Thermohydrodynamic Analysis of Dynamic Characteristics for the Third-Generation Carbon Dioxide (CO2) Gas Foil Journal Bearings Under High Pressure and Temperature

Abstract

Abstract. Nowadays, gas foil bearings (GFBs) have become a key enabling technology for S-CO2 turbomachinery applications, due to their capacity to withstand extreme operating conditions, and without pollution. Furthermore, GFBs eliminate the complicated external facilities and the speed limitations. While extensive research exists on air-lubricated GFBs, a few studies focus on the dynamic performance of CO2-lubricated GFBs under high-pressure and high-temperature conditions, particularly considering the complex thermal and centrifugal deformations. This lack of research impedes the application of GFBs in S-CO2 turbomachinery. Therefore, a thermohydrodynamic (THD) analysis method was proposed and validated in this study for the dynamic characteristics of the third-generation CO2 gas foil bearing (GFB). In this paper, a novel numerical method was presented for THD analysis of the dynamic characteristics of the third-generation CO2 bump-type GFBs, considering thermal and centrifugal deformations. The proposed numerical method was validated by the experimental data of the eight dynamic characteristic coefficients of air-lubricated bump-type GFBs under different rotating speeds (21,600 rpm and 27,600 rpm) and static loads (20 N–40 N). The present method was further validated based on the measured power loss of CO2 bump-type GFBs for different rotating speeds. The dynamic characteristics were calculated and analyzed for the third-generation CO2 bump-type GFBs at different operating conditions, including ambient pressures (1.0–3.0 MPa), ambient temperatures (300–600 °C), and eccentricity ratios (0.3–0.9). Calculations were performed for a wide range of perturbation frequencies (dimensionless perturbation frequencies 0.1–4.0) to encompass all possible excitation frequencies encountered in the bearing. The influences of operating parameters (high pressure and high temperature) and static loads on the dynamic characteristics of the third-generation CO2 bump-type GFBs were presented and discussed. The numerical results demonstrate that thermal and centrifugal deformations are critical for accurately predicting the dynamic performance of the third-generation CO2 bump-type GFB at high-pressure and high-temperature conditions. Both dynamic stiffness and damping coefficients decrease with rising ambient temperature. At an ambient pressure of 2.0 MPa and a dimensionless perturbation frequency ν¯=2.0, the direct stiffness exhibits 21.1% and 15.8% reductions as the ambient temperature rises from 300 °C to 600 °C. Meanwhile, the direct damping coefficients Cxx and Cyy decrease by 11.7% and 9.7%, respectively. To maintain sufficient direct stiffness and damping coefficients, a lower ambient temperature is recommended for high-temperature S-CO2 turbomachinery applications. The direct stiffness and damping coefficients increase rapidly with the rising ambient pressure below an ambient pressure of 2.0 MPa, while exhibit a diminished growth rate or decrease above this threshold. Thus, a critical ambient pressure threshold (2.0 MPa) is recommended to optimize rotor-bearing system stability while declining windage losses in the rotor cavity. Furthermore, the third-generation CO2 bump-type GFB has enough direct dynamic stiffness and damping coefficients, and a positive energy dissipation factor for all perturbation frequencies and eccentricity ratios, enhancing the stability of the rotor-bearing system.