Numerical design and optimization of casing treatments for transonic axial compressors

A Casing Treatment consisting of axially orientated semi-circular slots was applied to a transonic axial compressor stage, known as the Darmstadt Rotor 1. When trottling, three dimensional flow phenomena near the rotor tip seem to be the critical mechanisms for stall. Parameters for the identification of the critical flow mechanisms indicate that the compressor is suffering from tip blockage stall. The Casing Treatments effectively shifted the stall margin towards lower mass flows, increased the total pressure ratio and also improved efficiency at 100% speed. The benefitial effect of increasing the efficiency however cannot be observed at part speed when the rotor blade tip operates at subsonic conditions. Nevertheless, the stall margin and also the total pressure ratio were increased. In order to gain a better insight of the ruling mechanisms, some geometrical parameters of the Casing Treatment were altered. The effects on the flow were numerically investigated using unsteady 3D-FANS simulations with a mesh of approximately 4.8 Mio grid points. The numerical results were also validated with experiments conducted at Technische Universität Darmstadt.