Experimental and numerical investigation of endwall flow control through air injection in a contra rotating fan

This study reports on an experimental and numerical investigation of tip leakage flow control and its effects on the endwall flow structure and stall evolution of a contra-rotating fan. The control method involves the annular injection of high-momentum fluid in the axial direction, upstream of the front rotor. Five-hole probe measurements have been performed at different blade span locations to obtain the hub-to-shroud velocity distribution and also detect stall. Half-annulus transient simulations have been further conducted and validated against experimental tests. Two injection mass flow rates, 1.6% and 3.2% of the main mass flow rate, have been evaluated. Fast Fourier Transform and wavelet transform analyses have been applied to the experimental data. These analyses show that stall disturbances are intensified near the endwall, indicating the tip-critical nature of the studied fan. Under stall conditions, a modal wave in the tip region has been identified, rotating at 40% of the rotor speed. Numerical contours confirm the presence of circumferential disturbances. Also, the results reveal that rotor-2 is responsible for stall initiation. The findings further show that a 3.2% injection has sufficient momentum to eliminate the tip leakage vortex in the endwall flow. With 3.2% tip air injection, the total pressure rise coefficient and the stable operating range increased by 21% and 59%, respectively.