A safe and reliable operation of a reactor system relies on the accurate thermal-hydraulic design in the rod bundles. With the dramatic progress in the computer processing power, computational fluid dynamics (CFD) methodology can be applied in investigating these thermal-hydraulic characteristics in details. The flow mixing devices on a grid spacer are generally designed to enhance the turbulence and heat transfer in the sub-channels. Therefore, the majority of this paper is to investigate the effects of different mixing vane configurations on flow pattern and heat transfer in the downstream of the mixing vanes in the rod bundles through CFD methodology. This CFD model is first assessed by the previous experiment of Holloway et al. In this study, six different designs of split-vane pairs are simulated, including (a) the split-vane 1 pairs (b) the split-vane 1 pairs without weld-nugget (c) the split-vane 2 pairs without weld-nugget (d) the Hybrid vane (e) the Swirl vane and (f) the SSVF vane. The thermal-hydraulic characteristics within the rod bundles with the split-vane pairs can be reasonably captured by the present CFD model, which includes the swirling secondary flow, turbulent mixing increase, and heat transfer enhancement, etc. Based on the comparison of Nusselt number distribution downstream the grid, the design of split-vane 1 pairs can enhance the heat transfer capability than other vane designs.

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