EXPERIMENTAL INVESTIGATION FOR ENHANCED CONTROL OF ROTATING UNSTEADY FLOW INSTABILITIES IN AN UNSHROUDED CENTRIFUGAL COMPRESSOR IMPELLER

Unshrouded industrial centrifugal compressor impellers operate at high rotational speeds and volume flow rates. Under such conditions impeller blade excitation is dominated by high frequency interaction with stationary parts, i.e. vaned diffusers or inlet guide vanes. In a previous study conducted on two full compression units of the original equipment manufacturer (OEM), the authors also identified, characterized and quantified resonant blade vibration caused by the interaction of the impeller blades with sub-synchronous rotating stall cells during severe off-design conditions. The resonant impeller excitation lead to significant dynamic stress in the blades. In a follow-up study the authors have reproduced this phenomenon under representative off-design conditions in a downscaled test rig and successfully reproduced the phenomenon with unsteady Computational Fluid Dynamics (CFD) and structural mechanical Finite Element Method (FEM) analysis. The gained knowledge of these studies was translated into a new diffuser design philosophy, based on a sectorwise circumferential variation of the leading edge angle. In this paper, the patented philosophy by the OEM is presented and verified experimentally on the same test rig configuration in terms of flow path geometry and measurement equipment that was used in the mentioned prior study to assess resonant blade interaction. The results confirm the design aims: rotating stall onset was delayed without affecting the aerodynamic performance of the stage. Resonant blade interaction with rotating stall observed in the baseline diffuser could not be avoided with the two new diffuser designs. However, with the two new diffusers, the induced mechanical stresses in the impeller and the excitability were reduced by up to 12%.