Numerical Investigation of Flow Properties in a Hollow Shaft of a FlywheelEnergy Storage System under Vacuum Conditions

This study numerically investigates the multiphase flow characteristics inside a hollow rotating shaft under near-vacuum conditions, aiming to clarify the fundamental flow mechanisms governing rotor cooling in flywheel energy storage systems (FESS).Computational fluid dynamics (CFD) simulations were conducted to compare the stationary and rotating configurations, focusing on the influence of shaft rotation on phase distribution, pressure evolution, and velocity field characteristics.In the stationary case, the impinging jet induces a strong local pressure peak and downward flow, accompanied by pronounced air entrainment, incomplete gas removal, and localized cavitation near geometric transitions.When rotation is introduced, centrifugal force dominates the flow dynamics, leading to a stable annular flow structure with liquid adhering to the wall and air confined to the shaft core.The air phase develops a helical backflow path driven by the lowpressure core region, while near-wall backflow and cavitation are significantly suppressed.The findings reveal a transition from a gravity-and inertia-driven, chaotic two-phase regime to a rotation-dominated, stratified annular flow regime.This transition enhances flow stability and liquid film continuity, offering important insights into the coupled flow-phase interactions under vacuum and providing practical guidance for the thermo-fluid optimization of high-speed flywheel rotors.