Abstract
This study is aimed at evaluating the feasibility of a spring-mass-damping model which reduces both the amount of RAM (random access memory) usage and the time taken to calculate results when analysing the effect of turbomachine impeller mistuning on both vibrations and fatigue life. The mistuning is the result of asymmetries in the assembled turbomachine impeller. The use of a spring-mass-damping model of a mistuned impeller tailored to calculate forced vibrations by introducing the variable of exciting load, as well as the finite element method and methods for calculating the fatigue life, allowed us to assess the fatigue life of a turbomachine impeller. To assess the accuracy of the model, calculated results for vibration frequencies and fatigue life were compared with the results of experiments with a turbomachine impeller at the Brandenburg University of Technology. The calculated natural vibrations demonstrated good accuracy (3-4% for lowest vibration modes) compared with the experimental results. The fatigue life was calculated for five different mistuning schemes simulated experimentally by attaching additional masses to the impeller rim. The calculated results were again compared with the results of experiments on the modified impeller. Recommendations on the assembly of such impellers were given. The error between the calculated results and experimental results of the natural vibrations is sufficiently small, which permits the use of the proposed model in the analysis of the effect of the vibrations of turbomachine impellers caused by mistuning on their fatigue life. It is shown that the amount of RAM required for the calculation is reduced by orders of magnitude, since the proposed model contains only two degrees of freedom per sector. As a result, the proposed model is of great utility in modelling modern structures with a complex shape. Using the results of this study, the fatigue life of impellers can be maximised by following the "sawtooth" law of the locating of the mistuned blades on the impeller rim when assembling turbomachine impellers.
Highlights
The rotor impeller of a turbomachine has a cyclically symmetric design
Representing this mistuning in a finite element method (FEM) model of an impeller is a serious complication requiring a significant increase in computer memory usage and processing time
The mistuning was simulated by the addition of masses to the periphery, outer circumference, of the blades
Summary
Computer models designed to study impeller vibrations often represent the impeller rotor blades as a set of idealised symmetrical sectors, in which case, no deviations in symmetry can be accommodated. Nipkau[1] was selected following a review of spring-mass models designed to represent these small imperfections. Many authors assert [1–12] that mistuning often has a negative effect on the stress-strain state of the impeller due to vibrations significantly reducing impeller service life. Representing this mistuning in a finite element method (FEM) model of an impeller is a serious complication requiring a significant increase in computer memory usage and processing time. Development of an effective model to analyse the mistuning effect on the service life of impellers, which uses computer resources more efficiently, is an urgent requirement
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