Rotordynamic Performance Measurements and Predictions of a Rotor Supported on Multilayer Gas Foil Journal Bearings for Microturbomachinery
Authors: Lee Jongsung , Kim Young Min , Park Moon Sung , Kim Tae Ho , Ha Kyoung Ku , Lee Changha , Kim Sae Hoon 
Published: September 2016

This paper presents rotordynamic performance measurements of multilayer gas foil journal bearings (GFJBs) supporting the rotor of oil-free microturbomachinery, and a comparison with the model predictions. A series of rotor coast-down tests from 60 krpm were conducted to compare the rotordynamic performances of three previously developed multilayer GFJBs: types A, B, and C. During the tests, two sets of orthogonally positioned displacement sensors recorded the horizontal and vertical rotor motions, and an axially positioned displacement sensor measured the thrust of the runner axial motion. The test results revealed that the type C GFJBs have a superior rotordynamic capability over the other types. The additional coast-down tests from 100 krpm for the type C showed that the synchronous motions of the rotor are dominant at up to ∼50 krpm, but that large amplitudes of subsynchronous motion associated with the natural frequency of a rotor-GFJB system occur above this speed. Thermal transient response measurements were conducted using four k-type thermocouples at increasing rotor speeds of 20 to 100 krpm with increments of 10 krpm. The operation time required to establish steady-state temperatures was approximately 25 min for each speed. For most of the speeds tested, the front GFJB near the rotor impeller end showed the lowest temperatures, and both the rear GFJB near the thrust runner end and the permanent magnet (PM) motor showed the highest temperatures. The GFTB showed the lowest temperature at low speeds of below 50 krpm, and the highest temperature at the top speed of 100 krpm owing to the increasing axial load caused by the impeller force. The measured impeller pressure and motor output power increased nonlinearly with the increasing rotor speed and fits best with the second-order and third-order polynomial equations, respectively. The measured axial displacement revealed that the rotor moved axially up to ∼ 270 μm toward the impeller side as the speed increased to 100 krpm. Further experiments using a decrease in radial clearance of 30 μm demonstrated a suppression of the large amplitude of the subsynchronous rotor motion to a certain degree. In addition, the onset speed of the subsynchronous motions increased to 80 krpm for the type C GFJBs with the decrease in the radial clearance. Rotordynamic model predictions with the predicted GFJB stiffness and damping coefficients were benchmarked against the test data. The predicted natural frequencies, onset speed of instability (OSI) where the damping ratio became negative, and synchronous rotor response versus speed agreed reasonably with the measured whirl frequencies of the subsynchronous motions, the onset speed of subsynchronous motions (OSS), and the filtered synchronous rotor motion versus speed, respectively. The predictions also showed that the OSI increased from 50 krpm to 80 krpm with a decrease in the radial clearance, thus validating the present rotordynamic model.