Numerical investigation of pressure drop and flow behavior in a 9 × 9 helical-cruciform fuel assembly

The helical-cruciform fuel (HCF) design features helically twisted rods with a four-lobed cross-section, increasing the surface-to-volume ratio compared to conventional cylindrical rods. This geometry enhances heat transfer and coolant mixing while reducing peak fuel temperatures. The self-supporting rod arrangement eliminates spacer grids, reducing flow obstruction and pressure losses. These characteristics enable higher reactor power density and lower operating temperatures in current and next-generation reactors. This study numerically investigates pressure drop and flow characteristics in a 9 × 9 HCF assembly using Reynolds-Averaged Navier-Stokes (RANS) simulations with the $k$-$\omega$ Shear Stress Transport (SST) turbulence model. Preliminary analyses of 3 × 3 to 11 × 11 assemblies demonstrated that the 9 × 9 configuration is the minimum bundle size required to accurately represent essential flow behavior in larger assemblies. Entrance length analysis at Re = 21,121 shows fully developed flow after one helical pitch. The CFD model is validated against experimental pressure drop data. Flow regime boundaries are estimated at Reynolds numbers of approximately 861 (laminar-to-transitional) and 9,755 (transitional-to-turbulent). A friction factor correlation covering Re = 119 to 21,958 is developed and compared with existing correlations. The flow characteristics at Re = 21,121 were analyzed within subchannels and inter-subchannel gaps.