Impeller Stall Induced by Reverse Propagation of Non-Uniform Flow

Abstract

LecturesIn the case of centrifugal compressors, minor non-uniform flow upstream of the impeller is induced by an asymmetrical configuration in the circumferential direction at the compressor suction casing. This non-uniform flow is transmitted to the impeller discharge, but this minor non-uniform flow does not usually cause an adverse effect on the impeller stage performance. However, we found this is amplified at the return channel due to flow separation at reduced flows (depending on return channel geometry), and the amplified non-uniform flow did induce impeller stall by reverse propagation from the return channel to the impeller. These non-uniform flows caused a significant operating range reduction for a large flow coefficient impeller. The aerodynamics issues were mitigated using CFD analysis techniques, and eventually confirmed by the compressor performance during shop performance testing. The OEM conducted the CFD analyses using two (2) return channel geometries with several CFD models to verify the effect of the return channel geometry on impeller stall and to Copyright© 2018 by Turbomachinery Laboratory, Texas A&M Engineering Experiment Station confirm the most suitable CFD modeling method for stall evaluation. Shop performance tests utilizing both return channel geometries were conducted and compared to the CFD analyses. These studies were conducted while collaborating with the end-user. The steady CFD calculation was conducted with frozen rotor interface between full annulus impeller and stator parts. The modeling of diffuser and return channel was varied as follows: (1) 1-pitch model for the return channel with mixing plane at diffuser (2) Full-annulus model for the return channel with a mixing plane at the diffuser (3) Full-annulus model for the return channel without a mixing plane at the diffuser From the above studies and the shop performance testing, it was confirmed that the proposed CFD modeling method could simulate the measurements taken during the shop performance tests and that the CFD modeling met