Performance and Stall Margin Evaluation of Axial Slot Casing Treatment in a Transonic Multistage Compressor

Adverse pressure gradients are intrinsic to the flow behavior in compressors and are further ex-acerbated by secondary effects arising from rotor tip clearance flow interactions, a phenomenon common across various turbomachinery configurations, axial and radial. Tip clearance induces leakage flow, which leads to the formation of tip leakage vortices—a principal contributor to aerodynamic losses in axial compressors. These vortices exert a significant impact on both com-pressor performance and the operational stability range. Extensive prior research has demon-strated that passive casing treatments, particularly the implementation of axial slots, can sub-stantially enhance stall margin in axial compressors. The recirculating flow established within semi-circular axial slots acts to mitigate the strength of the tip leakage vortex by redirecting low-momentum leakage flow upstream of the rotor leading edge, thereby introducing a resistance mechanism that suppresses flow leakage through the clearance gap. Many existing studies on axial slot implementation focus predominantly on high-performance single-stage axial compressors. Consequently, there remains a gap in the literature regarding the application and performance implications of such treatments in multi-stage compressor systems. In the present study, a passive casing treatment employing axial slots was integrated into a multi-stage axial compressor. The slot geometry was selected based on recent experimental findings involving single-rotor configura-tions. The casing treatment was applied over the first rotor row in a three-and-a-half-stage (3.5-stage) axial compressor configuration, comprising an inlet guide vane (IGV) followed by three rotor-stator stages. Three-dimensional steady-state numerical simulations were conducted to investigate the aerodynamic effects of the axial slot treatment across various rotational speeds. The results obtained for the slotted configuration were compared against a baseline case featuring a smooth casing. The introduction of axial slots induced noticeable modifications to the internal flow structure, particularly in the tip region, leading to an improvement in overall compressor stability within the operating range between 85% to 100% of design speed. Stall margins improved 3.55% at design speed. Significant stall margin improvement occurred at 95% design speed with ΔSM of 11.24%. At 85% design speed, the efficiency starts to deteriorate and axial slots did not improve the stall margins with ΔSM of -1.03 %.