Investigation Of The Mechanisms For Stall Margin Improvement And Design Methods Of Casing Treatments In Transonic Compressors

The stable operational range of gas turbine engines is restricted by the flow instability phenomena including compressor rotating stall. The formation of the compressor stall is closely related with the flow near the blade tip. As a passive flow control method, casing treatments can effectively improve the flow field in the blade tip region and thus increase the stall margin of compressors. For transonic compressors, the interaction between the casing treatment and the tip leakage vortex, the shock, the boundary layer separation leads to extremely complex flow structures near the blade tip. At present, the mechanisms for stall margin improvement and the effective design methods of casing treatments are still hot problems in the research field of transonic compressors. This paper deals with the problems which need to be further improved on the casing treatments in transonic compressors. The main research contents are as listed as follows:1. The multi-objective optimization of circumferential groove casting treatment based on the artificial neural network surrogate model for a transonic compressor is conducted. The groove casing treatment configurations which give consideration to both the stall margin and the peak efficiency are obtained. And the reliability of the method is validated by comparing the model predicted results and the numerical results. The effect mechanisms of the optimized casing treatment configurations on the performance of the compressor are further studied by analyzing the interaction between the casing grooves and the flow field near the blade tip.2. The effects of the depth of the casing grooves, the tip clearance and the stagger angle at blade tip on the stall margin and peak efficiency of a transonic compressor are synthetically evaluated based on the orthogonal test and range analysis. The optimal design variables and the sequence of the design variables that affect the test indexes are obtained. And the effects of the optimal scheme for stall margin and the optimal scheme for peak efficiency on the performance and the flow field of the compressor are comparatively analyzed.3. The mechanisms of stall margin improvement due to the groove casing treatment in a transonic compressor at design and 60% design speed are quantitatively analyzed with the control volume method. Based on the effect of the grooves on the axial distribution of the axial forces applied on the control volume, the contribution of individual groove to the stall margin improvement is evaluated. On this basis, the more effective grooves are selected, and the casing treatment configuration with the less effective grooves removed is tested. It indicates that the effectiveness of the casing treatment on stall margin improvement is not significantly altered by removing the less effective grooves, which validates the reasonability of the analysis approach.4. The effects of the axial position of single groove on the stable operating range and the unsteadiness of the tip flow are studied with unsteady numerical simulation. And the mechanisms that the groove controls the tip unsteady fluctuation are analyzed. The results show that groove located in the middle-front part of the blade tip can effectively reduce the axial momentum ratio between the leakage flow and the main flow, and weaken the effect of the low pressure region and blockage due to the tip leakage vortex on the blade loading near the casing. Thus the unsteadiness of the blade tip flow is significantly suppressed and the stall margin improvement is higher.5. The effect of the axial position of axial skewed slot casing treatment on the performance of a transonic compressor is studied. The relative importance of the factors that affect the effectiveness of the slots on the stall margin improvement is obtained with the relative weight method. And the sequence of relative importance of the factors is the intensity of the recirculation in the slots, the position of the shock near the blade tip, the inlet axial velocity near the blade tip. The study shows that when the front part of the slots covers the initial position of the tip leakage vortex and the rear part of the slots covers the boundary layer separation zone, the recirculation in the slots is stronger due to larger driving force. Thus, the blockage due to the tip leakage vortex and the boundary layer separation can be effectively reduced, which is beneficial to stall margin improvement of the compressor.