The Effects of Axial Slots with Radial Skew Angles on the Performance and Stall Margins in a Transonic Multi-Stage Axial Compressor

In the present study, three-dimensional steady-state Reynolds-Averaged Navier–Stokes (RANS) simulations were performed to investigate the influence of radial skew angle in axial slot casing treatments (CTs) applied to the first rotor of a transonic three-and-a-half-stage axial compressor. The investigation focuses on three skewed slot configurations with radial skew angles of 35°, 45°, and 60°, while the axially aligned slot configuration (0° skew angle), previously reported and validated, is adopted solely as a reference baseline. All configurations share identical geometric proportions, enabling the isolated assessment of skew-angle effects on compressor aerodynamic behavior. The results demonstrate that the radial skew angle significantly influences the balance between stall margin improvement and efficiency variation, with the magnitude of these effects strongly dependent on operating speed. Under off-design operating conditions, skewed axial slots provide substantial stall margin enhancement relative to the smooth casing configuration, with larger skew angles generally yielding greater improvements in compressor stability. Conversely, at the design speed, increasing skew angle is associated with a progressive efficiency penalty, highlighting the trade-off between aerodynamic stability enhancement and additional aerodynamic losses. Among the skewed configurations investigated, the 45° radial skew angle provides the most favorable compromise, delivering consistent stall margin improvement across the examined operating range while preserving compressor efficiency under nominal operating conditions. Overall, the present study demonstrates that radial skew angle constitutes a critical design parameter for axial slot casing treatments in multistage compressors and should therefore be carefully tailored to the intended operating regime. The findings extend the current understanding of axial-slot flow-control mechanisms and provide practical guidance for the aerodynamic design of casing treatments in transonic axial compressors.