In-depth numerical investigation on three-dimensional flow phenomena in a MONJU wire-wrapped 169-pin fuel bundle using vortex core identification

This investigation presents a novel computational approach to analyzing three-dimensional flow phenomena within a MONJU wire-wrapped 169-pin fuel assembly. Complex vertical flow patterns generated by wire spacers have been investigated using RANS simulations with an SST turbulence model, introducing an innovative vortex structure identification methodology based on critical point theory. Numerical results show excellent agreement with experimental measurements from JNC, achieving validation across identical pin configurations. The study reveals distinct flow patterns between interior and edge subchannels, identifying periodic vortex formation synchronized with wire spacer positioning. Edge subchannels have enhanced flow characteristics, with velocity profiles showing marked differences from interior regions. Results demonstrate significant variations in both axial and tangential flows across subchannel types, with edge subchannels showing higher velocities and intensified turbulence patterns due to longitudinal vortex structures. Longitudinal vortex structures in the edge subchannels have the opposite direction to the rotation of the wire-spacer, as confirmed through vortex identification analysis. These structures exist over approximately 20% of the wire-spacer pitch length, with negative normalized helicity values validating our hypothesis from previous studies. While corner subchannels also show vortex structures, normalized helicity values indicate these are primarily local separation vortices. Inner subchannel structures generally consist of separation vortices, but at wire-spacer onset points, longitudinal vortices are also generated for certain periods and lengths. The analysis validates numerical predictions within 2% of experimental data, with edge regions demonstrating 16% higher axial velocities and 12% stronger tangential flows compared to interior subchannels. This comprehensive investigation advances our understanding of complex flow dynamics in wire-wrapped fuel assemblies, contributing valuable insights for nuclear reactor design optimization.