Effect of casing treatment axial position on a transonic axial fan rotor at varying Reynolds numbers

This study employs an experimentally validated numerical simulation method to investigate the effects of casing treatment (CT) axial positions on a transonic axial fan rotor at different Reynolds numbers (Re). The stall mechanisms and loss characteristics of the rotor are analyzed at five different Re to determine the approximate range of critical Re. Two Re, Re = 4.55 × 105 and Re = 0.34 × 105, representing different stall mechanisms, are selected for the study of a semi-circular axial slot casing treatments (ASCT). The mechanisms of stability enhancement and stall characteristics are examined in detail for different axial positions of the CT at these Re conditions. The results indicate that the maximum stall margin improvements for Re = 4.55 × 105 and Re = 0.34 × 105 are 44.0% and 44.4%, respectively. At Re = 4.55 × 105, the stall is triggered by blockage caused by the interaction of the shock wave and tip leakage vortex (TLV). Significant stall margin enhancement is achieved when the CT effectively suppresses this blockage while suppressing the leading-edge tip leakage flow (TLF). Shifting the CT upstream causes flow separation on the suction surface, while positioning it too far downstream intensifies the TLF. At Re = 0.34 × 105, the stall is driven by suction surface separation and TLF spillage caused by radial transport flow. Maximum stall margin enhancement is achieved when the CT covers the radial transport flow region downstream of the shock wave and the origin of the TLV. However, shifting the CT upstream negatively impacts the radial transport flow, while positioning it downstream similarly increases TLV intensity, thereby reducing the stall margin improvement.