Full annulus simulation on a multistage axial-centrifugal combined compressor

Axial-centrifugal combined compressors are widely used in small and medium aero-engines. This study aims to investigate the performance and fl ow characteristics of a multistage axial-centrifugal compressor using annulus unsteady simulation. The compressor comprises three axial stages and one radial stage, including the inlet and outlet guide vanes. The simulation employs an in-house URANS solver with a total of 357 million cells. The study analyzes two operating points, at the highest mass fl ow rate (choke) and near peak efficiency, fi ciency, respectively. The results show satisfactory agreement with the experimental data for the performance parameters of the compressor, such as pressure ratio, isentropic efficiency, fi ciency, and static pressure distribution. The simulation reveals the fl ow chokes in the radial diffuser, which is characterized by the presence of sonic surfaces occupying the throats of fl ow passages. Furthermore, two distinct vortices are observed in the radial diffuser, with one vortex axis being oriented vertically to the suction surface and the other being along hub-to-shroud direction. In contrast, the fl ow approximately maintains a smooth trajectory from the inlet until it exits the impeller. Moreover, even in scenarios where the compressor operates at choked conditions, the tip vortices of the fi rst rotor manifest a distinctive trait of rotating instability, substantiated by the noticeable presence of a "hump" within the frequency spectrum of pressure. The study can contribute to a better understanding of the fl ow behavior in axial-centrifugal compressors and provide insights for their design and optimization.