A Model for Squeeze Film Dampers Operating With Air Entrainment and Validation With Experiments

Squeeze film dampers (SFDs) reduce vibrations and aid in suppressing instabilities in high performance rotor-bearing systems. However, air ingestion and entrapment, pervasive in open-ended dampers with low supply pressures, leads to a bubbly lubricant that severely reduces the dynamic film forces and the overall damping capability. Analyses based on conventional film rupture models, vapor or gaseous lubricant cavitation, fail to predict the actual performance of SFDs, and thus lack credibility in engineering practice. A modified Reynolds equation for prediction of the pressure in a homogeneous bubbly mixture flow is advanced along with an empirical formula for estimation of the amount of air entrained in an open-ended damper. Careful experimentation in a test SFD operating with controlled bubbly mixtures and freely entrained air evidenced similar physical behavior, guided the analytical developments, and provided the basis for validation of the model forwarded. Comparisons of predictions and test results show a fair correlation. A simple equation to predict the amount of air ingestion is also advanced in terms of the damper geometry, supplied flow and operating conditions. The criterion may lack practical implementation since the persistence of air entrainment increases with the frequency and amplitude of journal motions, unless enough lubricant is supplied at all operating conditions.