The paper describes a stalldetection criterion based on the use of symmetrised dot pattern (SDP) visual waveform analysis and the stallwarning methodology based on a recently developed analysis. The experimental study explores the capability of the SDP technique to detect the stall incipience and evolution in the presence of low signal-to-noise ratios, that is, a noisy working environment. Moreover, the investigation presents a systematic analysis on the probe position’s influence with respect to the fan section. As such, the SDP technique in combination with an acoustic measurement is able to create a visual pattern that one can use to detect stall from potentially any location around the fan/duct system. 1. Introduction Aerodynamic instability arises when flow rate throttling constrains the operational envelope of fans, blowers, and compressors. To avoid instability, aerodynamicists must provide adequate stability (stall) margin to accommodate inlet distortions, degradation due to wear, throttle transients, and other factors that reduce fan, blower, and compressor stability from the original design baseline. Because aerodynamic stall, resulting in increased alternating aerodynamic loads, is a major potential cause of mechanical failure in axial fan stall, detection techniques have had wide application for many years. Researchers have studied the detection and analysis of the different forms of aerodynamic instability for several decades [1–4]. The standard classification of aerodynamic stalls in axial fans and compressors [5] distinguishes between rotating stall (in which reversed flow regions occur locally) and surge (which is characterised by periodic backflow over the entire annulus involving violent oscillations in the air flow which can result in mechanical failure such as fan blade breakage). The latter is unusual in fan applications due to the system’s insufficient counterpressure. Rotating stall is a mechanism by which the rotor adapts to a reduction in flow rate. This results in circumferentially nonuniform flow patterns rotating in the annulus. In reviewing the evolution of rotating stall, Cumpsty [6] noted that the drop in overall performance can occur as a so-called “progressive stall” or an “abrupt stall.” Engineers usually associated the former with a part-span stall which results in a small performance drop, whereas they associate the latter with a full-span stall and a large drop in performance. Notably, the part-span rotating stall occurs typically in single blade rows [6] and usually leads to more complex disturbances in single-rotor
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