Numerical investigation of rotating instabilities development leading to stall in a wide tip gap centrifugal compressor

Authors

Xavier Flete  - Liebherr-Aerospace Toulouse SAS, France; ISAE-SUPAERO, France
Nicolas Binder - ISAE-SUPAERO, France
Yannick Bousquet - ISAE-SUPAERO, France
Sandrine Cros - Liebherr-Aerospace Toulouse SAS, France

Abstract

In this paper, full stage unsteady simulations are performed to investigate stall inception mechanisms in a large tip clearance centrifugal compressor (about 10% of the blade span) with a vaneless diffuser and a volute. Four operating points along a speed line are analysed to understand the influence of the mass flow reduction on flow structures. It appears that the tip leakage flow has a major role in the stage destabilisation. At design condition, the flow passing through the tip clearance leads to a long tip leakage vortex development which interacts with splitter blades leading edges. By further throttling, this first loss mechanism is followed by a large scale vortex shedding from main blades leading edges. It is demonstrated that shed vortices are caused by the combination of the radial gradient of tangential velocity under the tip vortex and the reverse backflow near the casing. These vortical structures identified as backflow vortices according to previous studies performed on axial compressors, lead to a strong increase of the incidence angle in the tip region. Furthermore, by forcing the boundary layer separation at the main leading edges, such disturbances meet the necessary conditions for stall onset. This statement is illustrated by the emergence of a stall cell at the impeller inlet. Unlike for axial compressors, the present centrifugal configuration proves to be stall-resistant and operates in stalled conditions without significant pressure drop.