Thermal-hydraulic comparisons of 19-pin rod bundles with four circular and trapezoid shaped wire wraps

Coolant in wire-wrapped fuel assembly flows not only in axial direction but also in a transverse direction. This special flow is critical for momentum exchange and heat transfer in fuel rod bundle. To obtain a better performance wire-wrapped fuel assembly for advanced reactor cores, in this paper, the characteristics of turbulent flow and heat transfer inside 19-pin heated rod bundles where each pin is wrapped with four circular shaped and four trapezoid shaped wires are numerically studied, respectively. Considering the geometric complexity, the advanced hybrid-grid technique that combines the structured hexahedron grids and unstructured pentahedral grids is adopted to divide the modeling assembly. The validity of both RSM and SST k-ω model is assessed by comparing calculated pressure loss with the available experimental data to determine a proper turbulence closure. Special attention has been paid to understand the effect of secondary flows created by the wire wraps on the reduction of coolant temperature variation. Additionally, in order to assess the effectiveness of the different shaped wires, comparisons on both global heat transfer and associated penalty in pressure loss are evaluated. Results indicate that the frictional pressure loss of the rod bundle with circular shaped wires is smaller than that with trapezoid shaped wires, while the global heat transfer is equivalent. However, the trapezoid shaped wires give a lower maximum temperature as well as a uniform temperature distribution within the subassembly in comparison with circular shaped geometry.