Controlling Non-linear Axial Vibrations of a Turbine Rotor

The rotor of a 3-stage axial turbine designed for an ORC plant, supported by two axial/radial lubricated roller bearings, has some axial clearance in between the outer bearing ring shoulder in the casing to allow for thermal expansion. The rotor can move freely within the clearance before the bearings on both sides are able to apply a restoring force. In order to avoid axial vibrations excited by possible operating fluid pressure fluctuations, an unilateral axial squeeze film damper has been designed. CFD calculations have allowed to characterize the damper, which is highly non-linear. The damper has been introduced in the model of the machine and its performance has been analyzed by comparing the behavior of the damped rotor to the un-damped rotor, at the different exciting frequencies. The comparison has been performed necessarily in the time domain due to the presence of two non-linearity: unsymmetrical damper and non-linear elastic restoring force. The results of the comparison have shown the efficiency of the damper especially in conditions close to resonance.