Session: 28-07 Dynamic Response of Bladed Disks

Paper Number: 128647

128647 - Structural Dynamics of an Axial Compressor’s Rear Blisk Drum and Multi-Stage Coupling 

Modern turbomachinery compressor design favors the introduction of multi-stage drums to reduce the components’ weight and increase the machine efficiency. The drum design creates a direct connection between the stages, which can lead to a collaboration of the stages in the system structural dynamics. This motivates the investigation of multi-stage coupling for the structure. The work focuses on an industrial axial compressor’s rear drum blisk for civil aviation application. The geometry of the rear drum’s disks and aerofoils have similar dimensions for neighbouring stages, contrary to the front drum blisk where the aerofoil size significantly decreases with stage number. Furthermore, drum recesses for the inner shrouds of stator vanes are missing, resulting in a more direct connection between the disks. These features are likely to create overlaying natural frequency bands between stages, leading to inter-stage coupling.

To investigate the multi-stage coupling, the analysis of the structural dynamics has to consider the drum system. This, compared to the investigation of individual stages, requires a significantly higher computational cost. Moreover, also when considering the tuned reference geometry, the full drum loses the cyclic symmetrical property of the individual stages due to the different aerofoil counts. A reduction method to compute the system modes is investigated, taking advantage of the nominal individual stages’ periodicity. This allows to significantly reduce the computational cost when investigating a limited number of harmonic indexes. The advantage can be exploited in the investigation of the coupling, as coupled modes appear on a limited set of nodal diameters for limited frequency bands. The quality of the result is assessed with respect to a full annulus model, considering the number of harmonics required to correctly represent natural frequencies and mode-shapes.

The identification and quantification of the coupling is finally crucial to describe the structural dynamics of the drum system. Modes with high coupling present a co-participation of the stages in the dynamics. This implies vibrational responses on a stage due to the excitation of a different drum row. Therefore, while uncoupled modes can be analysed stage-wise, in presence of coupled modes the drum system must be considered. In particular, the excitation forces on all the coupled stages and the respective phase will contribute to the forced vibration amplitude and therefore dynamic stresses. An automated method to identify coupled modes is presented, considering an arbitrary number of drum stages. The methodology is applied to the investigation of the industrial rear drum blisk, analyzing the coupling of a real geometry.

Presenting Author: Marco Gambitta Brandenburg University of Technology (BTU)

Presenting Author Biography: Marco Gambitta is a mechanical engineer, currently working as PhD student at the Brandenburg Technical University (BTU) in Cottbus, Germany. He completed his studies at the University of Trieste, Italy, with the master in Mechanical Engineering in 2018. He wrote his mater thesis in Rolls-Royce Deutschland on manufacturing geometrical variability and robust design for axial compressors. Since 2018, he is employed at the BTU Cottbus as a PhD student, conducting his research in collaboration with Rolls-Royce Deutschland. The research focuses are axial compressor blades geometrical variability, aeroelasticity and vibration.

Authors:

Marco Gambitta Brandenburg University of Technology (BTU)
Bernd Beirow Brandenburg University of Technology (BTU)
Thomas Klauke Rolls-Royce Deutschland