Acoustic Analysis of the Transfer Parameters of Leading Edge Serration as an Application for Low Pressure Axial Fans

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At high-turbulent inflow conditions, the blades of axial fans are known to emit a significantly increased amount of leading edge broadband noise due to the impingement of turbulent structures on the solid surface. Recent research has firmly confirmed sinusoidal leading edges (Leading Edge Serrations) to be an effective passive treatment in both, noise reduction and in increasing specific parameters of the aerodynamic performance. However, albeit intended for turbomachinery, fans and blowers, Leading Edge Serrations were up to now mainly analysed in rigidly mounted settings by investigating the noise radiation of mostly NACA65(12)-10 high-lift aerofoils. This project aims at describing the acoustic and aerodynamic effects of Leading Edge Serrations as an application for low-pressure axial fans but also considering a more universal transfer to axial flow machines in general. In particular, it aims at defining and quantifying transfer parameters from the rigid to the rotating frame and focusses on insights in the underlying physical principles, leading to a noise reduction. In order to benefit from the previously carried out extensive analyses of rigidly mounted aerofoils with leading edge serrations, NACA65(12)-10 aerofoils are utilised and scaled accordingly to serve as rotor blades of a low-pressure axial fan, where the rotor itself allows for an exchange of the blades and a continuous adjustment of the stagger angle. With this purpose a test rig according to DIN ISO 5136 and ISO 5801 is established, allowing to measure both, acoustic and aerodynamic performance of the fan simultaneously. Subsequent to analyses with straight rotor blades, leading edge serrations are introduced by varying the serration amplitude and wavelength in order to define the noise reduction capability and to draw conclusions on transfer losses from the rigid to the rotating domain. During the further procedure, the complexity of the rotor in form of blade sweep and skew is increased successively to gain knowledge on possible masking effects and to assign noise reduction phenomena to the tested influencing parameters.Consecutively, Leading Edge Serrations are increased in complexity by adding additional design parameters, such as a flow-specific spacing and alignment of serration amplitude and serration wavelength in accordance to the fan design point. Leading Edge Serrations featuring secondary amplitudes and a combination of serrations and porous materials are tested in a one-to-one scale in a rigid setting using phased-array beamforming as well as in the rotating setup, giving direct conclusions on the efficiency and losses/ winnings due to the transfer.