Dynamic Characteristics Of Gas Foil Bearing-Rotor System Based On Trajectory Method

The ultra-high-speed centrifugal compressor supported by gas foil bearing(GFB)has become a hot research topic in various countries,but it always encounters an important problem at the ultra-high speed,that is,the dynamic instability of the system.Engineering practice shows that three-pad bidirectional GFB has better stability,In order to reveal the dynamic characteristics of the three-pad bidirectional GFB-rotor system,a coupled field model of the bearing rotor system was established and solved by using the time-domain trajectory method.The details are as follows:A solution strategy of "dichotomy search+fixed point iteration" was proposed,and the elastohydrodynamic coupling model of "foil-gas film" was established to determine the static equilibrium position of the bearing.Then,the static performance of single-pad bearing with rigid surface(SPBRS),single-pad bearing with flexible surface(SPBFS),three-pad bidirectional bearing with rigid surface(TBBRS)and three-pad bidirectional bearing with flexible surface(TBBFS)were calculated and compared.The influence of different pad position angle and foil pre-deformation on the static performance of three-pad bidirectional GFB was studied.The results show that the flexible surface bearing has relatively low bearing capacity,but the gas film pressure distribution is relatively "uniform distribution".Smaller pad position angle and foil predeformation can improve bearing capacityBased on the time-domain trajectory method,a coupling model of "gas film-foil-journal"was established.The transient Reynolds equation of gas lubrication was solved by alternation direction implicit iterative method.The model of the top foil was bulit on Mindlin plate theory and was calculated by finite element method.The bump foil was modeled by linear spring.The journal motion parameters varying with time were obtained by coupling iteration.The dynamic characteristics of the journal of SPBRS,SPBFS,TBBRS and TBBFS were studied,when the journals fall freely from the bearing center under the same rotational speed and without unbalance,respectively.Moreover,the effects of rotational speed,unbalance and pad position angle on the dynamics characteristics of three-pad bidirectional GFB-rotor system were studied.The research showed that the flexible surface bearing has better operation stability,especially for the three-pad bidirectional GFB,it can reflect higher anti-subsynchronous vibration characteristics.For the three-pad bidirectional GFB-rotor system,the dynamic characteristics of the system under non-periodic load excitation(e.g.,step load,rectangular pulse load,sinusoidal alternating load)were studied,and the response time,amplitude and axis trajectory of the transient vibration of the system were compared.The results show that GFB has strong impact resistance and adaptability to changing load.