Fuel assembly is one of the key components of a nuclear reactor that significantly impacts the thermal-hydraulic performance of the pressurized water reactor. The helical tri-lobe fuel (HTF) design has a better heat transfer performance compared with the mature rod-type fuel, hence has drawn much attention and deserves to further illustrate the enhanced heat transfer mechanism of helical structure.

This study aims to employ numerical simulation to examine the single-phase flow and heat transfer properties within HTF assemblies, investigating the influence of structural parameters on flow and heat transfer.

Firstly, a 7 HTF elements arranged in a triangular lattice was taken as analysis object in this study. The models of the HTF elements with various structural parameters were constructed, including different helical pitches, gap distances and ratio of lobe root arc to lobe tip arc radius (R₂/R₁). Then, the Integrated Computer Engineering and Manufacturing (ICEM) was adopted to generate a high-quality hexahedral structured mesh, achieving high mesh quality to accurately calculate the complex flow dynamics within the helical fuel flow field. Mesh independence check was conducted to confirm the satisfactoriness of the mesh scheme. Subsequently, ANSYS Fluent 2021R1 was adopted as the calculation platform, with the shear stress transport (SST) k-ω turbulence model and wall symmetry model being selected. The calculation model was set up with boundary conditions of a velocity inlet, pressure outlet, and uniformly heated wall surfaces. Finally, the essential thermal parameters, such as secondary flow velocities, vorticity of the cross-section, temperatures, and heat transfer coefficients of helical fuel flow field with different spiral shapes during the flow and heat transfer processes, were extracted from simulation output to elucidate the precise influence of these structural parameters on the flow and heat transfer characteristics.

Simulation results show that the helical structure of the HTF significantly augments the lateral mixing flow of the coolant and therefore intensifies the heat convection. The secondary flow intensity near the cladding surface area of the HTF is enhanced by reducing the helical pitch, and the heat transfer capacity of the HTF is improved. Meanwhile, with the decreasing of the helical pitch, the flow resistance of the coolant channel increases. However, a helical pitch exceeding 240 mm markedly amplifies fluid temperature non-uniformity and cladding surface temperature variations. Reducing the minimum distance between fuel elements can enhance the heat transfer capacity, while having little influence on the non-uniformity of fluid and cladding surface temperature. The increase of the R₂/R₁ of the HTF strengthens the heat transfer capacity, weakens the temperature concentration in the concave arc and increases flow resistance of the coolant channel.

Results of his study provide insights into optimizing fuel assembly design for enhanced thermal-hydraulic performance and reactor safety.