Study of Design of a Coupled Casing Treatment for a Two-Stage High-Loaded Axial Flow Compressor

A coupled casing treatment was designed and optimized with three-dimensional numerical simulation to improve the stall margin of a two-stage high-loaded axial flow compressor. The design was based on the understanding of the failure of traditional slot-type casing treatment in the experiment. The flow mechanism of stability improvement was analyzed with the method of unsteady simulations for the optimized coupled casing treatment, and the compressor's stall mechanism under the effect of the casing treatment was also studied. The results show that the stall of the two-stage compressor is induced by the boundary layer separation that is caused by the passage shock at the rotor tip of the first stage. The slot-type casing treatment cannot effectively diminish the blockage induced by the boundary layer separation, and thus cannot improve the compressor's stability. The coupled casing treatment, which takes advantage of slot-type casing treatment and recirculating casing treatment, bleeds the low-energy fluid at the rotor tip into the passage of inlet guide vanes and improves extremely the flow condition of the rotor tip. The stable operating range of compressor is improved by 49.3% and the adiabatic efficiency at the design point is improved by 0.54%. The compressor's stall is triggered by the blockage induced by the concentrated shedding vortex in the stator rather than the rotor under the effect of coupled casing treatment.