Characterization of unsteady leakage flow in a rotating axial fan

Authors

Edward Canepa - University of Genova, Italy
Mehrdad Moradi - University of Genova, Italy
Matteo Dellacasagrande - University of Genova, Italy
Andrea Cattanei - University of Genova, Italy

Abstract

The present work reports the experimental characterization of the unsteady leakage flow of a low-speed ring fan. Three experimental conditions have been investigated varying the dimensionless pressure rise across the rotor. For the lowest and the highest pressure rises, the leakage flow has been found to be re-ingested by the rotor and to move radially outwards, respectively. For these two operating conditions, a stable flow pattern has been observed in terms of the spatial distribution of the flow leaving the gap. Differently, a strong radial oscillation of the leakage flow has been observed for the intermediate flow case, with continuous switching between the two former configurations. To investigate the origin of such unsteady behavior, Particle Image Velocimetry (PIV) measurements have been performed in a meridional plane that was located in front of the rotor gap. PIV data has been then processed using the Proper Orthogonal Decomposition (POD). Particularly, a POD based phase-averaging procedure has been adopted for the inspection of the main sources of periodic unsteadiness in the analyzed flow, whose dynamics are not necessarily related with the rotor rotational speed. Interestingly, the POD decomposition here applied showed that the unsteady behavior characterizing the leakage flow for the intermediate pressure rise is not phased with the rotor period. Moreover, the coherent motions captured by the POD modes have been found to promote turbulence transport at different radial positions, thus possibly affecting the noise generation of the rotor. The same behavior has been not observed for the rotor related fluctuations. The POD based procedure presented here can be generally applied to all of turbomachinery flows for the characterization of their unsteady behavior beside the classical phase-averaging methods based on rotor related quantities.