Thermally Induced Vibrations due to Rub in Real Rotors

The analysis of the effects of the rotor-to-stator rub in the rotor dynamics field has produced many models, often nonlinear or with particular attention to chaotic behaviour, but with scarce correspondence with real rotors since Jeffcott-like rotors are mainly used without thermal transients, or sometimes real rotors are used but with very simple thermal modelling. One of the most remarkable aspects of the rotor-to-stator rub at operating speed is the possibility of observing spiral vibration phenomena in the synchronous vibration, i.e. the vibration vector changes its amplitude and rotates even at the rated speed. This is due to the combined effect of the contact forces which introduce heat and of the resulting thermal bow. The phenomenon is also called vector turning or thermally induced vibrations. It is interesting to analyse the dynamical behaviour of the machine: the spirals can have a self-propagating trend or conversely a self-correcting trend. The model presented in the paper for the analysis of the spiral vibration has a unique characteristic in that it allows real machines to be analysed, since it uses a fully assembled machine model (rotor, bearings and foundation) and implements a rather sophisticated thermal and contact model. The experimental validation of the proposed model is performed on data coming from a real machine: the generator of a 50 MW combined cycle power plant, in which unstable spiral vibrations were observed at the operating speed during a power increase.