The influence of uneven blade spacing on the spl and noise spectra radiated from radial fans

The influence of irregular blade spacing of car alternator radial fans on the total sound pressure level (SPL) and the noise spectrum has been investigated. For this purpose, the SPL and spectra were computed theoretically and the values were compared to measured results for several types of fans with various blade spacing. As theoretical background, the theory describing discrete frequency sound radiated by axial fans in open space was adopted. This was done in order to model the SPL and the spectra of alternator radial fans, placed inside a casing. Furthermore, due to the low blade tip Mach number, blades were modelled as dipole point sources. It was found, in accordance with previous results from known literature, that alterations in blade spacing do not significantly alter the total SPL nor the cooling capacity of the alternator radial fan. However, significant dispersion of the sound power over several harmonics was found with irregular fan blade spacing, thus allowing for a reduction of the siren effect. This phenomenon was predicted theoretically and confirmed experimentally. (C) 1998 Academic Press.