Characteristics of End-Wall Flow at Spike and Modal Stall Inceptions in a Variable-Pitch Axial-Flow Fan

Abstract

The flow mechanisms for spike and modal stall inceptions in a low-speed axial-flow fan with a relatively large tip clearance were studied. The pressure and velocity fluctuations were measured to clarify the influences of blade loading, end-wall flow, and tip-leakage flow on stall inception at two stagger-angle settings for the rotor blade, which are the design and small stagger-angle settings. A rotating instability was observed near the maximum pressure-rise point at both design and small stagger-angle settings. This instability was induced by the interaction between the incoming flow, tip-leakage flow, and end-wall backflow. The stall inception patterns were a spike type at the design stagger-angle setting and a modal type at the small stagger-angle setting. At the design stagger-angle setting, the interface between the incoming flow, tip-leakage flow, and end-wall backflow became parallel to the rotor leading edge plane and reached the pressure side of adjacent blade. The interaction between these flows generated the large end-wall blockage in the rotor blade passage, and this blockage developed leading edge separation on the overloaded rotor blade at the tip. The leading edge separation that developed then grew into a spike, which traveled upstream of the rotor. At the small stagger-angle setting, the rotating instability and modal disturbance were also induced by the interaction between the incoming flow, tip-leakage flow, and end-wall backflow. However, the interface between the tip-leakage flow and end-wall backflow surrounded the suction surface of the rotor blade at the tip and neither became parallel to the leading edge plane nor reached the pressure side of the adjacent blade even though the rotor blade at the tip had stalled. Spikes did not therefore appear. The modal disturbance periodically decreased the inlet velocity and induced a long length-scale stall cell including a spike. It is concluded from these results that the stall inception patterns, which were characterized by the interaction between the incoming flow, tip-leakage flow, and end-wall backflow, depended on the stagger-angle settings for the rotor blades.