CFD analysis of flow field in a 5 × 5 rod bundle with multi-grid

In order to understand the characteristics of flow field in a fuel bundle with multi-grid, in this paper detailed CFD analyses were performed against a 5 × 5 rod bundle with two spacer grids and one mid span mixing grid. To represent the actual grid as accurately as possible, in our calculation the grids are described as a structure of stripe, dimple, spring, and mixing vane. Based on the calculated results, characteristics of flow field at different channels along the length of fuel assembly, flow information at important specific sites as well as the influence of mixing vane geometry (namely its deflection angle and vane length) are investigated and compared. It is shown that the upstream grid has no remarkable impact on the flow field formed at its downstream grids if the cross flow is sufficiently developed. Both the spacer grid and mid span mixing grid are verifiable to play an important role in changing the flow behavior and enhancing the heat transfer among the bundle channels. In addition to the mixing vane, the structure of dimple and spring is also observable to have some impact on the local flow field at the grids. The performed analyses in this work suggest that increasing the deflection angle and length of mixing vanes will lead to much strengthened mixing performance and enhance the local heat transfer, along with an appropriately increased pressure drop simultaneously.