The transonic compressor with non-uniform tip clearance

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.