Reduction of the tonal noise annoyance of axial flow fans by means of optimal blade spacing

A method for the reduction of tonal noise annoyance from axial flow rotors is presented which is based on the uneven circumferential blade spacing. After a short description of the basic theory of the tonal noise generation from turbomachinery bladings, the existing methods for the circumferential spacing choice are analyzed and their practical drawbacks pointed out. A new systematic approach is presented in which a large number of balanced geometries are generated and then evaluated by means of an arbitrary objective function. To test the method, several objective functions have been calculated and the effect of the main parameters has been analyzed. Finally an optimized rotor representative of automotive applications has been realized and tested in the Johnson Electric- GATE semianechoic room in presence of a strong upstream velocity perturbation. The resulting acoustic pressure power spectra have been compared with those of an evenly spaced rotor equipped with the same blades, thus allowing for estimating the interference function of the unevenly spaced rotor. The good agreement with the theoretically expected trend proves that even in presence of strong velocity perturbation the obtained acoustic pressure power spectrum changes according to the theory which may thus be employed