Rotating Stall Inception From Spike and Rotating Instability in a Variable-Pitch Axial-Flow Fan

Abstract

End wall flow fields at the two stagger-angle settings for the rotor blades in the low-speed axial-flow fan are experimentally and numerically investigated to elucidate the mechanism of stall inception. Rotating instability is confirmed near the maximum pressure-rise point at both design and large stagger-angle settings. This instability is induced by the interaction between the incoming flow, tip leakage flow, and backflow from the trailing edge. The stall-inception pattern, however, differs at the two stagger-angle settings. The stall inception from a spike is observed at the design stagger-angle setting, and the stall inception without the spike and modal disturbance is observed at the large stagger-angle setting. The rotating instability seems to influence the formation of stall cell at the large stagger-angle setting. Tip-leakage vortex breakdown occurs at both design and large stagger angle settings. This breakdown induces the three-dimensional separation on the suction surface of the rotor blade at the tip. Three-dimensional separation at the design stagger-angle setting is stronger than that at the large stagger-angle setting. The strong separation grows into a three-dimensional separation vortex, which crosses the blade passage near the trailing edge. This separation vortex seems to be one of the conditions for spike initiation.