Aerodynamics of a Tandem-Bladed Axial Compressor Rotor under Circumferential Distortion at Different Rotational Speeds

For most aircraft engines, inflow distortion is inevitable. Inflow distortion is known to degrade the aerodynamic performance and stable operating limits of a compressor. Tandem rotor configuration is an arrangement that effectively controls the growth of the boundary layer over the suction surface of the blade. Therefore, a higher total pressure rise can be achieved through this unconventional design approach involving the splitting of the blade into forward and aft sections. It is expected that the effect of inlet flow distortion would be more severe for a tandem-rotor design due to the greater flow turning inherent in such designs. However, this aspect needs to be thoroughly examined. The present study discusses the effect of circumferential distortion on the tandem-rotor at different rotational speeds. Full-annulus RANS simulations using ANSYS CFX are used in the present study. The performance of the rotor at a particular flow coefficient and different rotational speeds is compared. The total pressure and efficiency are observed to drop at lower mass flow rates under the influence of circumferential distortion. The loss region in each blade passage is mainly associated with the blade wake, tip leakage vortex, secondary flow, and boundary layer. However, their contribution varies from passage to passage, particularly in the distorted sector. At the lower span, the wake width is found to be higher than that at a higher span. Due to the redistribution of the mass flow, the circumferential extent reduces at a higher span. In the undistorted sector, the strength of the tip leakage vortex is significantly higher at the design rotational speed than at lower speeds. The distortion near the tip region promotes an early vortex breakdown even at the design operating condition. This adversely affects the total pressure, efficiency, and stall margin. Under clean flow conditions, this phenomenon is only observed near the stall point. At the design operating condition, the breakdown of the forward rotor tip leakage vortex is detected in four blade passages. The axial velocity deficit and adverse pressure gradient play a significant role in the behaviour of tip leakage vortex at lower rotational speeds in the distorted sector. A twin vortex breakdown is also observed at lower speeds.