The Unsteady Mechanism And The Role Of Tip Clearance Flow In Distortion Induced Instability In An Axial Compressor

The presence of inlet distortion adversely affects both the performance and the stable operating range of an aero-engine compressor. For a long time, the research of inlet distortion is focused on how to get a reliable prediction of stall margin, while, the flow mechanism of how distorted flows deteriorate stability of a compressor is, as yet, unknown. Therefore, the aim of this dissertation is to propose a blade-passage-scale flow mechanism of how distorted flows deteriorate stability of a compressor which exhibiting spike stall inception and based on the mechanism provide a possible approach to improve the anti-distortion ability of a compressor.This dissertation is organized as follows: First, in order to get to know the elementary effects of inlet distortion on rotating stall, the stationary inlet distortion experiment is carried out. Second, based on the results of stationary distortion, an experimental and numerical study of rotating inlet distortion is carried out and by the comparison of the TLV form between distorted sectors and non-distorted sectors, and then, the flow mechanism of how distorted flows induce flow instability of compressor is proposed. Third, a modified Moore-Greitzer model is proposed to prove that the behaviors of stall inception are whether the same for both stationary and rotating inlet distortion. Finally, the tip micro injection is applied to improve the compressor stability under rotating inlet distortion which verifies the distorted flows mechanism proposed in this dissertation.The results of stationary distortion show that the characters of rotating stall, such as the stall inception and stall cell frequency, are not alerted by the introduction of inlet distortion. It's also found that the initial disturbances originate from highly loaded distorted sector. The rotating inlet distortion tests demonstrate that the heavier blade loading in the distorted sectors cause the TLV in the distorted sectors move closer to the leading edge of the rotor blade and be the first to initiate the spike-like disturbance which propagate with a relative fixed speed(about 0.5 rotor speed). 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.The modified rotating distortion model proved that the growth rate of any traveling disturbance is independent from the rotating speed of the distortion screen and also proved that the propagation speed of any steady traveling disturbance in such system is independent from the rotating speed of the distortion screen. These two proofs guarantee that the stall inception processes in both stationary and rotating inlet distortion are qualitatively the same at system scale. In addition, the model results are qualitatively the same with the experiment results, which indicates that it's feasible to use the model to predict the stall margin of compressors under rotating inlet distortion.The tip micro injection experiment results show that the injection alters the characteristics of TLV in distorted sectors and then, improves the stall margin effectively. This phenomenon demonstrates the blade-passage-scale flow mechanism proposed in this dissertation.