Detection Of Stall Regions In A Low-Speed Axial Fan. Part II – Stall Warning By Visualisation Of Sound Signals

This two-part study describes the development of a novel stall-detection methodology for low-speed axial-flow fans. Because aerodynamic stall is a major potential cause of mechanical failure in axial fans, effective stall-detection techniques have had wide application for many years. However, aerodynamic stall does not always result in mechanical failure. A sub-sonic fan can sometimes operate at low speeds in an aerodynamically stalled condition without incurring mechanical failure. To differentiate between aerodynamic stall conditions that constitute a mechanical risk and those that do not, the stalldetection methodology in the present study utilises a symmetrised dot pattern (SDP) technique that is capable of differentiating between critical and non-critical conditions. The SDP for a stall condition is different from that for a non-stall condition providing, a basis for differentiation of the two. Part I of this study presented the azimuthal experimental data which established the stall characteristics of a variablespeed fan. Part II describes a stall-warning criterion based on an SDP visual waveform analysis and developed stall-detection methodology based on that analysis. The study presents an analysis of the acoustic and structural data across the nine aerodynamic operating conditions represented in a 3 x 3 matrix combination of: (i) three speeds (full-, half-, and quarter-speed) and (ii) three operational states (stable operation, incipient stall and rotating stall). This differentiates critical stall conditions (those that will lead to mechanical failure of the fan) from noncritical ones (those that will not result in mechanical failure), thus providing a basis for an intelligent stall-detection methodology. Copyright © 2010 by ASME.