Aeroacoustic Interactions of Blade Skew and Leading Edge Serrations Applied to LowPressure Axial Fans

Abstract

Leading edge serrations are well known for their ability to reduce turbulenceinduced noise of single airfoils while also providing aerodynamic advantages under certain operating conditions. Continuatively, applying leading edge serrations to rotating machinery such as axial fans proved the validity to generally transfer the obtained aeroacoustic benefits of single airfoils. However, for the rotating applications, besides the discrete and broadband characteristics of the investigated machines, the noise reduction potential highly depends on the point of operation. This work aims at assessing the aeroacoustic effects of serrated leading edges under the increased geometrical complexity of the fan blades through blade skew, a combination of blade sweep and blade dihedral. Therefore, the question is whether combining two potentially effective noisereducing treatments through blade skew and leading edge serrations results in leveraging or obstructing effects. By varying the skew angle from 0 deg to 38 deg, four different prototypes of the fan impeller are designed, manufactured, and tested experimentally in a test rig according to ISO 5136 and ISO 5801, allowing for simultaneous assessment of the aerodynamic and acoustic performance of the machines. All configurations are tested with original blades of straight leading edges plus five sets of serrations each, parameterized by the serration amplitude and the serrations wavelength. The intensity of the incoming turbulence ranges from 2.6% to 12.1%. The results obtained show the skewed blades to improve both the aerodynamic performance and the noise radiation after exceeding an initial skew angle, complemented by a significant onset of stall. Moreover, no contraindication between blade skew and serrated leading edges is encountered, showing the potential to further extend the noise reduction capabilities by combining effects of blade skew and leading edge treatment.