Non-Synchronous Forced Response due to trapped acoustic modes in an axial multi-stage compressor

Authors

Anne-Lise Fiquet - Ecole Centrale de Lyon
Christoph Brandstetter - Ecole Centrale de Lyon
Nicolas Buffaz - Safran Helicopter Engines
Agathe Vercoutter - Safran Helicopter Engines
Stéphane Aubert - Ecole Centrale de Lyon

Abstract

Non-Synchronous-Blade vibrations have been observed in an experimental multi-stage high-speed compressor setup at part-speed conditions. High amplitude acoustic modes, propagating around the circumference and originating in a highly loaded downstream stage have been observed in coherence with a structural vibration mode. In order to understand the occurring phenomena, a detailed numerical study has been carried out to reproduce the mechanism. Unsteady full annulus RANS simulations of the whole setup have been performed using the solver elsA for several operating conditions. Based on the simulation results, it is possible to identify the physical source of the non-synchronous blade vibration. The results indicate that an aerodynamic disturbance appears in a highly loaded downstream rotor and excites a spinning acoustic mode. The aerodynamic disturbance is identified as a separation of the blade boundary layer near the leading edge around midspan which adapts its oscillation phase to the acoustic mode that is partially trapped in the machine. The wave characteristics of the spinning mode are determined by acoustic propagation conditions. From the numerical simulations, the modal forcing on the rotor blades is derived. It is shown, that these fluctuations are critical as they are coherent with a structural eigenmode. This phenomenon is highly relevant, because the appearance of axially propagating acoustic waves can lead to a complex coupling mechanism that leads to an excitation of rotor blades in upstream stages due to aerodynamic disturbances occurring downstream. The paper will include a brief description of the experimental results and a detailed analysis of the numerical results. Furthermore, an analysis on modal forcing on the different rotor blades will be presented. The study shows the capability and the necessity of a full annulus multistage simulation to reproduce the complex phenomenon which could not be resolved using a reduced model.