Large-scale unsteady flow structures in the leakage flow of a low-speed axial fan with rotating shroud

The leakage flow in low-speed fans provided with rotating shroud has been surveyed by means of two-dimensional LDV and PIV. Measurements have been taken upstream of the rotor and in the gap area at different operating conditions. The flow contains both periodic and non-periodic large-scale flow structures, which are responsible for the noise generation. The former structures have been extracted by means of the ensemble average technique, and, in order to study the latter ones, the non-periodic part of the instantaneous velocity field has been analysed by means of the proper orthogonal decomposition technique. In the present fan, the leakage flow consists in a jet which leaves the gap in the diagonal direction. Depending on rotational speed and blade loading, the jet may form a recirculation bubble anchored to the rotor ring or it may turn radially, forming a large recirculating flow zone. Large-scale, periodic vortical structures leaving the gap are convected by the leakage flow, either wrapping around the recirculation bubble or flowing radially outwards with a spiral pattern. In the former case, such structures, though weakened, likely reach the rotor, while, in the latter case, they have not been found at the rotor entrance. Hence, a different degree of mixing takes place which is consistent with the extension of the blade part where the leakage flow is reingested. This is consistent with the differences in acoustic measurements. The POD technique has shown that, at high loading, large-scale non-periodic vortical structures convect towards the rotor inlet. They are weaker though comparable to the periodic ones released from the gap; at low rotational speed they are stronger than at high rotational speed. Non-periodic flow structures may be present in the flow leaving the gap also, but it has not been possible to identify them clearly. Likely, non-periodic flow structures are originated by the ripping of the periodic ones. A possible explanation for the noise generation in fans with rotating shroud is that the periodic pressure field anchored to the rotor results in the release of periodic flow structures from the gap. Then, such structures may reach the rotor or rip during convection, likely resulting in non-periodic ones which are ingested by the rotor. In both cases, the interaction with the rotor blades likely results in noise generation. The degree of prerotation of the leakage flow is consistent with the frequency shift of the narrowband humps observed in the sound pressure spectra.