Detection Of Stall Regions In A Low-Speed Axial Fan. Part I – Azimuthal Acoustic Measurements

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; indeed, 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 developed in the present study utilises a symmetrised dot pattern (SDP) technique. Thus, the resulting methodology 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 establishes the stall characteristics of a low-speed fan using flush-mounted microphones placed at four azimuthal positions around the casing of a fan. Acoustic data are collected from these positions at full-, half-, and quarter-speed. These data are then processed to establish, for each speed, regions of: (i) stable aerodynamic operation; (ii) stall incipience; and iii) rotating stall. Spatial and temporal correlations between rotating instabilities are established, which facilitates a full analysis of stall inception. Part II of the study describes a stallwarning criterion based on visual waveform analysis and the stall-detection methodology that subsequently emerges from the analysis. Copyright © 2010 by ASME.