Experimental identification of rotating instability of an axial fan with shrouded rotor

The rotating instability phenomenon is investigated experimentally on a laboratory low-speed axial fan. Unsteady pressure measurements are performed close to the rotor leading edge, in the far-field in the upstream-duct section, and within the rotating frame by using Kulite sensors near the blade tip. As reference, the occurrence of rotating instability is proven for a rotor configuration with large clearance size. Existing explanations assume that rotating instability is induced by the tip clearance vortex. Following these models, the characteristic spectral and modal signatures of rotating instability are generated by the circumferentially coupled unsteady vortex systems within the individual blade passages. However, some early findings on a shrouded rotor setup and a study on an annular cascade without clearance have raised questions about the validity of the existing theories. To answer these questions, a second configuration is investigated using the identical rotor in a shrouded version. Here, a shrouding band covers the blade tips over the whole circumference. By using a comprehensive sensor arrangement in a wide variation of operating conditions, the existence of spectral and modal rotating instability signatures is proven on the shrouded-rotor configuration, as well. In this case, the tip clearance vortex system and, respectively, the leakage flow are suppressed. The present study underlines that novel explanations are required to describe the physical mechanism that generates rotating instability.