Sound Reduction by Leading Edge Serrations in Low-Pressure Axial Fans
2.3 Design for Low-noise Fans
In recent years a trend towards modifications of the leading edge geometry in airfoils for reducing the sound emission and improving the stall behavior has been observed. However, low-pressure axial fan design methodologies for a decreased sound radiation almost rely exclusively on the use of fan blade skew rather than on leading edge modifications so far.
Hence, with this study we aimed to investigate the sound reduction capabilities of leading edge serrations applied to low-pressure axial fans in. For this, we designed a generic fan with interchangeable flat-plate fan blades. We then did a parametric study with focus on the aerodynamic and the acoustic fan properties with a reference fan blade set with straight leading edges and four fan blade sets that featured sinusoidal leading edges with varying serration wavelength and serration amplitude. The investigations were made under free inflow conditions and distorted inflow conditions with an increased inflow turbulence intensity, induced by a turbulence grid. On studying the inflow parameters, Laser Doppler anemometer measurements showed that the inflow turbulence intensity was increased by a factor of 2 compared with free inflow conditions, owing to the installation of the turbulence grid.
All fans with leading edge serrations showed better aerodynamic characteristics (pressure coefficient and efficiency) than the reference fan with straight leading edges along the aerodynamic characteristic curves. The fans with leading edge serrations additionally showed substantially lower overall sound pressure levels than the reference fan with straight leading edges over a broad operating range. The sound pressure spectra for both free and distorted inflow conditions revealed that tonal and broadband sound components are reduced by the leading edge serrations. Additionally, subharmonic tip noise that occurred for the reference fan was greatly suppressed by the leading edge serrations. We found that the serration wavelength was the main factor for the sound reduction, opposed to the serration amplitude for airfoils. Based on the parametric study, physical mechanism on the sound reduction by leading edge serrations are explained and recommendations on the optimal choice of leading edge serration parameters for decreasing the sound emission of low-pressure axial fans are given.