Numerical investigation on the effect of transversal fluid field deformation on heat transfer in a rod bundle with mixing vanes

Spacer grids of fuel rod assemblies are equipped with vanes, which promote flow mixing and turbulence within and across the sub-channels, thereby enhancing the heat transfer. First, a literature study about the various effect of the spacer grid has on the sub-channel thermo-hydrodynamics is provided. It follows, that the multiple effects on the vane angle are insufficiently understood. The effect of the vane angle on design parameters, namely the evolution of the Nusselt number, the pressure drop, the cross and swirl flows, is here further discussed and supplemented by own simulations. The effect of the velocity gradient tensor V circle times u, decomposed into a strain and a vorticity contribution, is also looked at downstream of the spacer grid. The RNG k-epsilon turbulence model was found to provide results best matching the experimental data available in the literature. The use of vanes results in the formation of a downstream vortex. As the flow develops downstream of the spacer grid, the vortex migrates away from the sub-channel center and eventually weakens. In line with the presented literature survey, it is confirmed that a vane angle of about 30 degrees provides optimal swirl and cross flows, resulting in an enhanced heat transfer