Abstract
To reduce the tonal noise of axial fans loudspeakers are typically used to generate the required anti-phase sound field. The space requirement and the weight of the loudspeakers inhibit practical application of this method. In the present study the secondary sound field is generated by perturbing the flow in the blade tip regime by using high speed air jets. It has been shown, that this approach is very effective in controlling higher-order mode sound fields at blade passage frequency and its harmonics. The focus of the present work is to study the effect of various nozzle parameters on the generated secondary sound field. From the experimental data presented it can be concluded that when using circular nozzles an injection angle of α = 45° relative to the casing wall combined with an injection direction of β = 90° relative to the main flow (against the impeller rotation) are most efficient in reducing the tone levels. While it is principally possible to control acoustic modes at the blade passing frequency as well as at its harmonics, the injected mass flow rates and circumferential nozzle positions necessary for reducing two different blade tone harmonics are generally not the same. However, when using slit nozzles the differences between the circumferential nozzle positions where significant level reductions of two dominant acoustic modes can be observed can be reduced down to 4% of a stator vane pitch.
Future studies will involve two rings of injection nozzles at close axial distance to reduce two dominant blade tone acoustic modes simultaneously. Finally, experiments are also planned with a model fan stage of a modern high bypass ratio aircraft engine.