Stall detection using near-field low frequency pressure modulation in turbomachines

This work investigates on the use of unconventional sensors to measure the near field unsteady pressure on the casing of a fan in a view to detect rotating stall. Rotating stall is an aerodynamic issue with a frequency signature usually half the rotor frequency. In low speed turbomachines, such as industrial fan, this turns in very low frequencies, even lower than 10 Hz. The authors developed and set-up a measurement system able to acquire low frequency pressure signals using dynamic microphones. Traditional methods use piezoelectric sensors, e.g. pressure transducers or microphones respectively in the near and far-field, to detect instability from the signal patterns with broad frequency ranges. Recently electret microphones have been proposed, but with a cut-off frequency of 20 Hz as such not suitable for signal in infrasound region. The sensor used in this work, have a narrower frequency range than more advanced technologies. In this paper the authors developed a measurement chain based on dynamic microphone and pressure transducer in order to create a stall warning system. They tested the system on a low speed axial fan and they validated the work against state of the art acoustic control techniques. For this reason those devices represent candidate solutions for the detection of the patterns typical of rotating stall in turbomachines.This work investigates on the use of unconventional sensors to measure the near field unsteady pressure on the casing of a fan in a view to detect rotating stall. Rotating stall is an aerodynamic issue with a frequency signature usually half the rotor frequency. In low speed turbomachines, such as industrial fan, this turns in very low frequencies, even lower than 10 Hz. The authors developed and set-up a measurement system able to acquire low frequency pressure signals using dynamic microphones. Traditional methods use piezoelectric sensors, e.g. pressure transducers or microphones respectively in the near and far-field, to detect instability from the signal patterns with broad frequency ranges. Recently electret microphones have been proposed, but with a cut-off frequency of 20 Hz as such not suitable for signal in infrasound region. The sensor used in this work, have a narrower frequency range than more advanced technologies. In this paper the authors developed a measurement chai