Numerical Study on the Effect of Slot-Type Casing Treatment in a Transonic Compressor

This paper presents detailed numerical analysis of shallow reversed blade-angle slots casing treatment(RSCT) in a transonic axial compressor stage operating at design and off-design conditions. Previous studies have shown that the present compressor rotor is tip-typical and suitable for the use of casing treatment to improve flow stability and enhance stall margin. The unsteady simulations are performed with a three-dimensional time accurate Favre-averaged Navier-stokes flow solver. The numerical results with solid casing demonstrate that the numerical stall is initiated from rotor tip blockage caused by tip clearance vortex(TLV). At both design and off-design conditions, low energy flow zone caused by TLV and the interaction of shock and boundary layer bring about severe passage blockage, the stall characteristics contain of the tip blockage stall and blade tip stall. However, instantaneous flow field visualization analysis shows that the unsteady characteristics of tip blockage at two operating conditions are very different. The RSCT casing treatment improves effectively the stable operating range of the present compressor stage at both design and off-design conditions, the stall margin improvement(SMI) up to 8.7% and 15.6% separately, with the peak efficiency dropped slightly (less than 1.2%). The unsteady suction and injection effect of the RSCT slots removes observably the low energy blockage induced by TLV and weakens the separation of the blade suction surface boundary layer, playing a stabilizing role in the rotor blade tip flow. However, the comparison between SC and RSCT of the radial distribution of absolute rotor outflow angle and stator passages flow show RSCT brings about negative influence to compressor stator. At RSCT near stall condition, the absolute rotor outflow angle increases by about 3 degrees, and stator suction surface vortex shedding causes severe unsteady stator passages blockage, which is likely to trigger the new inception for compressor stall.