Sound Generation Mechanism in Low Speed Axial Fans

Because of the global civil regulation concerning acoustic emission, manufacturers aim to substantially decrease the noise radiated by the low speed fans, without degrading their aerodynamic performance. To accomplish this target, engineers must evaluate the unsteady aerodynamic sources in the fan rotor responsible for the noise emission. This paper presents the results of the research team's experimental analysis based on a cause-effect method. The process focuses on the experimental comparison between the novel prototypes of a family of low speed fans and enables the evaluation of the noise sources in the rotor tip near-field domain. The near-field aerodynamic sources accounted in this work correspond to the zone with high pressure fluctuations amplitude located in the wake region of the rear flow. The team measured the sources at the tip and then correlated them with the fan's noise emission in the far-field, using a Fourier-based methodology. The paper also investigates the differences in human perception of noise emission to the environment using a dot-pattern visualisation. The study concludes that the modified tip configurations have a marked effect on tip-leakage vortex formation by altering the near-wall fluid paths on blade surfaces. The team correlates the modification in rotor noise emission with the control of tip-leakage flows produced by the tip end-plates.