Theoretical Analysis And Experimental Investigation On The Performance Of A Novel Gas Foil Bearing With High Structural Damping And Its Rotordynamic Response

Gas foil bearings(GFBs)are self-adaptive hydrodynamic gas bearings with compliant substructure supporting the bearing surface.GFBs have many advantages,including high rotational speed,long service life,oil-free operation,compact size and so on.Consequently,GFBs are considered as ideal substitutes of the oil-lubricated fluid film bearings and rolling-element bearings in high temperature and high speed operation.Bump-type foil bearings(BFBs)have a large load-carry capacity and better machining precision,and are widely applied in high speed turbomachinery in many fields,including avigation,aerospace engineering and national defense.The rotor system which supported by traditional BFBs has insufficient system damping and poor operation stability in high speed region because of the low gas viscosity,which limits the development of the foil bearing technology.To remove these barriers,this dissertation introduced a novel foil bearing with high structural damping which adopted metal mesh dampers to increase the structural damping coefficient of foil bearing and improve its ability to dissipate the mechanical vibration energy,thus improving the stability in high speed operation.The main research works and achievements are as follow:The two-dimension finite element model of top foil was established based on the thin shell theory which can predict the local deformation of the top foil.A theoretical model of metal mesh foil bearings(MMFBs)was proposed which took the main material and structural parameters of this type of foil bearings into consideration.In this model,the metal mesh material was treated as the assembly of springs and dampers in parallel which including the influence of dry-friction joints inside the metal mesh material.Meanwhile,the theoretical model was verified by a series of circle load tests on a static load test rig.Consequently,the theoretical and experimental results can provide guidance on the design and performance prediction of this type of foil bearings.Based on the theoretical model of metal mesh structure and foil structure,the structural stiffness and damping coefficients matrix of this novel foil bearing was calculated.The steady state Reynolds equation of compressible gas film in foil bearing was solved by using the finite difference method(FDM)and Newton-Raphson algorithm.The gas pressure and film thickness distribution was calculated by solving the Reynolds equation and elastic equation of the bearing substructure continuously.The influence of relative density of metal mesh material,bearing load and rotational speed on the distribution of gas film along circumferential and axial direction was investigated and discussed.Meanwhile,the eccentricity ratio,attitude angle and equilibrium position of rotor center was calculated to bulid a database on static performance of the novel foil bearing and to guide the design of the bearing substructure.A series of experimental tests on the novel foil bearing was conducted with static and dynamic circle loads in order to investigate the structural force performance of bearing substructure.The test results show that the novel foil bearing has higher structural damping coefficient than BFBs.The dynamic stiffness and damping coefficients of the novel foil bearing were measured using impact load excitations.The dynamic bearing force coefficients were predicted by using the perturbation method,which solves the transient Reynolds equation of gas film and the motion equation of bearing substructure continuously.The predicted results were verified by the experimental data.Moreover,a nonlinear bearing force model of the novel foil bearing is introduced to predict the orbit of rotor centre and to analysis the influence of the friction coefficient and relative denstiy of metal mesh structure on the bearing critical mass.The predicted results show that high friction coefficient in metal mesh structure can enhance the operation stability of the rotor system.A complex thermal boundary of hollow rotor and bearing substructure in the novel foil bearing was established based on the fundamental theory of heat transfer.Furthermore,the thermohydrodynamic lubrication model was built by coupling the steady state Reynolds equation and the energy equation of gas film.The temperature distribution along bearing circumference and axial directions with different rotational speeds and thermal management methods was predicted using the thermohydrodynamic lubrication model.A test rig was built to measure the temperature field of the novel foil bearing.The predicted results were verified by the experimental data and the efficiency of different thermal management methods is investigated by the experimental and theoretical results.The primary role of heat transfer path in bump foil region was demonstrated by the comparison of heat transfer ratio of different heat transfer paths with cooling gas flow in the bearing substructure.To investigate the rotordynamic performance of rigid rotor supported by the novel foil bearings,a test rig including two journal foil bearings and a pair of thrust BFBs was built to investigate the effects of relative mesh density,unbalance mass,radial clearance and bearing load on the rotordynamic response.Meanwhile,rotor responses for the traditional BFBs-rotor system were also measured in order to be compared with the experimental results of the novel foil bearings-rotor system.The test results show that the novel foil bearings can delay the onset speed of subsynchronous vibration and suppress the motion amplitude of subsynchronous vibration efficiently at high rotational speed region,thus improving the stability of the rotor-bearing system significantly which demonstrated the design propose of this novel foil bearings.Based on the nonlinear stiffness model of foil bearing and rigid rotor theory,a rotordynamic model of the studied rotor system was built to predict the rotor responses under different level of unbalance masses.The predicted results agree well with the test data,which verified the applicability of this rotordynamic model.In conclusion,the novel foil bearing which is introduced in this dissertation can improve the structural damping of rotor-bearing system by placing the metal mesh damper into the bearing substructure and be parallel with the bump foil structure.The static,dynamic and nonlinear performance,thermal field and rotordynamic performance of rotor-bearing system are investigated theoretically and experimentally with different structure size,material parameters and operational conditions.All the theoretical and experimental results show that the novel foil bearings have good assembly accuracy,excellent thermal characteristic and can provide sufficient structural damping to the rotor system.The novel foil bearing is new foil bearing technology with good prospect in engineering application.