Influence of circumferential casing treatment groove on leakage vortex breakdown and unsteady flow characteristics in a transonic compressor rotor

The study reveals that leakage vortex breakdown is the primary cause of unsteady flow phenomena in the tip region of a transonic compressor rotor. Under near-stall conditions, significant unsteady flow characteristics are observed in the multi-channel configuration, with the tip unsteadiness closely linked to variations in tip loading. Under shock wave interaction, the tip leakage vortex undergoes expansion and generates reverse flow, leading to vortex breakdown. The leakage flow outside the vortex core circumvents the large-scale reverse flow region, accompanied by secondary leakage phenomena. Due to blockage in the tip flow field, leading-edge spillage occurs, forming a self-sustained unsteady cycle. After implementing circumferential casing treatment, leakage vortex breakdown is suppressed even under stall conditions, and the unsteady phenomena in the tip region are significantly mitigated. The scheme of groove parameters (CT-Z1, h6, W2) improve stall margin by 29 %. Regarding the stall mechanism: For the solid casing, the leading-edge spillage induced by leakage vortex breakdown serves as the precursor to rotor stall. As the outlet pressure increases, substantial leakage flow spills over the blade leading edge, thereby increasing the incidence angle of the mainstream and further destabilizing the flow in adjacent blade passages. When multiple channels experience large-scale blockage clusters, the rotor enters stall, accompanied by a rapid drop in mass flow. Unsteady RANS simulations reveal that leakage vortex breakdown is the primary cause of unsteady flow in the transonic compressor rotor tip region under near-stall conditions, and circumferential casing treatment can suppress such breakdown to mitigate unsteady phenomena. Although casing treatment grooves can eliminate leakage vortex breakdown and enhance compressor stability margin, the stall mechanism remains governed by leading-edge spillage of leakage flow. Consequently, casing treatment delays stall onset but does not fundamentally alter the underlying stall mechanism.