Experimental investigation of the interplay between the sound field and the flow field in skewed low-pressure axial fans

The type of fan-blade skew used in axial fans has a strong impact on the sound field, the flow field and their interrelations. In this study, the sound emission and the velocity distributions of three low-pressure axial fans with a similar design point and forward-, backward- and unskewed fan blades are investigated. The forward skewed fan is found to have best aerodynamic performance as well as the least sound emission over a broad operating range. More detailed investigations at the fan design point, based on sound pressure spectra and beamforming evaluations, reveal very dominant subharmonic narrowband components and increased turbulence-ingestion noise for the backward- and the unskewed fan. Moreover, an increase in the meridional velocity in the tip region is observed for the forward-skewed fan, which weakens the tip-vortex formation. For the backward- and the unskewed fan however, a reduced meridional velocity in the tip region is found, which is an indicator of an intensified tip gap flow. Additionally, values of the turbulent kinetic energy are substantially increased for the backward- and the unskewed fan, which also hints at unsteady flow phenomena in the tip region. These flow processes are linked to the subharmonic narrowband sound emission. Finally, unsteady flow features in the tip region lead to an increase in turbulence-ingestion noise.