Effect of axial slot casing treatment on the flow instability for different speed ratios in a counter-rotating axial flow compressor

This paper numerically investigates the effectiveness of axial slot casing treatment (ASCT) at different speed ratios and its flow control mechanism in a counter-rotating axial flow compressor (CRAC). The effect law of ASCT on the stall margin with speed ratio is revealed. Furthermore, the impact of ASCT on the first stall stage of CRAC is explored. The results show that the compressor stall margin at different speed ratios is improved by the ASCT. The combined effect of suction and injection of the ASCT suppresses the spillage at the blade leading edge in the rear rotor (R2) and delays the occurrence of stall inception. Quantitative analysis shows that the ASCT reduces the reverse axial momentum and deflection angle of the tip leakage flow (TLF), which contributes to improving the flow stability. Moreover, the blockage at the rotor tip with the ASCT is reduced, which improves the tip flow field of the R2. Although the stall inception of spike-type keeps unchanged with the ASCT, the first stall stage of the CRAC under the ASCT is switched from the R2 to the front rotor when the speed ratio is not greater than 1.0. When the ASCT is employed on the R2, the significant stall margin improvement can only be obtained if the R2 is still the first stall stage under the ASCT.