Evaluation of Foil Bearing Performance and Nonlinear Rotordynamics of 120 kW Oil-Free Gas Turbine Generator

Abstract

This paper presents a design approach of airfoil bearings (AFBs)fora 120 kWe gas turbine generator, which is a single spool configuration with gas generator turbine and alternator rotor connected by a diaphragm coupling. A total of four radial AFBs support the two rotors, and one set of double acting thrust foil bearing is located inside the gas generator turbine. The rotor configuration results in eight degree of freedom (DOF) rotordynamic motions, which are two cylindrical modes and two conical modes from the two rotors. Stiffness of bump foils of candidate AFB was estimated from measured structural stiffness of the bearing, and implemented to the computational model for linear stiffness and damping coefficients of the bearing and frequency-domain modal impedances for cylindrical and conical modes. Stiffness of the diaphragm coupling was evaluated using finite element analysis and implemented to nonlinear rotordynamic analyses of the entire engine. Analyses show the conical mode of the turbine rotor is the main source of instability of the entire engine when AFB clearance is not selected properly. Optimum AFB clearance is suggested from frequency domain modal analyses and nonlinear transient analyses.

On the manufacturing of compliant foil bearings

“…There is also a number of proof of concepts for other systems running on foil bearings. Examples are microturbines and auxiliary power units [4][5][6][7], turbocompressors [8][9][10][11], and turbo expanders [12,13].…”

Nonlinear Dynamic Analysis of High Speed Oil-Free Turbomachinery With Focus on Stability and Self-Excited Vibration

“…For example, the above-mentioned works [1][2][3][4][5][6][7][8] assumed a simple symmetric rigid rotor-bearing system. Other works considered rigid rotors with four degrees of freedom (DOFs) (corresponding to translation and rotation in each of the xy, yz planes) [11] or five DOFs (where an additional axial DOF was considered to account for an air foil thrust bearing) [12].…”

Efficient solution of the non-linear Reynolds equation for compressible fluid using the finite element method

“…The method described in this paper was derived to be used for nonlinear analysis as well as to be extended by incorporating more complex foil structure models based on FEM [18]. Non-linear dynamic simulation tools applied to complex industrial rotors supported by CFB is still demanding faster numerical methods [2,7,13,33,35]. …”