Study on the stall mechanism of variable cycle engine fan under a small bypass ratio and the adjustment method of local radial variable pre-rotation of inlet guide vane

This paper investigates the stall mechanism of the first-stage fan of a variable cycle engine (VCE) at a small bypass ratio and the corresponding method for radial variable pre-rotation adjustment. Steady and unsteady three-dimensional numerical simulations of the fan are performed. The results indicate that, under conditions of a small bypass ratio, the pronounced throttling effect of the outer bypass will amplify the interference caused by shock waves, tip leakage, and the boundary layer in the rotor's tip region. This phenomenon is likely to result in substantial flow separation and fan stall. The radial variation trend of the inlet relative airflow angle of the VCE fan in the process of stall with a small bypass ratio is analyzed, and it is found that it is necessary to apply inlet variable pre-rotation to the blade height of the rotor above 60%. A variable inlet guide vane, which can apply variable pre-rotation to the local blade height of the VCE fan, is proposed for the first time. With more than 60% of the guide vanes and a local deflection of 8°, the stability margin of the VCE fan can be increased by 4.33%, and the peak efficiency can be improved by 1.86%.