Unsteady Effects of Casing Treatment on Tip Flow Structures in a Subsonic Compressor Rotor

Abstract

Abstract With the help of the dynamic mode decomposition (DMD) analysis, the unsteady effects of casing treatment on tip flow structures in a subsonic compressor rotor were numerically studied. Taking the change of casing treatment geometric parameters into account, the frequency characteristics and dynamic modes were compared and discussed, for further revealing the stability enhancement mechanism of casing treatment. At smooth casing, the mode analysis showed that the tip flow structures were dominated by high absolute mode values alternately emerging along passage streamwise, which is consistent with the evolution of tip leakage vortex (TLV), and the mode frequency is 0.42 BPF. After applying casing treatment, the unsteady characteristics of tip flow fields changed significantly. The mode analysis indicated that the tip flow structures were dominated by intermittent high-energy excitation, which was caused by suction and injection actions of casing treatment. As a result, the relative position of slot and rotor determined the flow features inside casing treatment slots, and an significant induced vortex structure varied in one mode period. By comparing the casing treatment configurations with different slot number, it could be found that the suction and injection effects of casing treatment were closely related to the dominant frequency brought by the slots. When the slot number was not adequate (less than 2 per passage), the excitation energy of suction and injection effects on tip flow field were limited. With the slot number increasing, the excitation energy raised gradually, so the stability enhancement brought by casing treatment was better. This paper demonstrated the effectiveness of DMD method on investigating the unsteady effects of slot-type casing treatment, giving a encouraging prospect for the application of DMD in the internal flow fields with passive control strategies.