Leakage flow flutter in a low-speed axial-flow fan with shrouded blades

A large-amplitude axial vibration of a rotor fan with shrouded blades has been experimentally observed. The analysis of the Campbell diagram shows that the vibration is related to a backward-whirl vibrational mode of the rotor which is always present, with different amplitudes depending on the operating conditions and configuration. Modifications of the shroud roughness and insertion of small obstacles in the gap region have independently shown that leakage flow fluctuations constitute the excitation of the large-amplitude vibration. This indicates that the phenomenon is likely a flutter, as it is also suggested by the observed intermittency and aerodynamic stiffening. The feedback of the vibration on the flow is likely due to the variations of the gap size.