Experimental and Numerical Investigation of Axial Fan Aeroacoustics at Disturbed Inflow Conditions

2.7 Installation Effects

In customer applications axial fans are usually operating at disturbed inflow conditions. A typical inflow disturbance is a heat exchanger upstream the fan, which is mounted in a box. A simplified configuration of this installation is the fan with a rectangular box in front. This test case is investigated experimentally at ebm-papst using standardized aero acoustic measurements, microphone array and flow visualization of a 5 blade axial fan with 910 mm diameter. The inflow disturbance increases the broadband noise level over the whole spectrum. Especially in the low frequency range up to 1000 Hz the sound power increases around 10 dB. There is no noticeable effect on fan performance. Pressure rise as well as efficiency is nearly the same with and without box in front.
The effect of increased noise is investigated via CAA simulations of the axial fan with and without box. For this purpose LES simulations are done using FW-H propagation for far field aero acoustics. As emission surfaces the rotating blades as well as a permeable emission surface on the fan suction are applied. Simulations accurately predict the difference in broadband noise up to 4 kHz. The absolute noise levels are also well predicted, with a slight over prediction of 5 dB in the range of 2000 to 4000 Hz. Due to short simulation time of 2 rotor revolutions, the effect on tonal noise is not resolved in the simulation.
The simulations indicate that large scale vortical structures separate from the corners of the box. These structures unite to large regions of high turbulence at roughly middle of top and bottom walls of the box. This area of increased turbulence interacts with the fan blades. The existence of these large scale structures is supported experimentally by sound source localization with array technique and flow visualization on the box surface. Measurements as well as CAA show increasing sound sources at the leading edge every time the blade passes the vortex.