Hybrid Slot-Groove Casing Treatment for Stall Margin Improvement on an Axial Flow Compressor with Circumferential Total Pressure Distortion

The hybrid slot-groove (S-G) casing treatment (CT), which combines the advantages of slot and groove in consideration of stall margin enhancing and efficiency penalty, was experimentally investigated under circumferential distorted inflows. Previous experiments showed that the hybrid S-G CT can extend the stability by 19.79% with uniform inflow condition. To further estimate its stability enhancement ability with distorted inflow, three types of circumferential total pressure distortion inflow that the distorted intensities (DC(60)) are equal to 0.90%, 4.12%, and 24.75%, are selected to conduct a serial of experiments. Results demonstrated that the stability of the compressor were deteriorated by 7.87%, 9.19% and 39.08% respectively under three distorted inflows. It was founded that, under the above-mentioned distorted inflows, the hybrid S-G CT was able to extend the stability by 18.48%, 17.81%, and 13.80%, respectively, which proved the strong anti-distortion ability of the hybrid S-G CT. By using the dynamic pressure sensors fixed on the casing wall, the unsteady measurements demonstrated that the stall precursor with uniform and circumferential distorted inflows are always spiky-wave; thus the hybrid S-G CT can play an excellent stability enhancement capability. The analytical results with power spectral density proved that, when at the same flow point, the perturbation, with frequency being around the rotating stall frequency band, was distinctly suppressed by hybrid S-G CT, thus delaying the rotating stall. The stall precursor detected in the casing wall depicted that unlike the short length-scale of stall precursor (5–6 blade passage) under smooth casing; the hybrid S-G CT can create a stall precursor with long length-scale. Under the distortion intensities from 0 to 4.12%, the length scale of the stall precursors occupies approximately 10–12 blade passages. As the distortion intensity further increases to 24.75%, the length scale of stall precursor increases to occupy 16–17 blade passages. This phenomenon can guide the stall warning studies with compressors using CT in the future.