Testing of axial fans with microperforated housings

The noise radiated by electronic devices often results from the axial fans used to cool them. One component of axial fan noise is created by fluid shearing in the tip clearance region. In aero engines and other turbine applications, tip noise can be reduced by installing a finite flow resistance strip in the housing around the fan circumference. The finite level of flow resistance created by the slightly permeable housing may dissipate small-scale fluid motions, which may in turn reduce turbulence levels in the tip region and so reduce the tip noise. In the present work, the housing of a 120 mm axial fan was modified to accommodate strips of finite flow resistance, microperforated materials. The fan performance with and without the microperforated treatment was measured so that modified and unmodified fans could be operated at the same performance points and acoustical testing was performed using the ISO 10302 plenum. The fan noise was quantified on the basis of the blade passage tone level and tests were performed using microperforated materials spanning a range of flow resistances. It has been found that there are areas in the fan performance map where tonal noise levels can be consistently reduced by the use of a microperforated housing element.