Investigation of modal evolution and flow instability characteristics during radial diffuser stall in high-load centrifugal compressor with a throttle model

In high-load centrifugal compressors, radial diffusers are particularly susceptible to flow instability, which significantly restricts the stable operating range. This paper investigates the three-dimensional flow-field evolution in the radial diffuser from near stall to stable stall using full-annulus transient numerical simulations coupled with a throttle model. The results reveal that the most pronounced flow instability occurs in the vaneless and semi-vaneless (VLSV) regions of the radial diffuser, which evolve through three stages: near stall (S1), stall development (S2), and stable stall (S3). During this process, the dominant unstable region shifts from the vaneless section to the semi-vaneless section on the casing side of the diffuser. Through dynamic mode decomposition (DMD), the dominant mechanisms of each stage of unstable evolution are analyzed. The stall frequency increases from 396.3 Hz (0.41RF) in S2 to 684.2 Hz (0.7RF) in S3, with the corresponding energy contribution rising from 5.1% to 11.3%. Notably, in S3, the energy of low-frequency modes (1.4 RF, 2.1 RF, etc.) is significantly accumulated, accounting for 94.5% of the total, highlighting the critical role of low-frequency mode energy accumulation in diffuser instability. Further analysis reveals that in S1, unsteadiness is primarily driven by rotor-stator interactions dominated by the blade passing frequency between the impeller and diffuser. In S2, boundary layer separation caused by the interaction between the inlet shock wave and the boundary layer becomes the primary instability mechanism. In S3, the separated flow evolves into separation vortex structures, forming a blockage zone near the casing and ultimately inducing stable stall.