Stall control and recovery in a low-speed axial fan through the use of variable pitch in motion blades

Stall induced vibrations place fundamental limitations on a fan stage performance and remains a persistent problem in the development of axial compressor and fan stages. Rotating stall is purely a fluid mechanic instability, whilst blade flutter, stall and surge flutter, and their variants, are aeroelastic instabilities that involve coupled fluid-structure interaction. Stall oscillation frequency lays in a relatively low-frequency band (less than 0.7-0.5 shaft frequency), whilst mild surge oscillation frequency occurs usually in a much lower-order of frequency (typically <0.25-0.30) in high solidity industrial power fans. These mild surge oscillations can couple with fan blade aeroelastic modes. A loss in efficiency and high aeroelastic blade vibrations characterises fan performance in stall that can significantly increase stress levels in the blade. In this paper, the authors conducted an experimental study to investigate rotating stall recovery patterns in a high solidity axial power fan using different strategies. The authors drove the fan to stall at the design stagger-angle setting and then: i) operated a variable pitch mechanism in order to recover the unstable operation or recover from stall by increasing rotational speed. In both cases the recovery patterns entails the modification of the operating point of the fan along the throttle line of the system. They measured pressure fluctuations in the fan tip region using flush-mounted probes. The authors studied the flow mechanisms for the stall recovery associated with the two proposed methods. They cross-correlated pressure fluctuations and analysed cross-spectra in order to clarify the influence of blade pitch, end-wall flow, rotational speed and tip-leakage flow on stall recovery.