Numerical investigation of the first stall stage in a counter-rotating axial-flow compressor with large tip clearance and speed ratio effects under casing treatment

Single-passage steady-state numerical simulations were used to study the first stall stage of a counter-rotating axial-flow compressor (CRAC) under a large tip clearance of 1 mm and rotating speed ratio effects. The first stall stage is evaluated by assessing the effectiveness of the circumferential grooved casing treatment (CT) over the front and rear rotors. The variation of the first stall stage and the stability enhancement rule of the CRAC are analysed systematically under different speed matching schemes. The results show that, unlike the change law of the first stall stage of design tip clearance of 0.5 mm in the CRAC, when the speed ratio N2:N1 (N1 and N2 are the rotating speed of the front and rear rotor respectively) is less than 1.0, the front rotor R1 is the first stall stage; and when the speed ratio N2:N1 is greater than 1.11, the effect of CT on stall margin expansion is limited, and the CRAC may exhibit modal stall characteristics, pending future unsteady or experimental verification. Simultaneously, the flow field near the rotor tip can be effectively improved by CT at the first stall stage of the CRAC, including reducing the tip load, decreasing the relative airflow angle at the blade tip, moving the interface between the tip leakage flow (TLF) and the main flow backward, and reducing the degree of blade tip blockage, thereby enhancing its stall margin (SM). Additionally, the flow field of the treated rotor significantly affects the downstream rotor due to tip leakage vortex migration driven by the inter-rotor pressure gradient, potentially inducing localized flow alterations in the untreated rotor.