Heyns, P.S. (Philippus Stephanus)2014-07-302014-07-302014-04-162014Scheepers, R 2013, A Comparative study of finite element methodologies for torsional vibration response calculations of bladed rotors, MEng dissertation, University of Pretoria, Pretoria, viewed yymmdd <http://hdl.handle.net/2263/41013>E14/4/304/gmhttp://hdl.handle.net/2263/41013Dissertation (MEng)--University of Pretoria, 2014.Turbo-generator trains are susceptible to torsional vibration which can lead to fatigue cracking and failure. Methods are available for the measurement and calculation of the torsional natural frequencies of these systems for the purpose of design, monitoring and life prediction. Calculation methods are conventionally based on one dimensional (1D) finite element (FE) methodologies which require the simplification of a number of aspects including the participation of flexible blades in torsional vibration modes. The accuracy of 1D, three dimensional (3D) and three dimensional cyclic symmetric (3DCS) FE methods was investigated by the application thereof on a small test rotor. Experimental measurements of static and dynamic vibration responses were conducted with rotation and torsional forcing accomplished through the use of a DC motor and a digital control system optimised for fast transient and stable steady state response. Blade stagger angle was demonstrated to have a significant effect on torsional frequencies although no stress stiffening effects were noted in the speed range considered. Similarly, damping was measured to decrease with blade stagger angle but not with rotational speed. Step changes in torsional frequencies due to the activation of the motor field and armature currents required optimisation of the motor models for static and dynamic conditions. ii Shaft torsional vibration responses were found not to include all blade modes and vice versa. Full 3D parametric models with a high degree of geometric detail were generated and meshed using commercial software ANSYS ver. 14.0. No simplification was introduced other than for the armature motor where an equivalent material density and elastic modulus was obtained by measurement and frequency calibration. Calculated torsional frequencies agreed well with measured results for static and dynamic conditions. 3DCS models obtained by simplification of the full 3D models resulted in similar accuracy but lower solution times. Visualisation of torsional modes is enhanced by 3D modelling which also includes rigid shaft modes which is not possible in the 1D approach. Further reduction to 1D models requires a number of simplifications which result in smaller models with low solution times but generally reduced accuracy. Blade torsional participation was accomplished in the 1D approach using Euler-Bernoulli beam theory and the component mode synthesis technique to calculate equivalent mass and stiffness as well as the residual mass of each blade mode to be coupled. Simplifications for sudden diameter changes and shrunk-on disks were also made. It is concluded that all three FE techniques applied in this work are useful depending on the required accuracy, available information and resources. In cases, where a high level of accuracy is required, direct field measurements should be used for model calibration or model updating.en© 2013 University of Pretoria. All rights reserved. The copyright in this work vests in the University of Pretoria. No part of this work may be reproduced or transmitted in any form or by any means, without the prior written permission of the University of Pretoria.Torsional vibrationModal analysisSteam turbineFinite element analysisTorsional excitationComponent mode synthesisCyclic symmetricUCTDDissertation