Study on Mode Localization Induced by Material and Aerodynamic Mistunings in Impellers with Periodical and Cyclic Symmetry

The structure of ideal impellers of large-scale centrifugal compressors possess typical cyclic symmetry, which can be divided into exactly the same fan-shaped substructure, with respect to both material attributes and geometric shapes. Different from non-cyclic symmetrical structure systems, the ideal impellers own many unique dynamic properties. Nevertheless, in practical operation process, impellers are always mistuned due to manufacturing errors, running wears, adherence of working medium and corrosion etc., defined as mistuning. On the other hand, the impellers are sensitive to vibration mode localization induced by mistuning. Therefore, the existing results have shown that the vibration mode localization induced by the increasing diameter of impellers is responsible for the frequent happening of the catastrophic accidents, including high-cycle fatigue fracture of centrifugal impellers and instability of rotors. Under such background, the mode localization induced by material and aerodynamic mistunings in centrifugal impellers with cyclic symmetry are studied in this study. First, the finite element available for analyzing material mistuning and mathematical model for aerodynamic mistuning are given in detail. The material properties, such as Young’s modulus, Possion ratio, density, are considered in the mistuning, which have influences on the mode localization of impellers of centrifugal compressor, and probability distribution function is introduced to model the stochastic sequence of mistuning. Further, such function and model are applied to the element, and the mistuning of material and the elastic matrix etc. are then obtained. More, it is found that the non-uniform of flow field in each passage could lead to mistuning of aerodynamics, and a perturbation is introduced at the inlet of impeller to simulate the difference between each passage. Then, the results obtained are extended to software, ANSYS, and some programs are developed based on APDL. Further, the mode localization of centrifugal impeller introduced by material mistuning is studied numerically in detail. In particular, the influence of material mistuning on the mode localization is investigated, and the differences in natural frequencies between perfect and mistuning impellers are analyzed further. Following that, some important results related to material mistuning are obtained. Finally, the modal analyses of the impellers with steady aerodynamic mistuning are carried out using ANSYS and the developed program. The fluid-structure interaction is introduced to analyze the non-uniform flow field, with cycle periodic boundary conditions. In the study, the stress stiffening effect, which is induced by steady aerodynamic forces and rarely considered in the study before, is regarded as a main factor to mode localization. For impellers working under high pressure conditions, more attentions are paid to the aerodynamic mistuning, as it may lead to rather violent mode localization easily.