Passive Vibration Control by Frictional Energy Dissipation in Refrigerant‐Lubricated Gas Foil Bearing Rotor Systems

Abstract

Even though the dynamic performance of rotors supported by refrigerant-lubricated gas foil bearings (GFBs) is very sensitive to the amount of energy dissipated in the foil structure, almost none of the existing computational models really capture dry friction with typical stick-slip transitions. The presented work addresses this shortcoming by incorporating an elasto-plastic bristle friction law into the structural model, which is coupled in an interconnected solution approach to a Reynolds equation for non-ideal compressible gases and to a modified Jeffcott-Laval rotor model. Numerical results confirm that properly designed GFBs have the potential to benefit significantly from the foil structure acting as a passive vibration control device. Motivation and ModelingAs a benefit of the low-viscosity lubricant film separating the rotor journal from the supporting structure, gas foil bearings (GFBs) are characterized by almost wear-free operation with remarkably small power losses [1], which renders them a key technology in the development of energy-efficient and oil-free turbomachinery. On the other hand, the absence of any major source of damping combined with strongly nonlinear fluid forces makes the system prone to instability phenomena such as self-excited subsynchronous vibrations [2]. As a countermeasure, the compliant and slightly movable foil structure inside the lubrication gap ( Fig. 1) possesses the excitation-dependent ability of dissipating some of the excessive energy by deliberately introduced dry friction. While stiction between the foil structure and the bearing sleeve is the predominant state in stationary operation ( Fig. 2a) or with only small unbalances present (Fig. 2b), contact breakaway and subsequent dissipative sliding occur in response to heavy cyclic loading (Fig. 2c), thus calming rotor vibrations before their amplitudes become severe [3].Despite this seemingly simple mechanism, almost none of the computational GFB models found in literature are capable of really capturing the switching nature of stick-slip transitions in dry friction and hence suffer from substantial inaccuracies when assessing the dissipated energy. To address this shortcoming, the presented foil structure model (Fig. 3) extends a proposition by Feng et al. [4] by incorporating an elasto-plastic bristle friction law, which is capable of reproducing stiction without spurious drift [5]. With the particular aim of investigating GFBs lubricated by the working fluid of vapor-compression refrigeration turbomachinery, this structural model is coupled to a Reynolds equation for non-ideal compressible gases with small amounts of condensed liquid, which requires a transient formulation of [6], and to a modified Jeffcott-Laval rotor model. Fig. 1: Sketch of the rotor journal (orange) supported by a GFB with bump foil (green) and top foil (yellow) inside the lubrication gap filled with refrigerant (adapted from [7]). a) Stationary operation b) Moderate excitation c) Strong excitation Fig. 2: Basic principle of how frictional ener...

Dynamic Performances of Foil Bearing Supporting a Jeffcot Flexible Rotor System Using FEM

“…Numerical approaches to solve this fluid-structure interaction problem can be classified into two methods, monolithic and partitioned. The monolithic approach treats the dynamics of fluids and structures in the same mathematical model to form a single equation for the whole problem [27]. The equations of the fluid and the structure are solved simultaneously with a single calculation code.…”