Effect of rotor deformation and blade loading on the leakage noise in low-speed axial fans

The noise generated by the leakage flow in rotors provided with rotating shroud has been experimentally studied by means of acoustic measurements in an anechoic chamber and the basic features of the noise generating mechanism have been identified by means of LDV and PIV aerodynamic measurements. The acoustic tests, performed at constant rotational speed and during speed ramps, have shown a combined dependence of the received SPL spectra on the rotor axial position, on the rotational speed, and on the operating point. It has been shown that the confinement of the recirculating flow may result in a stronger noise, that the acoustic similarity is often not respected by the leakage noise, and that, in a number of situation, the pattern of the SPL spectrum has a sudden change as the rotational speed or the pressure coefficient exceed a certain value. The aerodynamic measurements have confirmed that the flow structures spouted from the gap have a non-periodic component and that, basically, two flow patterns exist which correspond to the two kinds of SPL spectra: a recirculation bubble anchored to the rotor shroud or a large recirculation zone. The former corresponds to a higher SPL than the latter, and the reason for this is that if the recirculation bubble is attached, a weaker mixing of the recirculating flow takes place. The suddenness in the SPL spectra transition is likely caused by an analogous flow pattern modification. Rotational speed and pressure rise may act on both the momentum of the recirculating flow and the change in shape of the gap geometry due to the rotor deformation. Possibly, the rotational speed acts on the Reynolds number of the gap flow, but it has not been possible to identify such an effect.