Aeroelastic study of the effect of the leakage flow in shrouded low-speed fans

A large-amplitude axial vibration of a rotor fan with shrouded blades has been experimentally observed. Various mechanical measurements have been performed to characterize this vibration. To this aim, a new test stand has been designed. 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 an aeroelastic flutter, as it is also suggested by the observed intermittency and aerodynamic stiffening. A complete series of aerodynamic measurements have been carried out, employing complementary techniques (PIV and LDA), to supply general information on the flow as well as deepen the unsteady flow involved in the flutter phenomenon. The PIV measurements have shown a recirculating flow downstream of the fan due to the presence of the obstruction disk which results in a backflow entering the gap between the rotating ring and the stationary shroud (the leakage flow). Large-scale eddies have been found at the edge and inside of this recirculating flow by investigating the PIV snapshots and have been proved by POD analysis. The LDA measurements close to the gap region have confirmed that the leakage flow enters from the gap downstream of the fan and mixes with the rotor inflow upstream of the fan when it leaves the gap. Using a new double phase ensemble average technique, the periodic part of the LDA signals related to the vibration has been investigated; it allows studying the effect of the precession motion of the rotor on the flow. Analyzing the flow using this method shows that the flutter also affects the relative flow angle at the rotor inlet, and consequently the angle of attack at the blade tips, which finally causes the aeroelastic phenomenon. Moreover, it has been found that the maxima and minima in the velocity field are located at angular positions different from the ones at which the gap outlet area is maximum and minimum, but a certain delay exists.