration amplitudes of the simulation agree well with those of the experiment.