Optimization of diffuser vane cavity in a transonic centrifugal compressor for improved aerodynamic efficiency and operating stability

Vaned diffusers are widely used in advanced transonic centrifugal compressors to improve the compressor stage performance. However, due to the physical nature of these diffusers, the boundary layer on the vane surface starts separating at lower mass flow rates, which reduces the efficiency and operating range of the compressor stage. The current study presents a novel approach of applying a 2D cavity to the suction side of the diffuser vane to enhance the aerodynamic performance and improve the stall margin of the transonic centrifugal compressor. A 3D compressible Reynolds-averaged Navier–Stokes (RANS) solver was applied to investigate the influences of the cavity geometry on compressor efficiency and operating stability. A Box–Behnken design of experiment (DoE) approach was used to sample thirteen different cases based on variations of three geometrical parameters of the cavity configuration, namely cavity width, cavity depth, and distance of the cavity from the leading edge of the diffuser vane. A genetic aggregation algorithm was employed as a surrogate model and multi-objective optimization was used to find the optimal cavity configuration that maximizes both compressor efficiency and stall margin. The numerical results showed that the compressor characteristics were significantly affected by variation of the cavity configuration. A relatively larger cavity located closer to the leading edge of the diffuser vane was more effective at reducing flow separation and increasing the compressor efficiency by forming a vortex inside the cavity. This vorticity redistributes the momentum and kinetic energy in the boundary layer on the suction side of the diffuser vane, leading to postponement of the separation further downstream inside the diffuser passage. Applying the optimal cavity configuration to the diffuser vane improved the design point compressor efficiency by 1% and stall margin by 3.94% compared to the reference compressor without a cavity.