A test rig is built in this study to measure the rotordynamic response of a rotor supported on porous-type gas bearings. A rotor with a double impulse turbine at one end is driven by compressed air and supported on two porous type journal gas bearings and a pair of bump-type thrust gas bearings. The rotor is accelerated to ∼25 krpm and coasted down in the test. The rotor dynamic response is measured for different bearing supply pressures (i.e., 0.40 MPa, 0.45 MPa, and 0.50 MPa) and imbalance masses (i.e., 85 mg, 150 mg, and 215 mg). Synchronous and subsynchronous amplitudes are extracted from the rotor responses. The critical speed increases as the bearing supply pressure increases, but the damping ratio decreases. The onset speed of subsynchronous motion increases, and the subsynchronous amplitude decreases as the bearing supply pressure increases. The deceleration time is more than 5 min for a bearing supply pressure of 0.5 MPa, which reveals the very low drag friction of the porous gas bearings. The synchronous amplitude increases as the imbalance increases for all the tested bearing supply pressures. The critical speeds for different imbalances are almost the same, except for the out-of-phase imbalance condition under a bearing supply pressure of 0.50 MPa, in which the critical speed increases as the imbalance increases. The normalized synchronous amplitude shows the rotor-bearing system behaves almost in a linear fashion for all in-phase imbalance conditions. Nonlinear behavior is shown around the critical speed for the 215 mg out-of-phase imbalance condition under a bearing supply pressure of 0.50 MPa. The onset speed of the subsynchronous motion decreases as the imbalance increases under the in-phase imbalance condition. The predominant mode of vibration changes from cylindrical to conical and then back to cylindrical as the rotor speed decreases during the coast down test for the in-phase imbalance conditions. However, the rotor vibration mode is predominantly conical during the whole coast down test for the out-of-phase imbalance conditions.

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