Dynamic Characteristics Of Mistuned Shrouded Impellers Of Centrifugal Compressors

Centrifugal compressors are important equipments in metallurgy, building material, electric power, petroleum, chemical, environmental engineerings and other industrial fields. They are broadly applied in many areas associated with the national economy as one of the key products in large-equipment manufacturing industry. Shrouded impellers are core device of centrifugal compressors. To a great extent, the safety of the equipment relies on the dynamic characteristics of the impellers.The structural systems of ideal shrouded impellers of centrifugal compressors possess perfect cyclic coordinated symmetry. Nevertheless, real impellers are always mistuned due to manufacturing errors, running wears, adherence of working medium and cankering, among others, which bring differences to sub-sector structures of the impeller. Based on previous theoretical and experimental researches, the dynamic characteristic of a mistuned impeller differentiates from that of an ideally tuned one. For the impeller that has some forms of mistuning under external excitations, the vibration energy can be concentrated to some local regions of the impeller and not be able to spread out even though the mistuning is tiny. Consequently, the local dynamic response increases to trigger the so-called localization of vibration. In general, the dynamic response in mistuned impellers is greater than that of tuned ones. The localized vibration may lead to fatigue failure in whole or in part even if it doesn't cause immediate damage to the impeller. Indeed, a great number of accidents have been reported on turbine machines which failed due to fatigue as the result of localized vibrations.In this thesie, the dynamic characteristics of shrouded impellers of centrifugal compressors are analyzed systematically. The degree of coupling of blade vibrations for the shrouded impeller is defined. Using the finite element method, the modal localization factor and the response localization factors are presented. Then, a shrouded impeller of a domestically made centrifugal compressor is taken as the example. A precise finite element model is built up through appropriate geometrical simplifications. The natural frequencies and steady-state response of the tuned impeller are obtained using the method of cyclic symmetry analysis. The mistunings originated from non-uniformities of stiffness and of mass are taken into consideration by changing the modulus of elasticity and density. The global mistuning in patterns is distributed on all blades, while the local mistuning is generated and applied to a single blade. Deep discussions are presented about the influence of mistuned strength on modal localization and response localization. Further, a particular type of local mistuning caused by ammonium-base salt crystallization is studied and discussed. Finally, conclusions are drawn and future researches on dynamical analysis for mistuned impellers are highlighted.This paper is funded by the Chinese National Programs for High Technology Research and Development (2007AA04Z405).