Sound Generation by Rotating Stall in Centrifugal Turbomachines

Experiments were conducted in order to investigate the relatively low frequency aerodynamic sound generating mechanisms in centrifugal turbomachines. A facility consisting of a centrifugal water pump impeller with various discharge configurations and an inlet duct was designed and built for the experiments. Air was used as the fluid medium. The inlet duct provided a controlled, quiet inflow to the impeller. Measurements of the acoustic noise radiated in the pump surroundings were made in parallel with fluid dynamic measurements in order to establish correlations. The most significant conclusion reached is that a form of rotating stall dominated the low frequency noise production in various configurations with no outlet diffuser or casing. Measurements of the inlet-radiated noise were made over a wide range of pump operating conditions; a method of scaling the data, based on similarity laws, was used as a means of separating source characteristics from the acoustic loading. Source spectra with no diffuser showed strong peaks centered at about 0·7, 1·4, 2·1 … times blade passing frequency. Aerodynamic pressure spectra measured at a single point on a rotating impeller blade were found to contain a cluster of harmonically related narrow-band tones, based on a fundamental around 0·7 times the shaft rotation frequency. Every seventh tone (harmonics 7, 14, 21 …) was strongly coherent with the acoustic signal. Measurements of unsteady discharge velocity were made with two stationary hot-wire probes at varying azimuthal separations close to the impeller exit. Spatial and temporal analysis revealed an almost-frozen multi-lobed pattern rotating at around 30% shaft speed. The following hypothesis is put forward on the evidence of the measurements: the prominent peaks in the acoustic signature of the diffuserless impeller represent rotating stall noise, generated by aerodynamic interaction between the rotating stall pattern and the impeller blades.