Design optimization and flow analysis of discrete tip injection in a transonic compressor based on nonlinear harmonic method and endwall blockage attenuation

Discrete tip injection (DTI) shows great promise for improving the operating stability of transonic axial flow compressor (AFC) rotors. However, the design optimization of DTI remains a challenging task because of both the reliance on computationally expensive unsteady simulations to calculate its effects and the lack of a flow physics-based index for assessing operating stability. The present study introduces a nonlinear harmonic method for the rapid simulation of the dominant unsteady effects caused by a DTI device, and it proposes the unsteady shroud endwall blockage attenuation as an operating stability optimization index for DTI design based on analyzing the stall flow mechanism in transonic AFCs. On this basis, an efficient optimization method for DTI design is proposed in combination with an adaptive kriging-based optimization technique. This design optimization method is validated by the Coanda injector design for the transonic rotor National Aeronautics and Space Administration Rotor 37, with improved operating range and reduced injection mass flow pursued simultaneously by a comprehensive objective function. The optimal DTI design significantly reduces the stalling flow coefficient of the compressor by 4.46% at a small injection mass flow (0.72% of the compressor stalling mass flow), with a slight increase in the aerodynamic performance of the compressor. Detailed unsteady flow-field analysis shows that the main reason for the improved operating stability of the transonic AFC is a significant attenuation and delayed recovery of shroud endwall blockage, and the underlying flow mechanism is elucidated well.