Effect of the rotor-stator gap variation on the tonal noise generated by axial-flow fans

The effect of the rotor-stator axial-gap on tonal noise generated by an axial-flow fan employed for automotive cooling systems has been studied. A fan equipped with a 9 blade rotor and a 18 vane stator positioned at several axial-gaps has been tested in a hemi-anechoic chamber. The acoustic measurements have been performed during rotational speed ramps. To analyze the experimental data, the propagation function, obtained by means of the spectral decomposition, has been compared with the velocity-scaled SPL and the phase angle, evaluated at the 1st and 2nd blade passing frequency harmonics. Opposite to what is commonly observed, the SPL due to the rotor-stator aerodynamic interaction does not monotonically decrease with the axial-gap and at the shortest gaps it may not be scaled with a single power of the rotational speed. The listed quantities have been plotted versus frequency, rotational speed, and axial-gap. Their analysis provides a detailed picture of the investigated phenomena and supports the assumption that the observed behavior is due to acoustic effects and not to the aerodynamic noise generating mechanism.