Investigation on Mode Characteristics of Rotating Instability and Rotating Stall in an Axial Compressor

Rotating instability (RI) and rotating stall (RS) are two types of aerodynamic instability in axial compressors. The former features the side-by-side peaks below the blade passing frequency (BPF) in frequency spectra, and the latter represents one or more stall cells rotating in the compressor. This article presents an experiment on the nearfield pressure and farfield acoustic characteristics of RI phenomenon in a low-pressure axial compressor rotor, which endures both RI and RS at several working conditions. To obtain the high-order modes of RI and other aerodynamic instability, a total of 9 or 20 Kulites are circumferentially mounted on the casing wall to measure the nearfield pressure fluctuation using a mode order calibration method. Meantime in the farfield 16 microphones are planted to measure the acoustic mode order using the compressive sensing method. Through calibration, the experiments acquire the mode orders generated by RI and the interaction between RI and BPF, which are higher than the number of transducers. As for RS, the mode decomposition shows a mode order of 1, indicating one single stall cell rotating in the compressor. This experiment also shows that the amplitudes of RI modes are decreased when RS occurs, but both RS modes and RI modes will be enhanced if the flowrate is further reduced. This experiment reveals that RI experiences three stages of "strengthen-weaken-strengthen," and hence, RI may not be regarded only as "prestall" disturbance.