Tip injection as a means for rotating stall control in an axial flow fan

The present experimental investigation is aimed at studying the effect of tip injection on the performance and flow characteristics in the tip region of an axial flow fan under uniform and distorted inflow conditions. A detailed parametric study was carried out by varying injection parameters such as pitch and skew angles, jet-to-free-stream velocity ratio, number of injectors, and the axial location of the injectors. The configuration with six injectors, jet-to-free-stream velocity ratio of 3.5, 0 degrees pitch angle, and injector skew angle of -15 degrees (in all orientation opposing the direction of rotor rotation) resulted in the highest stall margin improvement of similar to 9.4 per cent. This study was further extended to determine the effects of circumferential and combined circumferential and radial inflow distortion on fan performance and the effectiveness of tip injection under these conditions. A preliminary study of the effect of pulsed injection was also carried out at different pulsing frequencies in order to optimize injection flowrates. Interestingly, pulsed injection resulted in less stall margin improvement when compared with continuous injection. These results have been supported by flow behaviour studies carried out in the tip region using a seven-hole probe. A qualitative study was carried Out using unsteady pressure transducers, and it revealed that the trailing edge of the distortion screen acts as a source of disturbances.