Propagation of Acoustic Resonant Modes in a Multistage Axial Compressor

Acoustic resonances can occur under specific aeroacoustic conditions in turbomachinery. This phenomenon can cause high sound pressure levels, structural vibrations, and fatigue. In order to form an acoustic resonance, acoustic modes are reflected in such a way that they superimpose to form a standing wave with a very high sound pressure level. This phenomenon has been observed in a 4½-stage high-speed axial compressor near the stall limit at high rotor speeds. However, the physical mechanisms governing excitation and resonance are not yet fully understood. Reliable prediction and suppression of this phenomenon are important for future engine designs because acoustic resonance can restrict the operating range and compromise reliability and even safety. Previous numerical investigations identified the unsteady tip-leakage vortices as an excitation mechanism and the relevant acoustic mode. This paper investigates the propagation behaviour of acoustic modes under resonance conditions, with emphasis on a possible trapping mechanism. The propagation of the modes is evaluated by means of unsteady harmonic balance simulations and an analytical sound transmission model. The results show that the acoustic resonant mode is a cut-off/cut-on transition mode which gets mainly reflected by the rotor and stator blades. Remarkably, the reflection coefficient is found to stay constant when throttling the compressor.