The Role of Self-Recirculating Casing Treatment on Axial Compressor and Its Aeroelastic Behavior
Kumar, S Satish
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The modern-day axial compressor stages in a gas turbine engine play a vital role in generating the high-pressure ratios by operating at the limits of their design. Aerodynamic instabilities such as rotating stall and surge that are detrimental can occur at higher blade loadings as the operating mass flow is reduced. There are possibilities of aeroelastic instabilities such as forced excitations/stall induced blade flutter occurring in compressors that are operating at their design limits. The aim of the thesis is to understand the flow field of the compressor annulus with a detailed survey made near the tip region of the compressor. The knowledge gained from the baseline compressor stage is used in developing a new self-recirculating casing treatment passive flow control device for alleviating stall to a lower mass flow rate at a constant rotor corrected speed. The aeroelastic significance of the flow control device is also studied. Detailed steady and unsteady measurements are performed at different flow conditions for a compressor with asymmetric rotor tip clearance. Higher flow variations near the tip region indicate the presence of dominant secondary flow effects, and the stall inception is likely to occur near the tip section of the blade. The complex distribution of clearance levels existing in the transonic compressor stage has a negligible influence on the overall performance behavior. Unsteady pressure measurements made along the compressor casing from rotor inlet to rotor exit show the rotating stall behavior. The abrupt nature of rotating stall frequency occurring at about half of the compressor rotor frequency is captured for the solid casing. A strong link with the tip leakage vortex and shock structure interaction near the rotor tip is observed numerically. The spread and radial growth of the low momentum fluid, along with the tendency of leading-edge spillage and backflow near the trailing-edge, suggests that the compressor is about to enter into stall. A new self-recirculating casing treatment concept is proposed, and a parametric study is carried out to assess the best injection angle for attaining higher stall margin improvements with a negligible drop in compressor performance using steady and unsteady experiments and CFD. A minimum amount of annulus flow is bled that is self-regulating based on the prevailing conditions at the suction port. Injection flow rate increases with an increase in the rotor pressure, with maximum injection flow rate occurring near the stall point. For the 0-Deg skewed self-recirculating casing treatment (RCT) configuration, there is around 6.3% to 9.35% stall margin improvements. There is a gradual increase in peak pressure ratio ranging from 0.17% to 0.44%, which can be attributed to the possibility of the compressor that can be further loaded after the blockage fluid is blown away. There is an improvement in the efficiency ranging from 0.2% to 0.5% with the 0-Deg RCT over the solid casing for a few operating speeds investigated experimentally. The resulting improvements in stall margin and other overall compressor performance parameters can be attributed due to the RCT that is able to overcome a larger loss producing mechanism in comparison to a small quantity of high-pressure fluid being tapped for injection. The unsteady pressures for the casing treatment show that the compressor is un-stalled at the stalling mass flow rate of the solid casing, and the compressor can be throttled further. The structural dynamics of the compressor rotor are studied, and its modal parameters characterized. The aeroelastic nature of the self-recirculating casing treatment is discussed. At the solid casing stall flow condition, the rotating stall cells excite the blades in their fundamental mode for the compressor with baseline solid casing, and there is no excitation of the blades with the self-recirculating casing treatment. Also, the self-recirculating casing treatment compared to the solid casing is able to reduce the overall vibration levels of the blade at fundamental natural frequencies that are excited at the stalling mass flow condition. The casing treatment alters the flow field near the tip region of the rotor blade and hence influences the forcing function of the rotating cantilever blades. The present research work contributes to a better understanding of the stall inception process in a typical high-speed axial compressor stage that will assist in better predictions of the compressor stability limit. The asymmetric clearance study showing the negligible influence on the overall performance of the compressor stage will assist in planning for better maintenance schedules for the engine. This study also aids in providing an impetus for compressor designs that can harness the dual benefits of casing treatments for stall margin extension and also for suppressing the aeroelastic excitations.