Critical loading, a key parameter for evaluating the limit of pressure rise in cascade, plays a fundamental role in triggering compressor stall. This work identifies specified vortex aggregation as the primary mechanism driving critical loading, which subsequently accelerates stall onset. Through 2.5-dimensional (2.5D) cascade and three-dimensional compressor simulations, the mechanisms of specified vortex aggregation are comprehensively analyzed. In the 2.5D cascade, an increased angle of attack alters the trajectory of shedding vortices, causing them to spread circumferentially and converge with local vortices in adjacent passages. This convergence results in increased upstream blockage and redistribution of mass flow pitchwise, ultimately forming a spike inception. To validate these findings in areal 3D rotating case, the stall process in a highly loaded compressor is also investigated. It is found that in 3D compressor vortex aggregation is inherently more complex due to three-dimensional and rotational effects. The simulation results reveal that the Coriolis effect significantly accelerates specified vortex aggregation, especially enhancing interactions between the leading-edge vortex and the tip leakage vortex. The interaction between the two vortexes, forming a more intricate vortex system, increases passage blockage from the midchord to the trailing edge, thereby accelerating the onset of stall eventually. This work offers new insight into the role of critical loading, Coriolis effect, and vortex dynamics in a compressor, providing an earlier local unsteady flow structure for analyzing stall precursors and assessing compressor flow instability.
