Stability enhancement and hysteresis improvement of axial flow fan by discrete and distributed tip injection schemes

An experimental study has been conducted to compare the effectiveness of injection schemes applied on an axial flow fan rotor tip. Injection was applied from the casing by varying different parameters e.g. injection angle, number of injection ports, and location of injection ports. The study was carried out in a low speed axial flow fan test rig at the Aerospace Engineering Department, IIT Bombay. The study is aimed at understanding the effect of tip injection on immediate post-stall hysteresis phenomena and stall margin gain. The study includes discrete and distributed injection schemes applied: i) only at the rotor tip mid chord, and ii) both at the leading edge and at the mid chord of the rotor tip (combined injection). The effect of injection applied have been studied (i) during recovery only, and (ii) during the entire period from unstalled flow to the end of recovery (hysteresis). It is observed that various tip injection schemes are successful, in varying degrees, in alleviating hysteresis. While discrete injection provides more stall margin gain, combined injection scheme is better in hysteresis recovery