An Experimental Investigation of the Effects of Air Ingestion on Subsynchronous Damping Coefficients in an Open Ends SFD

Squeeze film dampers (SFDs) utilize a thin oil film between a non-rotating journal and stationary housing to reduce vibrational amplitudes while crossing critical speeds and improve stability. SFDs may experience air ingestion with certain end seal designs and motion amplitudes, leading to a reduction in damping coefficients. Most research into SFDs focuses on circular, centered orbits (CCOs), meaning the applied excitation consists of one frequency component; however, turbomachines frequently experience excitations containing both synchronous and subsynchronous frequency components. These experiments use a dual-frequency excitation with the smaller frequency varying from 9 to 42 Hz and being labeled as “subsynchronous.” A higher frequency is held constant at 100 Hz and is labeled “synchronous.” One test condition holds the subsynchronous amplitude constant with an amplitude to clearance ratio (r/c) equal to 5%, and the synchronous amplitude ranging from r/c = 5 – 60%. A second test condition sets both amplitudes to equal values ranging from r/c = 5 – 30%. Preliminary CCO tests are performed using the synchronous amplitude range outlined above and a frequency range of 10 – 100 Hz. The 100 Hz tests are used for evaluating the dynamic pressure. All testing is performed using an open ends configuration with a relatively large radial clearance of 279 μm (11 mils). Additional analysis is provided by calculating the average squeeze velocity due to both frequency components and plotting against damping coefficients. The results show a large increase in subsynchronous damping as synchronous amplitude increases, despite the onset of air ingestion. Synchronous amplitude provides the most significant contribution to the squeeze velocity, and the results show that subsynchronous damping increases with significant increases in squeeze velocity.