Impact of the large-scale environment on the tonal noise of axial fans

Abstract

The measurement of the sound emission of axial fans is standardized by internationally accepted guidelines. While it is known that the sound emission of fans is significantly influenced by the inflow conditions, the standards address this issue barely. A detailed definition of the inflow conditions for standardized acoustic measurements of axial fans does not exist. In this investigation, it is shown that disturbed inflow conditions exist even when the test rig meets the requirements of the current standards. Flow visualization and RANS simulation reveal unexpected vortex-like flow structures at the intake and inside the duct originating in the large anechoic chamber. These large-scale eddies in the inflow are also recognized by flush mounted pressure transducers on the blade surface of the impeller. It turns out that a hemispherical inflow control device may be suitable to get closer to the state of test rig independent acoustic measurements of axial fans.