Stall Precursor Study of High Frequency Data for Three High Speed, Swept Compressor Rotors

The frequency spectra of three single stage, high speed compressor rotors were derived and analyzed from experimental data. The compressor rotors included straight, backward-swept, and forward-swept blade designs. Data was acquired from four pressure transducers located on the compressor casing and arranged axially between locations 40% of blade chord upstream of the rotor face to just over the leading edge of the blades. Each rotor was throttled to stall at two constant speeds between 90% and 100% of design speed. Data digitization at 20 kHz revealed multiples of the rotor frequency up to and beyond a dominant blade passing frequency. Further analyses for characteristics of impending stall included examination of integrated signal power and cross-correlations of transducer signals from the upstream and downstream-most pressure transducers. Results of the integrated signal power approach showed promise as a potential pre-stall warning. This technique detected differences in the signal power at the two transducers in the immediate pre-stall time frame. Pre-stall warning times varied from several seconds to less than 0.5 second, with higher rotor speeds giving less warning time. Results for the forward-swept rotor, however, showed that integrated signal power did not warn of impending stall, and that surge energies played an important role in this rotor's stalling characteristics. An hypothesis for the behavior of the forward swept rotor, based on the possibility of stall development beginning in the near-hub radial region of this compressor, was developed.