Squeeze Film Damper Suppression of Thermal Bow: Morton Effect Instability

The Morton effect is a synchronous vibration problem in turbomachinery caused by the non-uniform viscous heating of the hydrodynamic bearing’s lubricant. This leads to hot and cold spots around the journal circumference and a resultant thermal bow. This thermal bow is mostly associated with overhung-type rotors which tend to generate larger inertial forces and asymmetric viscous bearing heating near the critical speed of the rotor. The non-uniform viscous heating, thermal bow, and inertial force may develop a positive feedback loop. This in turn causes the rotor-bearing system to experience a spiral-shaped locus of synchronous vibration. This may cause rubbing between the pad surface and journal if the machine is not tripped. Squeeze film dampers SFD are widely used to reduce rotor vibration by providing damping to the system when the bearing damping is inadequate to control resonance and instability.  Installing a properly designed squeeze film damper may change the rotor critical speed location, damping and mode shape, and thereby suppress the Morton effect. The paper treats the unconventional application of a SFD for Morton Effect suppression. The effectiveness of suppressing the Morton Effect with a SFD is tested via simulation studies employing a high fidelity 3D thermal-elasto-hydrodynamic tilt pad bearing model and a Timoshenko beam rotordynamic model. The model is for a compressor that experimentally exhibited the Morton Effect at 8,000 rpm. The squeeze film damper model includes fluid inertia and is installed on the non-drive end bearing location where the asymmetric viscous heating of the journal is highest. Nonlinear transient simulations are performed to compare the rotor dynamic performances and journal temperature differential with and without the squeeze film damper. In addition, the paper investigates the impact of the squeeze film damper’s squirrel cage stiffness and the amplitude of the film clearance on the Morton effect. The current work provides guidelines for designing the squeeze film damper to suppress the Morton effect.