Failure mechanism of casing treatment in improving stability of a highly loaded two-stage axial compressor

Abstract

Casing treatment is a powerful method for enhancing stability of certain aircraft compressors in which tip leakage vortex (TLV) triggers stall. An optimized slot-type casing treatment was installed on the first rotor of a highly loaded two-stage compressor. The experimental result showed that the casing treatment could not increase the compressor stability at three tested speeds. This study aims to understand the failure mechanism of the casing treatment for developing an effective method to depress stall. A “three-step method” is proposed to determine the stall-limiting location and understand the underlying stall mechanism. At 90% speed, there is a spike in the aerodynamic flow parameters in the tip region of the first rotor under the near stall condition compared with that under the design condition. The spike is induced by an interaction among the TLV, boundary layer at the blade suction surface, and passage shockwave. Both the TLV and the shockwave–boundary layer interaction contribute to the accumulation of tip blockage which triggers compressor stall. The TLV does not break down under the near stall condition, but unsteady forward spillage of the TLV at the blade tip leading edge can be detected, resulting in unsteady tip flow. When the casing treatment is installed on the first rotor, a typical flow circulation is established in the slots. The intensity of the TLV and the flow unsteadiness are greatly depressed, and the discharge capability in the tip region is remarkably improved. However, the flow circulation cannot manipulate the fierce boundary layer separation induced by the shockwave, which causes increased tip blockage and reduced compressor stability. A coupled casing treatment, which can bleed the shockwave induced blockage, is proposed and found to be very effective in improving compressor stability. Therefore, the technology for damping the compressor stall should be capable of manipulating both the TLV and the shockwave induced boundary layer separation. At 100% speed, a shedding vortex is initiated at the hub of the second stator. This vortex expands sharply in the stalling process and triggers compressor stall. Variation of the location at which the compressor stalls is the reason for the failure of the casing treatment at 100% speed.