EXPERIMENTS AND MODELS FOR OPERATION OF A SEALED ENDS SQUEEZE FILM DAMPERS: A STEP TOWARDS QUANTIFYING AIR INGESTION IN SQUEEZE FILMS
Abstract
Squeeze film dampers (SFDs) in high-performance turbomachinery reduce rotor motion amplitudes as it traverses a critical speed or when the system has a dynamic instability, thus ensuring system reliability. To improve the damping capacity in aircraft engines within a limited space, piston ring (PR) seals are installed at the axial ends of a film land. Even though PRs effectively seal a SFD, a significant amount of a lubricant exits through the gap at the abutted ends of the PR (PR slit). However, when the squeeze-film pressure is lower than ambient pressure, air ingests into the film and mixes with the lubricant. The advanced turbomachines have a larger operating speed with a smaller lubricant supply than traditional turbomachines; hence, air entrainment in a sealed ends SFD becomes significant. This dissertation presents a computational physics model for a sealed ends SFD and open to ambient, hence prone to air entrainment; and delivers predictions benchmarked against experimental test results. The first embodiment is a SFD with a PR and an O-ring (OR) sealing the film land. In the tests, a known gas (air) volume fraction (GVF or β) in a mixture of air and ISO VG2. The PR and the O-ring (OR) that seal the film land are located in the grooves at the top and bottom of the journal, respectively.
The supplied mixture discharges through the PR slit, located at the top axial end, into a vessel submerged within a large volume of lubricant. Another damper, which has same journal geometric parameters, is supplied with a pure lubricant of a supply pressure. Both the top and bottom axial ends are sealed with PRs and opened to ambient. Hence, the supplied lubricant exits through the PR slits into ambient, and the air ingests through the PR slits when the film pressure is below the ambient pressure. There are two distinctive models evaluating the evolution of gas volume fraction in a squeeze film land: (a) a volume of fluid (VOF) model and (b) a bubbly mixture. The models predicting the pressure field in the squeeze film implement the Reynolds equation, modified to include temporal fluid inertia effects, and uses physics-based inlet and outlet lubricant conditions through a feed hole and PR slits, respectively A parametric study produces the dynamic forced performance of the PR sealed ends SFD. The predictions show the time-space average GVF increases as the squeeze velocity (vs) increases. On the other hand, the GVF decreases as the supply pressure increases. The damper physical geometry also affects its dynamic forced performance. The GVF increases as the journal diameter increases; whereas the SFD axial length does not significantly change the GVF. The GVF reduces as the damper clearance increases. The GVF does not significantly change as the PR slit cross-sectional area varies. An oil supply pressure large enough to prevent air ingestion varies with damper geometry, lubricant inlet/outlet conditions, and the kinematics of the journal. The PR slits allows air ingestion even as the squeeze velocity is small. As the damper diameter to clearance ratio (D/c) increases, the GVF in the film increases. Most importantly, the location of the PR slit relative to the feedhole significantly affects the amount of air content in the film. When the PR slit faces to the feedhole, the film land is mostly filled with a pure lubricant. The GVF increases as the arc distance from the PR slit to the feedhole increases.
Citation
Koo, Bonjin (2019). EXPERIMENTS AND MODELS FOR OPERATION OF A SEALED ENDS SQUEEZE FILM DAMPERS: A STEP TOWARDS QUANTIFYING AIR INGESTION IN SQUEEZE FILMS. Doctoral dissertation, Texas A&M University. Available electronically from https : / /hdl .handle .net /1969 .1 /195929.