Influence of Inflow Turbulence on Aeroacoustic Noise of Low Speed Axial Fans with Skewed and Unskewed Blades

G1 - Installation Effect

The aeroacoustic noise radiation has become a key parameter to be taken into account during the design and development of low speed axial fans. On the one hand, noise radiation is dependent on the fan’s design and operating point, on the other hand it is highly influenced by the boundary conditions under its operation, like e.g. inflow turbulence intensity.
To investigate the influence of inflow turbulence on noise generation subject to blade geometry, three different fans with one common operating point were designed with a 2D blade element method: an unskewed fan, a forward-skewed fan with a constant sweep angle and a backward-skewed fan with the same sweep angle (without dihedral on both fans).
Performance and sound pressure distribution measurements were conducted in a test rig according to DIN EN ISO 5801. The fans were installed in a short duct with l = 3D. Different grid were mounted to the inlet in order to increase turbulence intensity. Velocity distribution and turbulence intensity measurements were performed in the ducted flow field on the suction side of all fans with a 2D Laser Doppler Anemometry system (LDA).
LDA results show increased turbulence intensities with mounted grids. Effects on the sound power spectra due to increased turbulence intensity can mainly be seen at frequencies below 2 kHz. The extent of this is dependent on the blade geometry. Explanations will be given, how aeroacoustic sound radiation of axial fans under influence of increased inflow turbulence can be reduced by blade geometry parameters.