Static And Dynamic Characteristics Study Of Multiple Sliding Beams Journal And Multi-layer Thrust Gas Foil Bearings

Gas foil bearing is the self-acting passive bearing,and has the characteristics of gas lubrication and elastic support.The traditional foil bearings have been applied to the turbomachinery such as the air cycle machine,and possess the advantages of long life,no contamination and good adaptability to rotor misalignment.Recently,the development of high speed turbomachine has put forward higher requirements for the bearing performance.Some new types of foil bearings have been developed;however,there are seldom related open publications.Meanwhile,the theoretical study of foil bearing is still behind experiments.Models are usually simplified and the complex contact constraints inside the bearing are neglected.In this paper,finite element models of the foil structures are developed for the journal foil bearing with multiple sliding beams and the multi-layer thrust foil bearing respectively.Different types of contact constraints are considered based on the contact mechanics.The static and dynamic characteristics of the bearings are studied in depth.Firstly,the calculation model of the foil bearing with multiple sliding beams is developed.The finite element model of foil structure is established,including the nonlinear sliding beam model,and linear models of the top foil and link-structures.The frictional contact model of sliding beam,the area contact model between top foil and sliding beam,and the 'node-element' contact model between top foil and the link-structures are developed.In addition,the rigid-flexible structural contact model between the rotor and top foil is developed to simulate rotor assembly and static rotor loading.The non-linear foil structural model with different types of constraints is solved by the Newton-Raphson iteration,and the Reynolds equation of each flow field is solved by the Finite Difference Method.The coupling between foil deformation and gas film pressure is iteratively solved.Static performance of the foil bearing with multiple sliding beams is investigated.Deformations of the sliding beam and the overall foil structure are calculated and analyzed.The influences of structural parameters on the stiffness and sliding distance of the sliding beam as well as the energy dissipation caused by the Coulomb friction are studied.Influences of the area contact between top foil and sliding beam on the overall foil deformation are also discussed.Simulations of the rotor assembly and static rotor loading as well as the calculations of rotor liftoff speed and start-up torque are carried out.The aeroelastic characteristics are calculated,including the gas film pressure and thickness distributions,and the foil structural deformation.In addition,effects of the thickness,structural layouts,and stiffness distribution of support foil on the bearing load capacity are also studied.Based on the small perturbation method,linear perturbation model of the journal foil bearings with multiple sliding beams is established.The perturbation equation of the nonlinear foil structure with different types of contact constraints is derived,which is further coupled with the perturbed Reynolds equation.The dynamic coefficients are calculated under different static loads,foil thicknesses and rotor preloads,and the effects of the relevant parameters on the linear rotor stability are analyzed.Meanwhile,the vibration model of the nonlinear foil structure with various contact constraints is developed in the time domain,considering the foil mass and the dynamic sliding behavior of the multiple beams with friction.The coupled solutions of the foil structural dynamic equation,the rotor dynamic equation and the transient Reynolds equation are integrated in time domain.The influence of Coulomb friction effect on the vibrations of sliding beam and the rotor-bearing assembly is obtained.Considering the aerodynamic effect,the effects of static load,foil thickness and rotor preload on the rotor dynamic responses are studied under the self-excitation and the excitation of rotor unbalanced mass.In addition,the calculation model of the multi-layer thrust foil bearing is developed.The bearing static performance is studied.The‘node-to-node'and'node-to-element' contact models between foil layers are established respectively to investigate the area contact effects between foil layers and the coordinate deformations of the multi-layer foil structure.The results show that under the aerodynamic force,adjacent foil layers may separate,which will affect the stiffness distribution of overall foil structure.Based on the area contact model of multi-layer foils and the Reynolds equation in polar coordinates,the effects of foil structural parameters,such as the anvil and plate thicknesses,and the vertex angle of support element on the bearing aeroelastic characteristics are studied.In addition,the researches on bearing loading carrying performance including load capacity,stiffness,ultimate load capacity and ultimate rotor eccentricity are also conducted.The test rigs of foil journal and thrust gas bearings are set up respectively.The loading and displacement measurement systems of the journal and thrust foil bearings are developed.Test pieces of the journal foil beairng with multiple sliding beams and the multi-layer foil thrust bearing are designed and manufactured.The loading tests of the journal bearing with static and the running rotors are carried out respectively.Through variable parameter design,the effects of rotor speeds and main foil structural parameters on the multi-layer thrust foil bearing capacity are studied experimentally.