The transonic compressor with non-uniform tip clearance: Effects on aerodynamics and aeroelasticity

This study experimentally investigates the aerodynamic and aeroelastic behaviour of a transonic compressor with non-uniform tip clearance. Current design trends for aero engines require more efficient compressors with high pressure ratio and reduced weight. An increased overall pressure ratio permits a downsized core engine while its power output is maintained. This increases the bypass ratio, which is crucial for engine noise and efficiency. However, a more compact core engine leads to larger relative gaps between rotor and casing, as the clearance cannot be scaled accordingly. For the same reason, casing asymmetry has become more important for future engines. On the one hand, the aim of the work is an aerodynamic analysis in the field of clearance sensitivity and thus engine deterioration. On the other hand, an analysis of the aeroelastic behaviour is carried out, since transonic compressors are used in front stages of the core engine, which tend to flow-induced vibrations at part speed. These vibrations can result in cracks or even blade failure. Stabilizing measures, such as casing treatments at the rotor tip, are common knowledge. However, their influence on blade vibration, especially of a non-uniform circumferential distribution, is largely unexplored. The results of the work relieve aerodynamic and mechanical concerns regarding the application of axial compressors in future small core engines. The destabilizing influence of casing eccentricity is less than previously assumed, which is why the intended safety margins can potentially be reduced. With regard to flowinduced vibrations that occur in concentric casings across the entire speed range, this dissertation shows for the first time that non-uniform rotor tip clearances represent a novel approach in the field of aerodynamic mistuning. The mass flow redistribution upstream of the compressor with a non-uniform clearance causes a circumferential variation of the rotor incidence, which in turn varies the aerodynamic force on the blades circumferentially. As a result, the rotorrelative flow recovers during one turn of the rotor, which ultimately leads to a reduction of the blade vibration amplitudes. Slight asymmetries in an engine have a stabilizing influence on blade vibrations. For the given case, e.g. an eccentric rotor clearance reduces the amplitudes of nonsynchronous vibrations by -25% of the rig operating limit, compared to a concentric casing with the same average clearance. Partial casing treatments benefit from the same effect and will hence be a powerful tool to avoid non-synchronous vibrations in the future. In contrast to a circumferentially uniform casing treatment that can amplify blade vibrations, non-uniform designs are now a smart solution for the problem of non-synchronous vibrations in front stages during off-design operation