Inception patterns of rotating stall in a low-speed axial flow fan have been investigated experimentally. Experiments have been carried out at two different stagger angle settings for rotor blades. Pressure and velocity fluctuations were measured to elucidate the features of the stall cells and the stall inception patterns. At the design stagger angle setting for the rotor blades, a short length-scale stall cell known as a “spike” and multiple short length-scale stall cells appear when the slope of pressure-rise characteristic is almost zero. These stall cells grow into a long length-scale stall cell as flow rate decreases. The spike and the multiple short length-scale stall cells do not make the slope of the characteristic positive. However, the long length-scale stall cell induces a full-span stall, and makes the slope of the characteristic positive. At the small stagger angle, a long length-scale disturbance known as a “modal oscillation” is observed first, when the slope of the characteristic is positive. Then the spikes appear together with the modal oscillation as flow rate decreases. The long length-scale stall cell is generated by the spikes without change in the size of the modal oscillation. Suction-tip corner stall occurs in the stator passage near the peak of the characteristic at both the design and the small stagger angle settings. At the design stagger angle, however, the corner stall does not induce the modal oscillation and does not make the characteristic positive. In contrast, the corner stall at the small stagger angle induces the modal oscillation and makes the characteristic positive because it is larger than that at the design stagger angle. It is concluded from these results that the rotating stall inception patterns depend on the rotor stagger angle, which influences blade loading and rotor-stator matching.
Rotor Tip Flow-Field near Inception Point of Short Length-Scale Stall Cell in an Axial-Flow Fan(Fluids Engineering)