Operating Range Limitations in High Speed Compressors
Aerodynamic instabilities, but also aeroelastic phenomena limit the operating range of realistic compressors at low mass flow rate. The Turbomachinery platform at LMFA has been set up in order to get accurate data characterizing those phenomena in high speed compressors. This talk will present some configurations studied at LMFA which demonstrate different onsets of instability and exhibit some coupling between aerodynamics, acoustics and structural vibrations. A focus will be made on Non-Synchronous-Vibrations (NSV) that are increasingly observed in the new rotor designs of HP compressor (blisks) but also in a UHBR fan with composite blades. NSV occur at stable operating points, close to the aerodynamic stability limit, and are a coupling between aerodynamic disturbances propagating in the rotor with an opposite direction of rotation and structural vibrations of the blades. The aerodynamic perturbations are characterized by a frequency spectra of unsteady pressure containing broad-band frequencies (referred as “rotating instabilities” in the literature). Their propagation speed is a key parameter in the process of the frequency lock-in. It will be shown that CFD is a great tool to help the understanding of this phenomena, but caution has to be taken in order to catch the correct physics. Finally, computational setups and impacts of turbulence modelling will also be discussed depending on what has to be highlighted.