Passive casing treatments for stall margin improvement in axial compressors

Axial flow compressors are mainly used in the power generation and aerospace industry. Axial compressors are prone to instability issues since it is the nature of compressors to impart work on a fluid across an adverse pressure gradient. Instability in the compressor will instigate rotating stall and ultimately surge within the compression system. For tipcritical compressor, rotating stall can be delayed using passive casing treatments such as circumferential grooves. The design of circumferential grooves for improving the stability limit of axial compressors is the main theme of this thesis. A transonic isolated axial compressor rotor is selected as the testbed for the application of the circumferential casing grooves. Prior to applying the casing grooves, the near-casing flow aerodynamics of that compressor rotor is numerically studied and validated using experimental test data. The shock-tip leakage vortex interaction is found to be responsible for the accumulation of the near casing blockage. The near casing blockage is quantified using a mass flow overshoot criteria. As the compressor approaches stall, the location of the peak blockage is found to move upstream towards the leading edge. Based on the findings from the numerical study of the smooth casing, a design optimisation procedure is developed for identifying an optimised groove design. This procedure is based on a surrogate-based optimisation method. The magnitude of the peak blockage location and adiabatic efficiency at conditions close to stall are used for finding the optimised groove design. This method is different from the ‘black-box’ approach used by many researchers as found in the literature. The stall margin improvement gained by the optimised groove is about 1%. Further studies are conducted by changing the optimised groove axial location and simulating the grooved casing at part-speed conditions. No detrimental effects to the stall margin improvement are found at part-speed conditions. The design optimisation method is numerically validated on a low-speed version of the testbed transonic compressor test rig. For this, the low speed blades are rescaled and designed using an inverse design method. The aim of the design is to mimic the same stalling criteria as the high speed version. Using the same design optimisation approach applied on the testbed compressor, a single casing groove design is obtained. The effect of the optimised casing groove on the stall margin improvement is studied using numerical simulation. The optimal groove design is found to improve the stall margin of the low-speed compressor by about 5.4%.