Dynamic instability in an axial compressor with mistuned blades

According to Emmons's classic theory, compressor stall is triggered by high incidence in one certain blade passage and its propagation is driven by flow diversion toward adjacent passages. In this theory, blades are assumed to be axisymmetric around the annulus. In practice, the assembly usually causes the non-axisymmetric distribution of the blades. The focus of this paper is to answer how mistuned blades affect stall inception and whether even can mitigate its propagation. This paper conducts detailed unsteady measurements on an axial compressor with mistuned rotor blades, where the stagger angle is of the priority of blades mistuning. The results indicate that blade loading around the annulus varies and correlates obviously with the stagger angle. Owing to the difference in blade loading, the flow in three specific blades (with higher stagger angle) shows less unsteadiness, compared to that of other blade passages. For the base configuration (the error of stagger angle within 1°), stall inception revolution does not show any difference. Of interest, when the error increases to 4°, disturbances surprisingly occur and propagate around the annulus at the stable operation condition near the stall boundary, which is triggered by the mistuned blade and will decay within one revolution in the rotor frame of reference. The phenomenon demonstrates the suppression effect of other blades with changed stagger angle on the propagation of stall inception. With the compressor throttled further, full rotating stall eventually occurs once disturbances can propagate around the entire annulus. Therefore, the aerodynamic mistuning has a clear effect on the stall inception and propagation, which can be further discovered for the stall control and stability design.