Stall Margin Improvement of an Axial Flow Fan with End Wall Injection and Suction.

The experimental studies are conducted to reveal the mechanism of stall margin improvement of an axial flow fan by injection or suction from the end wall. In case of injection, the largest improvement is obtained by the injection at about 0.14∼0.21 times axial chord length downstream from leading edge. The reason for large improvement is that stall vortex, shed intermittent separation vortex and tip leakage vortex are dissipated by this injection, and also that this blowing suppresses the separation of boundary layer. In case of suction, the largest improvement is found for the suction from the end wall near leading edge. The amplitude of periodic static pressure after stall inception becomes smaller in comparison with injection cases. These effects are increased with the increase of suction flow rate, because the discharge of the vortex occurs more easily. On the other hand, the suction from the upstream of leading edge reduces the axial velocity near rotor tip, and then it induces stall. Also we tried to visualize the tip region flow. The suppression mechanism is discussed based on the visualization. The suppression of stall is successfully photographed.