Aerodynamic Effects in a Transonic Compressor With Nonaxisymmetric Tip Clearance

Future axial compressor designs tend to be built with larger relative tip gaps and eccentricity, since the core engines are reduced in size. Our knowledge of the aerodynamic effects due to eccentric tip gaps is largely based on low-speed work. The aim of this study is to widen current knowledge by using the 1.5 stage Darmstadt Transonic Compressor, which is representative of the front stage of a high pressure compressor. Efficiency, peak pressure rise and stability margin of the compressor are reduced linearly at design speed when the tip clearance is increased from 0.9% to 2.5% tip chord length. This holds true for configurations with eccentric rotor tip gap, if their circumferentially averaged gaps are considered. For a compressor with 96% eccentricity and 1.7% average tip clearance, corrected mass flow at rotor exit varies locally with up to ±20% and ±10% at stator exit, which can result in inlet distortions for subsequent stages in a multi-stage configuration. Also, the redistribution of flow massively influences stall inception during throttling at constant speed. Propagating disturbances are damped in sectors with higher inlet mass flow and lower incidence. Thus, overall operation remains stable, even though some sectors are highly disturbed. Consequently, the maximum clearance of an eccentric stage is not limiting the stable operation of the whole stage.