Rotordynamic Analysis of Plain Journal Bearings in Vertical Machines

Sammanfattning

Plain journal bearings have been widely used in hydropower machines for many years, exhibiting different dynamic behavior and instabilities. They are beneficially used as turbine guide bearings due to their high damping ability and load carrying capacity. Modeling the dynamic behavior of plain journal bearings is important for predicting possible instabilities and improving understanding, which can strengthen the reliability of hydroelectric systems. Oil whirl are common and modern vertical machines are more prone to this phenomenon due to their lack of a static equilibrium position. These instabilities increase vibration levels and shorten the operational life of the machines, which is why it is essential to be able to predict behaviors by developing and testing models and comparing them with experimental data to verify their accuracy. This study used two different models to predict and simulate the dynamic behavior, a Reynolds short bearing model and a model in which the bearing coefficients were predefined using polynomial equations. The latter was done using a Navier-Stokes approach to create a simulation model for nine different rotational speeds for each bearing (80 mm and 27 mm height, both 80 mm wide). The two models were then validated with experimental measurements of the displacements and the forces acting on the bearing. The experiments showed that sub-synchronous instabilities are more likely to occur at lower bearing loads, while the shorter 27 mm bearing exhibited a more stable behavior compared to the 80 mm bearing. The Reynolds short bearing model showed the best applicability for the 27 mm bearing and followed the overall response measured in the experiments, while it underestimated the displacement for the 80 mm bearing. The simulation model based on the Navier–Stokes equations showed better agreement with the overall response trends measured in the experiments for the 80 mm bearing, but did not capture the instability region. The simulations also showed larger orbital motion and additional sub-synchronous frequencies compared with the experiments. The instability phenomena occurring in the simulations were investigated and showed a strong connection to the cross-coupled stiffness terms. Another parameter showing a large effect on the dynamic behavior was the oil temperature, where lower temperatures reduced the sub-synchronous behavior. The conclusions regarding the applicability and accuracy of the models used were that design parameters and precise temperature measurements are of great importance. It proves difficult to reproduce the behavior and predict the dynamic response of plain journal bearings in vertical machines, where current models show limitations in predicting instabilities and capturing displacement amplitudes. Understanding the relationship between bearing coefficients, design parameters, and modeling choices was shown to be critical to improving the predictive capability of plain journal bearing models.