Theoretical Analysis And Experimental Study On Vibration Characteristics Of Gas Foil Bearing Flexible Rotor System

Elastic foil of gas foil bearing structure effectively improved the bearing stiffness and damping characteristics,in addition,gas foil bearing with low viscosity of air as the fluid lubrication,has the advantages of no pollution,low friction loss,high efficiency,compared with the rigid gas bearing,gas foil bearing in high speed rotating machinery is widely used.However,the gas foil bearing is difficult to be applied to flexible rotors because of its strong nonlinear characteristics at high speed and limited bearing damping.Most of the researches on the gas foil bearing rotor system focus on the rigid rotor and less on the flexible rotor.If the technical difficulties of flexible rotors can be overcome,the application fields of gas foil bearings will be further expanded.In order to solve these problems,a gas foil bearing flexible rotor test platform is built,and a linear and nonlinear theoretical analysis model is established.The general vibration law of the bearing-rotor system is discussed theoretically and experimentally,and the relevant vibration reduction methods are summarized.The original gas foil bearing flexible rotor test bench which could not start is optimized and designed.In order to reduce the starting friction torque and increase the driving power,the thrust bearing and turbine are improved.Aiming at the problem of limited damping of gas foil bearing structure,A gas foil bearing with high damping wire mesh O-ring is proposed.Besides,the static push-pull experiments of common gas foil bearing and the gas foil bearing with O-ring were carried out.From the static push-pull curves,it is concluded that wire mesh O-ring can obviously improve the structural damping of gas foil bearing.A linear analysis model of the bearing-rotor system was established by coupling the dynamic 8-parameter solution model of the gas foil bearing with the rotor finite beam element model.Based on the three common rotor structures proposed,the modal analysis shows that the rotor structure layout will affect the mode shapes and natural frequencies of the rotor.It is found that increasing the bearing span can reduce the critical amplitude of rotor bending,which is beneficial for the flexible rotor to cross the critical state of bending.It is found that for the same unbalance,the critical amplitude of rotor bending is different when the unbalance acts on different positions of the rotor.However,the linear theory analysis can’t be used to analyze the subsynchronous vibration produced by the fluid incentive in the condition of high speed.Then,coupled with the gas film Reynolds equation,the foil deformation equation and the rotor dynamics equation of the finite element,a fully coupled nonlinear transient analysis model of the bearing-rotor system is established.Based on this model,the trajectory of rotor axis is simulated,and the frequency components of the trajectory are observed by Fourier transform.Through a lot of calculation,it is found that sub-synchronous vibration of rotor is easy to occur at high speed.However,sub-synchronous vibration can be suppressed by static loading and reducing nominal clearance of bearing.If the rotation speed is close to the critical bending speed,the rotor almost only exhibits the synchronous vibration.The results of linear and nonlinear analysis are combined to guide the design of vibration experiment of gas foil bearing-rotor system.At the same time,the theoretical model is proved to be correct and reasonable by experiments.However,the vibration of the rotor in the flexible critical is too large,which is prone to safety accidents.Therefore,through the near critical vibration experiment with smaller vibration,it can be concluded that the vibration of the rotor bending critical can be reduced by the following methods:(1)Constantly adjust the rotor to reduce the vibration of the rotor as much as possible through the dynamic balance experiment of the counterweight screw;(2)Reasonable allocation of nominal clearance of bearings;(3)Increase the bearing span;(4)Series high damping wire mesh O-ring,increase the structural damping of the bearing-rotor system.