Axial compressors can obtain substantial improvement on stall margin by using axial-slot casing treatments. However, this type of casing treatment usually yields large peak efficiency penalty due to the interaction between the slots and rotor tip region where the tip leakage flow plays an important role. Therefore, as a main factor that influences the peak efficiency, the tip leakage loss was examined in this paper with a variety of slot geometries. Unsteady numerical simulations were performed on both low speed and transonic compressors with axial skewed slot casing treatments with different geometric parameters. In addition, an equation which can be applied to evaluate the tip leakage loss under casing treatment cases was derived from Denton’s leakage mixing model. The leakage loss can be expressed in terms of the cube of the tip leakage flow rate. Combined with the simulation results, the effects of the number, depth and width of the slots on both the leakage loss and peak efficiency deficit were investigated. For the transonic compressor, the impacts of shock wave and its interaction with the tip leakage flow /vortex were assessed as well. Lastly, two axial-slot casing treatments with an isosceles-trapezoid shaped opening were designed to reduce the loss in the rotor tip region. It was shown that the newly designed axial-slot casing treatments were capable of improving the peak efficiency of both compressors.

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