An empirical correlation of rotating stall inception points of elementary compressors (isolated rotors, stages without prerotation, complete single stages, and multi-stage machines with repeating stages), modeled as equivalent diffusers, is presented. From it, two inception criteria for self-induced rotating stall are derived. Compressor blade rows are classified according to a geometric form parameter, (L/A)cor, into two groups, subcritical and supercritical. The subcritical geometries stall at a constant kinematic area ratio AE/A, in what appears to be a pure rotating stall mode, which occurs before the airfoil stalls. In supercritical geometries, the rotating stall is delayed until it is triggered by the airfoil stall. Thus, for the latter geometries, the airfoil stall and rotating stall are coincident. In contrast to other diffuser-analog methods, the divergence method determines the stall angle and the stalled flow coefficient rather than the stalled pressure rise.

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