Helium turbulent fluctuation characteristics in the multilayer helical tube bundle using improved delayed detached eddy simulation

Flow-induced vibrations pose significant risks to the long-term operation of the helical coil steam generator (HCSG). Developing effective mitigating measures heavily relies on clearly understanding the internal turbulent flow. This study investigates helium flow within a 30-degree central region of heat transfer assemblies in the HCSG of the high-temperature gas-cooled reactor utilizing the shear stress transport based on improved delayed detached eddy simulation. The complex vortex structures are effectively captured in the tube bundle, accompanied by a pressure drop of approximately 6.28 kPa. A non-uniform flow distribution influenced by frictional resistance is observed. According to power spectral densities, symmetric annular channels demonstrate similar fluctuation characteristics. A specific flow pattern with a smaller dominant frequency of 2.5 Hz is revealed, which conditionally stabilizes in internal channels L₂ and L₃ only after passing through the 10-20 rows of tubes and not being suppressed by the effects of shell walls in side channels. This observation offers valuable insights into the optimal scale selection of computational domains for macroscale applications on HCSG. Based on the wavelet spectrum, high-frequency turbulent structures persisting throughout the tube bundle region originate from detached vortices, while low-frequency turbulent structures result from periodic transverse flows triggered by vortex interactions, which may induce resonance in large-scale components and pose potential risks to the structural integrity.