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Abstract

Leading-edge or trailing-edge serrations have been used with some success as a noise-reduction technique applied to fixed airfoils in wind-tunnels. More recently, this technique has also been used on axial fans. A previous investigation by the authors also assessed the application of sinusoidal blade leading-edge serrations on a plug fan. ln this study, these previous results are re-evaluated and corrected. Furthermore, new research based on blade trailing-edge serrations is also presented.
RANS simulations on a non-serrated baseline impeller have been performed to define an assumed optimum geometry of leading-edge serrations to reduce the noise. This geometry has been used as a reference to manufacture three impeller prototypes with various geometries of serrations defined by their amplitude and wavelength. Three impellers with trailing-edge serrations were also designed, based on design criteria for fixed airfoils. All these impellers had an iron-shaped geometry, deemed to be better than the more prevalent sawtooth or sinusoidal shape.
All six prototypes have been tested in a reverberant room, where noise and air performance were measured simultaneously. For each impeller, six fan operating points were tested, and for each point, the sound levels in narrowband and one-third octave band were measured at the fan inlet and outlet. This allowed assessing the effectiveness of serrations at different operating points while checking their impact on the performance curve and fan efficiency. The results were then compared with the reference fan without serrations.
Leading-edge serrations have yielded a mitigated sound power reduction (up to 1 dBA reduction in the overall sound power level) and only for some configurations. One of the impellers slightly reduced the noise at all the operating points, another did it at some of them and the third one increased the noise at most of the operating points. Trailing-edge serrations, on the other hand, reduced broadband noise for all operating points and impeller prototypes (an overall sound pressure decrease between 1 and 5 dBA). Leading-edge serrations reduce noise for low to mid frequencies but increase it over 1 kHz, whereas trailing-edge serrations reduce noise along the whole spectrum.