Volume 60

Numerical Investigation of Flow Properties in a Hollow Shaft of a FlywheelEnergy Storage System under Vacuum Conditions Jiao Yuanyuan, Wang Yifei, Dai Xingjian, Xu Yujie, Chen Haisheng

Abstract

This study numerically investigates the multiphaseflow characteristics inside a hollow rotating shaft undernear-vacuum conditions, aiming to clarify thefundamental flow mechanisms governing rotor cooling inflywheel energy storage systems (FESS). Computationalfluid dynamics (CFD) simulations were conducted tocompare the stationary and rotating configurations,focusing on the influence of shaft rotation on phasedistribution, pressure evolution, and velocity fieldcharacteristics. In the stationary case, the impinging jetinduces a strong local pressure peak and downward flow,accompanied by pronounced air entrainment,incomplete gas removal, and localized cavitation neargeometric transitions. When rotation is introduced,centrifugal force dominates the flow dynamics, leadingto a stable annular flow structure with liquid adhering tothe wall and air confined to the shaft core. The air phasedevelops a helical backflow path driven by the low-pressure core region, while near-wall backflow andcavitation are significantly suppressed. The findingsreveal a transition from a gravity- and inertia-driven,chaotic two-phase regime to a rotation-dominated,stratified annular flow regime. This transition enhancesflow stability and liquid film continuity, offeringimportant insights into the coupled flow-phaseinteractions under vacuum and providing practicalguidance for the thermo-fluid optimization of high-speedflywheel rotors.

Keywords Flywheel energy storage system (FESS);hollow rotating shaft; multiphase flow; near-vacuumflow; CFD simulation