Abstract
This work investigates the use of unconventional sensors to measure pressure modulation interpreted as pseudo sound in the near field 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 fans, this turns in very low frequencies, even lower than 10 Hz.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 near infrasound region.The sensor used in this work, have a narrower frequency range than more advanced technologies. The authors developed and set-up a measurement system able to acquire low frequency pressure signals using dynamic microphones.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.
Highlights
Detection of aerodynamic instabilities, such as rotating stall, is still an open issue relevant to the design and operation of industrial turbomachines
The auto-spectra shows a comparable behavior for the frequency range above the first harmonic of the blade passing frequency (BPF), while in the frequency range of the rotating instability under scrutiny, i.e. which is in the order of the RF, it is evident how the pressure transducer features have a higher response than the dynamic microphone
15 Hz at RF, due to the increased power absorption at stall. This frequency threshold only the piezoresistive probe is able to measure the peak at about 50% of RF that could be linked with the presence of a rotating stall
Summary
Detection of aerodynamic instabilities, such as rotating stall, is still an open issue relevant to the design and operation of industrial turbomachines. Continuous monitoring systems, was proposed for industrial fans [7,8,9,10] to allow the early detection of the aerodynamic instabilities, so to enable a fast intervention. Nowadays the available technologies for sensing pressure instabilities, are based on piezoelectric effect used in high frequency response pressure transducers [11], and condenser microphones [8, 12].The benefit in using these sensors is their high sensitivity in frequency range, spanning from few Hz to 20kHz. in low speed fans, aerodynamic instabilities have a frequency signature typically lower than 100Hz, in particular rotating stall occurs at a frequency that is about half of the rotor frequency. The study were limited to far-field analyses [13]
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