Implementation of an acoustic stall detection system using near-field diy pressure sensors

In this paper the authors propose the use of an unconventional instrumentation, based on a DIY transducer, to measure the pressure instabilities in a low speed industrial axial fan, with the purpose of rotating stall detection. Rotating stall is an aerodynamic instability with a frequency typically half the rotor frequency, and in slow turbomachines such as industrial fans this frequency has a value even lower than 10 Hz. The authors carried out the pressure measurements using a dynamic transducer and a piezoelectric sensor to provide the measurement base-line. In turbomachinery standard methods, time-resolved pressure measurements use piezoelectric sensors such as microphones in the far field and pressure transducers in the near field. Other classes of sensors, such as electret microphones, May be not suited for pressure measurements, especially in the infrasound region since their cut-off frequency is about 20 Hz. In the present study, the authors compare a low cost and DIY technology to a high precision piezoelectric sensor as alternative technology to stall detection. They implemented and set-up a measurement chain that is the basis of a stall warning system able to identify the rotating stall typical pattern in low speed axial fans. The results have been validated respect to the state of the art of the acoustic control techniques described in literature. The signals acquired using the two technologies are discussed combining spectral analysis and time-domain reconstruction of phase space portraits. The acoustic patterns obtained through the phase space reconstruction shows that the DIY dynamic sensor is a good candidate solution for the rotating stall acoustic analysis.