NONLINEAR DYNAMIC ANALYSIS OF FOIL AIR BEARING MACHINERY AND INNOVATION TO THE BEARING DESIGN

Abstract

Foil Air Bearings (FABs, also known as air foil bearings or gas foil bearings) are oil-free bearings that allow reliable, compact, efficient, high temperature, and maintenance-free operation of high speed turbomachinery. Despite these numerous advantages over conventional oil-based journal or rolling element bearings, the dynamic response of FAB-rotor systems is more challenging to predict since it is governed by the nonlinear coupling of the rotor with the air films and associated foil structures that make up its bearings. Nonetheless, it is known that bearing design modification aimed at inducing and enhancing the hydrodynamic air film wedge effect can improve both the static and the dynamic performance, particularly the delay of the onset of instability speed (OIS) and suppression of sub-synchronous vibrations. Such enhancement can be achieved through foil stiffness variation in the axial or circumferential direction, or through the introduction of mechanical preload (radial clearance variability). The latter can be achieved either by altering the bearing shell shape from its nominal circular form (lobed bearing), or by shimming the underlying foil structure (bump foil) at certain locations. Shimming is arguably the most convenient, since it can be applied to an existing bearing, but passive shimming is laborious and restrictive, requires pre-determination of the optimal preload, and typically raises the friction torque at low speeds (before air film formation). This has motivated research into adaptive or active devices that allow the controlled application of preload. Such controllable preload FABs (CPFABs) have typically used PZT actuators to control either the shape of the bearing shell or the level of shimming of the bump foil. However, the compliance of the bearing shell and/or PZT actuators, and the use of any mechanisms to amplify PZT displacements, mean that such devices have issues with maintaining the prescribed preload under reaction loads in the loaded state, and the potential of introducing resonant vibrations under dynamic loading. Hence, there is the need for a novel design of CPFAB. The design of such a CPFAB needs to be guided by parametric studies based on rotordynamic performance simulations. Such simulations should involve both nonlinear and linearised analyses. The latter is needed for stability and modal analysis (determination of the OIS and Campbell diagrams) and its results should be consistent with low-amplitude transient nonlinear dynamic analysis (TNDA) of the nonlinear system in the vicinity of its static equilibrium condition. It has been recently shown that such consistency is perfectly achieved when the nonlinear system is expressed as a dynamical system that fully (simultaneously) couples the air film, foil and rotor domains and the linearisation is based on the Jacobian of such a system. There is the need to upgrade existing codes to allow for variability in radial clearance and/or foil stiffness. Moreover, such codes need to be made more computationally efficient to facilitate multiple simulations needed for parametric studies. Hence, the aims of this thesis are two-fold: the development of code for the solution of the fully coupled dynamical system for use in parametric studies into FAB design modification, focusing on radial clearance variability; the design, build and test of a novel CPFAB. The novel contributions are therefore: - A parametric study of the design of a single-pad FAB based on finite difference (FD) which considers: different preloads; foil stiffness variation in the circumferential direction. - The development of rotordynamic solver that uses an arbitrary-order Galerkin Reduction (GR) method to perform nonlinear and linearised analyses of a generic rotor on single-pad foil air bearings with different applied air film conditions; the GR eliminates the mesh needed for discretisation of the air film domain in FD or finite element (FE) modelling, leading to a considerable o

Date of Award	1 Aug 2022
Original language	English
Awarding Institution	The University of Manchester
Supervisor	Philip Bonello (Supervisor)