Development of Predictive Models for Synchronous Thermal Instability

The increasing demand of higher efficiency and increased equipment compactness is pushing the modern rotordynamic design towards higher and higher bearing peripheral speed. Due to the increased viscous dissipation, modern fluid film bearings are prone to the development of complex thermal phenomena that, under certain conditions, can result in synchronous thermal instability, often referred to as Morton effect. Although the phenomenon is known and studied from the late 1970s a lack of knowledge is highlighted in literature and the strategy to approach its prediction and analysis is yet debated within the scientific community. This work presents the development and validation of the numerical models for the prediction of the synchronous thermal instability. The proposed models are derived from a preliminary analysis of the physical time scales of the problem and of the orders of magnitude of the equations, which allowed an aware selection of the modelling strategies from a dual point of view: the physical genesis of the Morton effect (i.e., the differential heating of the shaft) and the assessment of the stability of the rotor-bearing system under the influence of the thermal effects. Particular focus is devoted to the fluid-dynamical problem with the description of two dedicated codes, developed, respectively, for the analysis of the thermo-hydrodyanmics of fluid film bearings and for the modelling of the differential temperature developed across the shaft. This latter phenomenon is due to the differential heating and results to be the driving parameter of the problem. Once the two codes has been individually validated, these have been inserted in more complex systems in order to evaluate their ability to enable the prediction of the Morton effect. A linear stability analysis has been firstly performed and results, although affected by discrepancies with respect to the experimental data, have shown the potential of the codes to reach the objective of the work. Better results have been finally obtained when the models have been inserted in a more complex architecture. This latter has been developed in collaboration with the MDMlab of the Department of Industrial Engineering of the University of Florence in order to model the synchronous thermal instability by means of an iterative approach. A comparison with available experimental data, derived from a dedicated test campaign carried out at the GE Oil & Gas facility in Florence, is shown in order to validate both the procedure and the models. Moreover, some key parameters driving the Morton effect are presented and a study of the sensitivity of the phenomenon to the thermal expansion coefficient is proposed in order to improve researchers’ knowledge on the topic.