Coupling method of stability enhancement based on casing treatments in an axial compressor

Abstract

Casing treatment utilized in aircraft compressors is characterized by its powerful capacity of enhancing stability but causes remarkable efficiency penalty. To deal with this problem, we tested several types of casing treatments, including slots, grooves, recirculating casing treatments, and a composite structure to understand their characteristics. The test results show that different casing treatments can be combined to obtain a higher compressor stability, and the efficiency penalty can be decreased by implementing a novel flow management technology. Based on the results, a coupled casing treatment (CCT) that is constructed with slots, injectors, and a plenum chamber is proposed and optimized in the present study. The optimized CCT improves compressor stability by 16.7% with no penalty on the compressor efficiency. Two flow circulations, namely, an inner circulation in the slots and an outer circulation from the slots to the injectors, which dampen the development of tip leakage vortex, account for the excellent stability enhancement. The outer circulation, defined as the coupling effect, has generally a positive effect on compressor stability and a negative effect on the compressor efficiency. The effects of the length and numbers of the slots on the coupling effect are more significant compared to the slot location. The hysteresis effect that the recovery of tip blockage lags behind that of the tip leakage vortex cannot be established due to inadequate injection energy actuated in the CCT. As a result, a circumferentially discrete distribution of recirculating loops cannot improve compressor stability satisfactorily. As the CCT covers the full annulus, a much higher stability is obtained, and the boundary layer at the rear part of blade passages is severely separated when approaching the stall limit, which ultimately triggers compressor stall rather than the tip leakage vortex. The stall inception behaves as a full-annulus-dimension collapse with the flow phenomena of backflow at the trailing edge and forward spillage at the leading edge of the blade.