Rotor-Tip Flow Fields Near Inception Point of Modal Disturbance in an Axial-Flow Fan

<jats:p>The rotor-tip flow fields in two rotors of a low-speed axial-flow fan were experimentally and numerically investigated to clarify the mechanism behind modal stall inception. A NACA 65 wing section and a controlled diffusion airfoil were applied to the two rotors. At the small stagger-angle setting for both rotors, which is ten degrees smaller than the design value, the modal disturbance is observed near the peak pressure-rise point, and the rotor blades at the tip stall before the modal disturbance is observed. In the modal stall inception, the interface between the incoming flow and the reversed tip-leakage flow does not become parallel to the leading edge plane, although backflow from the trailing edge initiates near the stall condition. The reversed tip-leakage flow does not spill from the leading edge at the stall condition. Moreover, the tip-leakage vortex breakdown does not occur near or at the stall condition. A three-dimensional separation vortex is induced by secondary flow on the suction surface near the stall condition and develops at the stall condition. It is concluded from these results that the rotor-tip flow fields in the modal stall inception differ from those in the spike stall inception and that the three-dimensional separation vortex induced by the secondary flow influences the initiation of modal disturbance.</jats:p>