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 objective is to propose a method to characterize such collaboration for coupled components. To investigate the multi-stage coupling, the analysis of the structural dynamics has to consider the drum system. In consequence, compared to the investigation of individual stages, significantly higher computational costs are expected. Moreover, also when considering the tuned reference geometry, the full drum loses the cyclic symmetrical property of the individual stages due to the different airfoil counts. A reduction method to compute the system modes is investigated, taking advantage of the nominal individual stages’ periodicity. The quality of the results is assessed comparing the computed mode-shapes and natural frequencies with the respective full model of the investigated drum. The identification and quantification of the coupling is finally crucial to describe the structural dynamics of the drum system. Modes with high coupling show 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. In this work, a formulation for an index is proposed, named coupling strength, to quantify the intensity of this collaboration for each mode. The formulation considers an arbitrary number of stages and can be extended in general to different coupled structures.