The Interactive Unsteady Mechanism Between Tip Leakage Vortex, Stall Inception And Micro Tip Injection In Low-Speed Axial Compressor

The phenomenon of flow instability in the compression system, has been a long-standing problem for the turbomachinery industry.There is a strong driver to provide an improved understanding of the aerodynamic mechanism of rotating stall and its inception.The emphasis of this dissertation is on the issues of rotating stall in axial compressor.The research aims are twofold: First,Micro injection has the influences of suppressing rotating stall in axial compressor in the previous experiments. But the mechanism of stall suppression of micro injection and its unsteady response are still kept unknowning. Recently, the unsteady feature of tip leakage vortices (TLV), which appears in some flow conditions, was shown out and its influences to the compressor performance were recognized. However, the detailed transient flow field, the originating mechanism and the relationship between the unsteady tip leakage vortices and rotating stall are unclear or not enough understood to date. These previous findings inspired the author to pursue the relationship between the unsteady response of micro injection and the self-induced unsteadiness of tip leakage vortices (TLV). So the first part of the paper is to detect the relation based on a series of experiments. So in the second part of the paper, micro injection,as a new practical example, can be an effective stall control technique. It is important how to maximize the stability for a given level of injected flow in a real case. Secondly , the paper is to explore respectively the injection parameters, such as the injection angles, axial gaps between injector and blade leading edge and the amount of injected air and so on, and then to analyze their effects on the stall margin of the compressors.So chapter 3 involves the design of compact injectors that are flush with the endwall and can fit over the rotor. The experimental results show that the angle of jet incidence is increased while the effect of compressor unsteady response to the micro injection would be reduced. The results were demonstrated that stability enhancement directly related to the momentum addition instead of the massflow through injector.Chapter 4 will focus on researching the relationship between the unsteady TLV and the rotating stall. It is key to clarify the developing process of such unsteadiness through experiments and to explore its effects on the stall inception. In order to detect the unsteadiness and ensure its existence in the experimental environment (not just by numerical simulation), phase-locked Mean and Root-Mean-Square (RMS) contours are used to track the time-averaged trajectories of the TLV, Fourier frequency analysis provides a method to identify the magnitude and the frequency of the oscillation. It is found that a spike stall precursor is originated from circumferential locations where the strongest unsteady TLV are.In chapter 5, all kinds of tip micro-injection are applied to test the effects of the unsteady TLV to stall inception. First, the injections are opened after the unsteady TLV happens, demonstrating that mirco injection could shift the position of TLV towards downstream and suppress the unsteady TLV. Further, the injections are arranged over the rotor tip in difference axial locations and at the locations with strong unsteady TLV, also demonstrating that micro injection alters the characteristics of TLV improving the stall margin effectively.In chapter 6, inlet distortions give examples that bring effects opposite to those of micro injection. It has been shown form test results that the same inlet distortion screen is placed in two different annular locations, those two cases are significantly different from each other. When inlet distortion screen is placed in the upstream where the unsteady TLV is stronger, the compressor becomes more unstable. Then, micro injection is experimentally verified for its influences on improving the compressor stability under different inlet distortions.In chapter 7, some test results can be provided based on a compressor with subsonic velocity. It also shows that micro injection can be an effective stall control technique with a great potential.