Research on the Effect of Helical Geometry on Flow and Heat Transfer Characteristics of Helical Tri-lobe Fuel

[Background] 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, which has drawn much attention and deserves to further illustrate the enhanced heat transfer mechanism of helical structure. [Purpose] This study employs numerical simulation to examine the single-phase flow and heat transfer properties within HTF assemblies. The purpose is to investigate the influence of structural parameters on flow and heat transfer. [Methods] This study conducted an analysis on 7 HTF elements arranged in a triangular lattice. 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). The 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 satisfactory of the mesh scheme. ANSYS Fluent 2021R1 was adopted as the calculation platform, with the 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. After the simulations, the study extracted the essential thermal parameters of helical fuel flow field with different spiral shapes during the flow and heat transfer processes. These parameters include secondary flow velocities, vorticity of the cross-section, temperatures, and heat transfer coefficients. The objective was to elucidate the precise influence of these structural parameters on the flow and heat transfer characteristics. [Results] 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 can be enhanced by reducing the helical pitch, and the heat transfer capacity of the HTF can be improved. Meanwhile, with the decrease 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 has little influence on the non-uniformity of fluid and cladding surface temperature. In engineering practices, it is necessary to take the increase of flow resistance at the tip of the lobe by reducing the minimum distance into consideration. 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. The influence of the R2/R1 value on the above parameters is within a certain range, thus the design value of the R2/R1 is mainly considered the influence on fuel inventory. [Conclusion] This research provides insights into optimizing fuel assembly design for enhanced thermal-hydraulic performance and reactor safety.