Rotor work characteristics and loss generation mechanism of an axial flow compressor at windmill operation

Abstract

Work characteristics and loss generation mechanism of a single-stage axial flow compressor at windmill operation were investigated by experiment and CFD analyses. When the inlet flow coefficient is gradually increased from the design point, the rotor blade gradually enters the windmilling operation from the tip toward hub. The research attentions were focused on two windmilling operations, those were free-windmilling and highly loaded windmilling conditions. Total pressure loss under windmilling operations was mainly caused by three flow structures, those were (1) tip leakage flow from suction to pressure surface near the leading edge and that from pressure to suction surface near the trailing edge, (2) the interaction of separation vortices due to the highly negative incidence and the rotor leading-edge vortex, and (3) the boundary layer separation near the hub wall. The distribution of the rotor operating mode was found to exist not only in the span-wise direction but also in the chord-wise direction under the windmilling operations. The turbine mode region was observed near the leading edge, while the compressor mode region was observed near the trailing edge even in the highly loaded windmilling condition. Therefore, driving force of the windmilling was dominated not by the area of the turbine mode on the rotor blade but by the static pressure difference of suction and pressure surfaces on the rotor.