Tonal noise reduction by unevenly spaced blades in a forward-curved-blades centrifugal fan

Abstract

Tonal noise is an essential characteristic of the noise generated from a forward-curved-blades centrifugal fan. Herein, methods to unevenly arrange blades, in particular, a random modulation method, were studied to weaken the undesired tonal noise and reshape the noise spectrum. First, the balance and aerodynamic performance requirements of the impeller were considered when designing the unevenly spaced blades. The modulated blade positions had to satisfy the dynamic balance condition, while the variation of spacing between every two adjacent blades should not exceed 30% of the original spacing in the even impeller. Next, the influence of non-uniform blades on the noise spectrum characteristics of the fan was theoretically predicted by calculating the interference functions at each discrete frequency. Finally, computational fluid dynamics and computational aeroacoustics analyses were performed to investigate the acoustic performance of each uneven blade spacing scheme. Both the pressure fluctuation on the tongue and the noise frequency spectrum from the acoustic receiver demonstrated that unevenly spaced blades could reduce the tonal noise by dispersing the acoustic energy at the blade passing frequency to other specific frequencies as a form of sidebands. Compared with the existing sinusoidal modulation method, the random modulation method has no limits on the blade number. Moreover, this method has a higher degree of freedom that could overcome all limitations of blade spacing. It is shown that a reasonable blade distribution designed by this method could reduce tonal noise without negatively affecting the aerodynamic performance and the overall noise level.