Numerical investigation on swirl flow and mixing behavior in a hexagonal-arranged dense-lattice helical cruciform fuel assembly

The forced convective heat transfer capacity of fluid-solid conjugate heating surfaces is closely related to the evolution behavior of the fluid flow field. The strong shear flow induced by complex non-canonical structures can promote the generation of vortices, thereby enhancing the heat transfer capacity and improving the safety margin of the heat exchanger. In recent years, helical cruciform fuel (HCF) has received extensive attention in the design of small modular reactors due to its high-power density, compact arrangement, and complex non-canonical structure. To investigate the refined swirling flow field and mixing behavior under complex twisted structure, based on the validation results of the mixing experiment, flow resistance experiment, and applicability analysis of turbulence models, the computational fluid dynamics (CFD) simulation on a 19-rod hexagonal-arranged dense-lattice HCF assembly was conducted. The causes of strong shear flows and swirling vortex distributions in the rod bundle flow field were analyzed. Meanwhile, quantitative analyses of transverse mixing and flow resistance characteristics of the HCF assembly with different flow rates were carried out. Different from cylindrical rods, the mixing coefficient of the helical fuel doesn't exhibit a monotonic variation trend with Reynolds number; instead, it mainly manifests as forced flow sweeping along complex twisted structures. The average mixing parameters of the central, side, and corner channels are 0.028, 0.027, and 0.04, respectively. Furthermore, the flow resistance model of HCF assembly based on Rehme's theory and a refined mixing model is developed, with an average prediction accuracy improved by 44.8%.