Conference proceedings article
Unsteady Secondary Flow in an Annular Compressor Cascade



Publication Details
Authors:
Hermle, G.; Lawerenz, M.
Editor:
ASME
Publication year:
2013
Book title:
Proceedings of the ASME 2013 Gas Turbine India Conference

Abstract
The flow in compressors is highly unsteady and determined by three-dimensional effects. Nevertheless theaerodynamical design is in general based on time averaged considerations of the flow field. Therefore theunderstanding of the unsteady phenomena is an essential step to further improve the efficiency of modernturbomachinery.Subject of the experimental investigations presented is the analysis of secondary flow in an annularcompressor cascade. Main focus lies on the so-called rotating instability (RI), which stands in closeconnection to rotating stall. RI can cause blade vibration and mechanical damage. Hence the furtherresearch of RI is of interest for the safe operation of turbomachinery regarding aerodynamical andmechanical aspects.The experimental setup consists of unsteady pressure measurements in the area of the blade tipclearance. The transducers were mounted along the leading edge around the circumference of the cascade.This arrangement provides the coherence and the phasing between the single transducers via cross spectraldensity function (CSD). The results give insight into the frequency-spectrum of the pressure field and itsmodal behavior. Furthermore the pressure signals are analyzed by means of the Wavelet transform, whichdisplays the time dependent evolution of the pressure field. The incidence angle and the inlet Mach-numberis varied to change the incoming flow in order to investigate the influence on the RI.The spectra show the significant effect of the incidence on the occurrence of the RI. Though thecoherence between the transducers rises slightly already at incidence angles of 8°, the spectrum shows thetypical RI related shape at an incidence of 12° for the investigated Mach numbers (Ma 0.2 - 0.5). Modalanalysis reveals that the RI-peaks in the frequency-spectrum can be assigned to circumferential modes ofthe phenomena. The phenomenon moves from pressure to suction side through the cascade similar torotating stall. The pressure fluctuations rise with inclining Mach numbers. Analogical to the CSD the Waveletanalysis shows the peaks of the characteristic RI-frequencies in the time-resolved spectrum but they appearin a rather stochastic way.In summary the onset of RI is basically influenced by the incidence angle, though the Mach numberinfluences the strength of the rotating pressure field. The results of the Wavelet transform for singletransducers don't show a periodic pattern for the RI frequency peaks as expected for a rotating phenomenon.

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