Modelling fluid-film bearings in high-speed rotating machinery considering the prehistory of dynamic phenomena

This article presents a concept for describing the dynamic states of a rotating machine, which accounts for the disruption of the continuity of the lubricating film in the slide bearings due to cavitation, and thus considers the time-varying boundaries of the positive hydrodynamic pressure region. Numerical procedures and an advanced computer programme have been proposed to account for the mutual couplings of the machine's dynamic states at preceding and succeeding moments. This approach has been referred to as prehistory or continuous description. The article also presents an example of the practical application of the computational tools developed. The subject of the analysis was a 100 kW Organic Rankine Cycle (ORC) microturbine. The calculation results showed that at a nominal speed of 9,000 rpm, hydrodynamic instability may occur, with the amplitude of journal vibration reaching values close to the radial clearance. Based on the results of the theoretical research carried out using the developed programmes, a decision was made to modify the bearing system. The proposed method is particularly useful for large and rapid displacements of the journal within the lubricating gap, especially after the stability threshold of the rotor-bearing system has been exceeded. In such cases, differences in the assessment of the machine's condition based on traditional research tools may be qualitative. This is demonstrated in the present work using the example of a 100 kW microturbine. Taking prehistory into account allows for a continuous, step-by-step mathematical description of the processes and represents a further refinement of the research tools being developed to capture the nature of the phenomena occurring in fluid-flow machines.