Gas-Film Lubrication, Leakage, and Secondary-Face Friction of a High-Speed Floating Ring Seal under Transient Operating Conditions

Gas-film floating ring seals are widely used in high-speed turbomachinery, where the establishment and decay of the gas film during transient operation strongly affect leakage performance, secondary-face friction, and rotor–ring contact risk. In this study, a transient tribological model is developed to investigate the lubrication and tracking behavior of a high-speed gas-film floating ring seal under startup, steady-speed, and shutdown conditions. The model couples a Hirs-type turbulence-corrected reduced-order representation of the gas-film force and leakage with a secondary-face friction model based on Majumdar–Bhushan fractal contact mechanics and a radial rub-impact model between the rotor and floating ring The model is validated using leakage-rate measurements and synchronous rotor–ring displacement responses obtained from a high-speed test rig. The maximum deviation of leakage prediction is 10.3%, and the deviation of the tracking gain is approximately 10.8%, indicating that the model can capture the main sealing and tracking characteristics. The results show that increasing rotational speed strengthens the gas-film support and reduces leakage by approximately 16.3% from 5000 to 35000 r/min, while no radial rub-impact is observed within the investigated speed range. Increasing rotor excitation amplitude reduces the minimum gas-film thickness and increases the relative contact-risk indicator, whereas increasing inlet pressure mainly increases leakage and secondary-face friction while slightly weakening the tracking response. Contact-risk maps further indicate that rotor excitation has a stronger influence on lubrication margin than inlet pressure. Within the investigated operating range, establishing sufficient rotational speed before applying full inlet pressure is suggested as a practical strategy for improving the non-contact lubrication margin and reducing transient contact risk.