Rotordynamic performance of a rotor supported by three pad hybrid air foil bearing with controlled air injection

Air Foil Bearing has found its place in the commercial applications of turbo machinery. They are cleaner and more reliable at high speeds than their conventional counterparts (Rolling Element Bearings). Low drag friction and apparently no contact during the operation results in a better service life and superior rotor-dynamic performance of the air foil bearings (AFBs). With all these advantages AFBs prove to be the suitable option for oil-free turbo-machinery. Further development in AFBs suggest that pressurizing these bearings externally results in remarkably better load carrying capacity, heat dissipation and more stable rotor. These externally air pressurized bearings are called Hybrid Air foil Bearing (HAFB).;The present study is a continuation of the work on Imbalance Response and Stability Characteristics of a rotor supported by HAFB by Kim, D and Varrey, M [1], wherein a three pad HAFB previously developed in the laboratory was tested. This study was carried out for a 4.84 kg rotor of 49 mm diameter. The cylindrical mode imbalance response was noted for a pressure range from 2.67-4.67 Bar. It suggested an increase in onset speed of sub-synchronous vibration with higher pressures. However, not much change was seen in modal stiffness and natural frequencies. The present work is to numerically investigate the feasibility of controlled air injection to stabilize the rotor. The numerical analysis is carried out by orbit simulation method and is later on compared with the experimental results. In orbit simulation, the journal dynamic equations are solved simultaneously with the unsteady Reynolds equation in time domain under given feed pressure and bearing parameters. Various case studies were also carried out experimentally including test runs with a different clearance between the rotor and the bearing and also at sub synchronous vibrations.