Squeeze Film Dampers in High-Speed Turbomachinery: Fluid Inertia Effects, Rotordynamics, and Thermohydrodynamics

Unbalance induced vibrations are the main source of structural vibrations in high-speed turbomachinery. This mass unbalance is associated with the limitations and imperfections in manufacturing rotor systems and leads to a synchronous load cycle in the rotor. Squeeze film dampers (SFDs) are essential components in high-speed turbomachinery, including aircraft jet engines, high-performance compressors, gas turbines, and automotive turbochargers, which are incorporated to attenuate or completely suppress the steady-state unbalance induced vibration amplitudes at the resonance frequencies, reduce the forces transmitted to the supports, and to ensure the stable operation of the system. Conventionally, the dynamics of SFDs are characterized by using Reynolds equation, where it is assumed that the effect of fluid film inertia is negligible relative to the viscous effects. However, at large velocities, the inertia forces are significantly large and the effect of fluid inertia can no longer be neglected. Additionally, the design and analysis of SFDs are generally based on isothermal conditions; however, the thermophysical properties of the bearing lubricant strongly depend on the local state of temperature. At high operating speeds, the bearings may experience significant temperature rise, since the viscous dissipation that is associated with the shear motion, as well as the heat transfer with the bearing surfaces, can generate significant temperature and viscosity variations within the lubricant film, which ultimately influences the static and dynamic performance of the bearing considerably. The objective of this thesis is to develop a comprehensive model for SFDs in high-speed turbomachinery. The proposed model incorporates the effect of fluid film inertia and thermal effects to provide a precise prediction of the SFD behavior at high velocities. In order to achieve this objective, the effect of lubricant inertia at different SFD operating conditions, including small and large amplitude motions of the journal center, are formulated by using momentum approximation and perturbation method, and for different damper geometries, including finite-length and short-length dampers. The results of the analysis demonstrate the significant influence of fluid inertia on the shape, phase, and magnitude of the SFD pressure distribution as well as the magnitude and direction of the fluid film reaction forces. Subsequently, a finite element based rotodynamic model is developed for multi-mass flexible rotors and the proposed SFD models are incorporated into the rotordynamic model to study the effect of SFD fluid film inertia on the unbalance induced steady-state vibration amplitudes and transient orbits of high-speed rotors. The results of the rotordynamic analysis confirmed the considerable influence of SFD fluid inertia on the attenuation of the unbalance induced vibrations at resonance zones. Finally, a detailed thermohydrodynamic (THD) model incorporating the full term Navier-Stokes equations, the energy equation for the lubricant, the Laplace heat conduction equation for the surrounding solids, including bushing and shaft, and realistic thermal boundary conditions is developed to investigate the effect of temperature variation on the dynamics of SFDs.

Keywords: Finite-Element Analysis
Fluid Inertia Effects
Jet Engine Vibration
Rotordynamics
Squeeze Film Damper
Thermohydrodynamics
Advisor: Behdinan, Kamran