Study of Tonal Fan Noise Reduction by Modification of the Volute Cutoff

Blade-passing frequency (BPF) tonal noise is usually the main contributor to centrifugal fan noise as opposed to broad-band noise; therefore, abating its strength can reduce fan noise significantly. The predominant BPF noise source is the surface acoustic dipole over the cutoff due to the flow interactions between the impeller and the volute. In this study, a modified cutoff with an inclined edge and enlarged cutoff clearance was successfully employed to reduce the BPF noise; the measured noise spectra at three different flow rates of the inclined-cutoff fan and the original fan were compared, and noticeable noise reduction was found. The cause of this tonal noise reduction was numerically studied, using computational fluid dynamics (CFD) technology and acoustic analogy theory to calculate the flow fields inside the fan and sound radiation respectively. Results showed that the cutoff, including the geometric configuration and it's clearance with the impeller, was crucial to impeller-volute flow interactions. As a consequence, this inclined cutoff with enlarged cutoff clearance could eliminate the BPF pressure fluctuation, i.e. BPF dipole source, at the design point effectively; although the BPF pressure fluctuation was still notable at large flow rates, the radiated sound power decreased drastically attributed to the phase cancellation of the dipole sources. This study shows that CFD combined with acoustic analogy can provide a reliable access to noise prediction, thus it will be employed in aeroacoustic optimization of centrifugal fans.