Energy characteristics of transient rear cavity vortex in a spherical-casing nuclear reactor coolant pump

The next-generation nuclear reactor coolant pump (NRCP) is transitioning toward a mixed-flow design, featuring a non-conventional spherical casing. Recent studies identified a novel vortex structure—the rear cavity vortex—within this geometry. This study employs Large Eddy Simulation (LES), combined with Proper Orthogonal Decomposition (POD) and Turbulent Kinetic Energy (TKE) analysis, to investigate the energy characteristics of this vortex both qualitatively and quantitatively. The innovation of this work is the integration of entropy production theory into LES, enabling a new approach to characterizing energy dissipation. Results show that POD modes in the rear cavity region exhibit distinct multi-scale energy structures, with the velocity components showing nearly isotropic energy distribution. An energy deficit phenomenon was identified and used to clarify TKE transport mechanisms. Moreover, direct entropy production was found to dominate total energy dissipation, while no simple proportionality exists between TKE and entropy production. This study presents a novel entropy-based framework to systematically investigate the energy structure of the rear cavity vortex in NRCP, offering new insights into its dissipation mechanisms. The findings offer new theoretical insights and provide engineering guidance for vortex control and performance optimization in advanced nuclear coolant systems.