Mechanism of the Interaction Between Casing Treatment and Tip Leakage Flow in a Subsonic Axial Compressor

The use of slots and grooves in the shroud over the tips of compressor blades, known as casing treatment, is known as a powerful method to control tip leakage flow through the clearance gap and enhance the flow stability in compressors. This paper present a detailed steady and unsteady numerical studies of the coupled flow through rotor blade passages and two different types of casing treatment for a modern subsonic axial-flow compressor rotor. Particular attention was given to examining the interaction between the tip leakage flow and the casing treatment. In order to validate the multi block model applied in the rotor blade end-wall region, the computational results for the modern subsonic compressor rotor both with and without casing treatment were correlated with available experimental test data for estimation of the global performance. Detailed analyses of the flow visualization at the tip have exposed the different tip flow topologies between the cases with casing treatment and with untreated smooth wall. It was found that the primary stall margin enhancement afforded by the casing treatment is a result of the tip clearance flow manipulation. The repositioning of the tip clearance vortex further towards the trailing edge of the blade passage and delaying the movement of incoming/tip clearance flow interface to the leading edge plane are the physical mechanisms responsible for extending the compressor stall margin.