Pre-stall tip-region flow instability mechanisms in a high-load centrifugal compressor using zonal large-eddy simulation

Pre-stall unsteady flow in high-load centrifugal compressors has a decisive impact on stability margin, yet the associated tip-region instability mechanisms are still not clearly understood, partly because overall performance curves and standard Reynolds-averaged simulations simply miss the highly localized near-tip dynamics. In this study, a zonal large-eddy simulation (ZLES) framework is applied to a high-load centrifugal compressor to examine these pre-stall tip-region instabilities in more detail. Large-eddy simulation is employed in the impeller passage, whereas the rest of the flow domain is handled with Reynolds-averaged Navier-Stokes, so that the tip-leakage vortex (TLV) and the leading-edge shock wave (LESW) can be resolved with reasonably high fidelity without an excessive computational cost. The computations show that pre-stall unsteady flow is strongly concentrated in the near-casing leading-edge zone (0.1 ≤ s/C ≤ 0.3); variations of the LESW, its interaction with TLV, and the downstream convection of the breaking vortex (BV) dominate the unsteadiness and generate low- and mid- frequency components that grow as stall is approached. Frequency slice wavelet transform (FSWT) analysis reveals broadband amplification at 0.26 f _BPF along the TLV and shock trajectories, indicating the onset and development of vortex-shock interaction and vortex breakdown. The results support a stall-inception scenario in which vortex-shock interaction at the blade leading edge triggers TLV breakdown, the BV periodically impinges on the adjacent blade and enhances tip blockage, and the subsequent upstream migration and disappearance of the LESW allow low-energy fluid to convect circumferentially toward the adjacent blade leading edge, causing leading-edge overflow and stall inception.