This study describes the assessment of a novel methodology that the authors recently proposed for the early detection of stall in low-speed axial-flow fans. Because aerodynamic stall is a major potential cause of mechanical failure in axial fans, effective stall-warning techniques have had wide application for many years. This paper presents an acoustic stall-warning system that can identify stall pre-cursors, and in so doing, establish that the stall is incipient. The study illustrates the application of the stall-detection technique to a class of heavy duty fans for induced draft service in coal-fired power plants. With an installed power up to 10 MW, this class of fans operates at constant rotational speed, utilising variable-pitch in motion blades to adjust fan duty point. The authors characterise the acoustic stall-detection system in an industrial laboratory environment exploring the fan operating envelope from maximum to minimum pitch angle.
The authors record the evolution of transient unsteady pressure events with microphones fitted into i) the fan casing over the fan blades and ii) in the inlet plenum one diameter upstream of the fan rotor. Whilst the authors know that the measurement of unsteady pressure evolution potentially facilitates the prediction of stall incipience, the present investigation demonstrates the possibility of detecting stall events with pre-polarised pressure transducers in the far-field.