Sound generation mechanism in low speed axial fans.

Abstract

Because of the global civil regulation concerning acoustic emission, the goal of the manufacturers is to substantially decrease the noise radiated by the low-speed fans, without degrading their aerodynamic performance. One of the main goal to accomplish this target is the evaluation of the unsteady aerodynamic sources in the fan rotor responsible for the noise emission. An experimental analysis, based on a cause-effect method, was carried out and the results are presented in this paper. The process focuses on the experimental comparison between the novel prototypes of a family of low-speed fans and enabled the evaluation of the noise sources in the rotor near-field domain. The near-field aerodynamic sources accounted in this work correspond to the zones with high pressure fluctuations amplitude, located in the wake region of the exhaust flow. The sources were measured along the rotor radius and then correlated with the noise emitted by the fan at the far-field domain, using a Fourier based methodology. The analysis was carried out looking at the cross-spectra polar form using the phase shift as revealing tool for aerodynamic noise sources. The results were compared to evaluate the merit of the aero-acoustic performance of each prototype.