The Flow Mechanism of How Distorted Flows Deteriorate Stability of an Axial Flow Compressor

The stalling behavior in a single-stage low-speed axial compressor under inlet distortion is investigated. A blade-passage-scale flow mechanism is proposed to explain the stability deterioration caused by inlet distortion for the tested compressor exhibiting spike stall inception. In contrast to the existing understanding of inlet distortion based on system scale dynamics, the main elements of this flow mechanism are the unsteady behavior of tip leakage vortices (TLV) under inlet distortion; its effect on the initiation of spike flow disturbances, and its interaction with distorted sectors. Rotating inlet distortion (RID) is used as a tool because RID makes it possible to directly compare the flows between distorted and clean flow sectors with fixed measurement stations on the casing, and the fact that the stationary inlet distortion is only a special case of RID makes the results generic. The tests demonstrate that the blade loading in the distorted sector is heavier than that in the non-distorted sector, causing the TLV in the distorted sector move closer to the leading edge of the rotor blade and thus be the first to initiate the spike-like disturbance. The unsteady CFD simulation further confirms that such a disturbance corresponds to a vortex spinning out of the leading edge of the blades. However, the initiation of this spike-like disturbance doesn’t necessarily trigger the full stall immediately. The tracking of the disturbances indicates that most of such spike-like disturbances will be smeared by non-distorted sector and the growth of the spike-like disturbances actually relate closely to how and how often the path of the propagating disturbances come across the path of the rotating distorted sector. The proposed blade-passage-scale flow mechanism also offers an alternative explanation to the “resonance” phenomenon in rotating inlet distortion research, which was explained with excitation-and-response theory for compressors that exhibit modal stall inception.