Stability Enhancement Technology Of Tip Air Injection In Axial Flow Compressor-Mechanism And Intelligent Control

Rotating stall and surge in axial compressors are one of the most difficult problems in the field of aerothermodynamics of turbomachinery, which thus have been the research focus for many years. After decades of numerical and experimental research, researchers have gradually recognized two important flow structures that are believed to be crucial to stalling mechanisms: the blockage induced by the flow separations inside blade passages, and the tip leakage flow in blade tip region. At the same time, the control methods have also been investigated, among which the tip air injection was proved to be effective although its stability enhancement mechanism is not fully agreed upon. Some researchers explained the effectiveness based on the steady theory of flow separation and blockage alone, while the others paid their attention to unsteady tip leakage flow. A unified view of stability enhancement mechanisms, not just the individual stalling mechanism itself, are needed in order to design more reliable tip air injections to guarantee effective stall margin extension and minimize the efficiency penalty and reduce the complexity of the injection system.To explore the different stability enhancement mechanisms, in this dissertation the experimental investigation was performed in both a subsonic and a transonic axial flow compressor. Firstly, the existence of demarcation points with different injected momentum ratios was confirmed in the low-speed compressor, and the different stability enhancement mechanism from micro injection to macro injection was clarified. Secondly, a technique to apply the aforementioned mechanisms to real compressors was proposed systematically. An innovative cross-correlation analysis was proposed to detect the unsteadiness in the pre-stall process. A proportional electromagnetic valve to regulate injected mass flow was chosen, and the corresponding DSP controller was designed. The stall margin of the low-speed compressor can be significantly and reliably enlarged as the new control approach smoothly utilizes the different stability enhancement mechanisms based on the needs of the flows. Finally, to reduce the complexity of the tip air injection system with external air sources, the same stability enhancement strategy was extended to the technique of air recirculation from the back of compressor stage to the leading edge of the rotor blade. Self-recirculation injection systems were designed to regulate the injected mass flow adaptively, and their effects on stability enhancement were tested in different compressors. The contents in this dissertation can be briefly summarized as below:1. Experiments on the stability enhancement mechanism from micro tip injection to macro tip injection (0.13%-3%) were preformed in low speed axial flow compressor. Firstly the existence of demarcation points was confirmed with different injected momentum ratio under different tip clearance and injected angle. Then the stability enhancement mechanism of the three injected momentum ratio level (micro tip injection, large tip injection, overlarge tip injection) were analyzed in detail base on the two basic understanding of stall mechanism: Micro tip injection can only weaken the unsteady tip leakage flow (UTLF) and the strength of tip leakage vortex (TLV), while keeping the flow velocity and flow angles along the entire span unchanged. Large tip injection is strong enough to alter the flows in tip region, such as reducing the local angle-of-attack and/or pushing the starting point of TLV further back towards the trailing edge. The overlarge injection has the same effects with the macro tip injection, and as the injection amount increasing, the injected air influences not only the tip region but also more lower-span region, which produces stronger influence on the whole engine performance and possibly brings new stability enhancement mechanism. It can provide guidance design criteria for realizing optimal effect of stability improvement with tip air injection in practical compressor.2. Detection mechanism and method research of pre-stall inception. The relevance of pre-stall inception and the unsteady tip leakage flow has been studied, and the pre-stall inception is associated with the unsteady tip leakage flow. Then the cross-correlation analysis was proposed to analyze the pressure time-trace; and its feasibility has been validated by tip air injection experiments. The solution is provided to detect the feedback disturbance signal in the stall active control.3. Active control base on tip air injection, proportional electromagnetic valve and DSP (Digital Signal Processing) controller. The tip injection control system was designed base on the stability enhancement mechanism of tip air injection and detecting mechanism of pre-stall inception. Firstly, the DSP controller was designed, and the cross-correlation algorithm was cured to CPU of the DSP controller. Combined with the proportional electromagnetic valve and the cross-correlation analysis method, the system can realize continuous transition of the injected mass flow form micro tip injection to large tip injection. Compared to the traditional tip injection system with on-off valve, this system is much better to enhance the compressor stability. At the same time, the DSP controller and the proportional electromagnetic valve can help to save a lot of injected energy. So this system is more economical and practical than the traditional steady tip air injection.4. Experiments of self-recirculation injection. Different from the tip injection with external compression system, self-recirculation injection can take full advantage of static pressure difference between inlet and outlet, and can regulate the injected mass flow adaptively. Stability enhancement effect with different suction angle and injected yaw angle has been investigated, and the stability enhancement mechanism of front injection has been analyzed. According to the experimental results in low speed axial compressor, the injected mass flow rate by self-recirculation was equivalent to the micro injection, but the stability extension effect was better than the injection by using external air. And the stall margin improvement (SMI) can reach to8%-15%high on the transonic axial flow compressor. So if the self-recirculation injection system is designed rationally, it can increase both the stall margin and the efficiency. The results provide opportunities to apply the self-recirculation injection practical compressors.The core content in this paper can be summarized as follows:Firstly, the existence of critical injected momentum/free stream momentum has been verified. The inlet and outlet parameters, tip leakage vortex (TLV), unsteady tip leakage flow (UTLF) below and over the demarcation point of injected momentum ratio were analyzed to clarify the stability extension mechanism of tip air injection.Secondly, the relevance between pre-stall inception and unsteadiness of tip leakage flow was clarified. The detection method base on cross-correlation analysis was proposed, and its feasibility was also verified by using tip air injection.Then, the DSP controller is designed which integrate cross-correlation algorithm and high speed acquisition channel, and the proportional valve was used to adjust the injected mass flow. Then the experiments about active control by using tip air injection were performed.Finally, the self-circulation injection with different suction angle and injected yaw angle were investigated in low speed and transonic high speed axial flow compressor. And the stability extension mechanism was also analyzed.