Numerical investigation into the effects of casing aspiration on the overall performance and flow unsteadiness in a counter-rotating axial flow compressor

The impacts of casing aspiration with circumferential slot over the rear rotor on the overall performance and unsteady flow behaviors have been studied with numerical simulations in a counter-rotating axial flow compressor. The results indicate that casing aspiration is an effective active flow control method to expand the safe operating range of the compressor. The peak efficiency is always decreased to a different extent in the aspiration schemes and the best aspiration scheme is located at near 10% tip axial chord without significant peak efficiency loss. After the casing aspiration, both the relative inlet flow angle and the leading edge blockage are reduced obviously in the tip region. The angle between the exit direction of tip leakage flow (TLF) and axial direction is also reduced over the whole blade chord range, especially below the aspiration slots due to the local blade unloading effect. Additionally, the interface between the TLF and incoming main flow is pushed more downstream in the aspiration schemes and it is more effective to improve the flow stability by controlling the TLF released around mid-chord through casing aspiration. The intensity of both TLF and double leakage flow is attenuated and the overall oscillations inside are alleviated to a different extent in the aspiration schemes. Frequency analysis shows that the fluctuation of TLF with lower frequency component is suppressed by casing aspiration which is thought to be beneficial for the enhancement of stall margin.