On some axial-flow fans, high-level Abnormal Tonal Noise components (abbreviated as 'ATN' in this paper) have been found which are perceived as a disharmonic screetching sound and can not be explained by the classical theory of fan noise. It was obeserved that: (i) ATN appears only in a limited interval of the fan's aerodynamic operating conditions; (ii) ATN consists of multiple tones, whose frequencie f_i can be derived from one main ATN frequency f_0 and the blade passing frequency BPF by f_i = n centre dot f_0+m centre dot BPF (n=1,2,3,;; m=0,+-1,=-2,..); (iii) in a limited interval of air flow the main ATN frequency does not change with the aerodynamic fan operating condition; (iv) under certain conditions the main ATN frequency f_0 alternates between two closely spaced frequencies. Modal analysis of the sound field in the duct and measurements of flow velocity using hot-wire probes were performed. The experiments showed that ATN is generated by periodic vortex shedding from the stator vanes' trailing edges which is azimuthaly syncronized to an acoustic resonant mode of the duct. The vortex shedding is amplitude modulated by the blade passing frequency. The alternating of the main ATN frequency is caused by the swirl component of the flow leaving the fan. ATN completely dissapears when the periodic vortex shedding is impeded (by sharp stator vane trailing edges or by tripping the flow around the stator vanes), or the azimuthal synchronization is blocked (by increasing the chord length of selected stator vanes.)
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