Rotor Dynamic Characteristics Of Foil Bearing System With Surface Micro Structure

The hydrodynamic compliant gas foil bearing possesses the features of high speed,wonderful shock resistance,favorable flexibility of temperature and high reliability,yielding a widespread application prospect in aerospace and cryogenic engineering fields.However,the load capacity and dynamic properties of gas foil bearing are lower than those of oil lubricated rigid bearing due to the low viscosity of lubricating gas.Besides,the gas fluid excitation and friction between foil structures are affected each other,which is prone to cause sub-synchronous vibration problem in gas foil bearing-rotor system.In order to improve the load capacity and dynamic properties of foil bearing,this dissertation proposes novel journal and thrust foil bearings with surface micro structure.The flow-thermal-elastic coupled model considering rarefied gas is established.By analyzing the static,dynamic and thermal characteristics,superiorities of the novel journal and thrust foil bearings are verified.In the case of sub-synchronous vibration,an efficient time series forecasting algorithm is presented for the nonlinear dynamic characteristics analysis of gas foil bearing-rotor system.Then,a foil bearing-rotor system test rig is built for rotor system experimental verification.All these works provide theoretical and experimental basis for design and application of gas foil bearing.In an effort to improve load capacity of journal foil bearing,a novel bump-type journal foil bearing with surface micro structure is proposed,in which a micro groove in several microns depth along circumferential direction is designed at mid-plane of top foil.Due to the more obvious rarefied gas degree in novel journal foil bearing,the gas pressure governing equation is established with the rarefied gas velocity slip flow boundary into consideration.Based on finite difference method,the bearing characteristics including load capacity,friction torque and dynamic coefficients under room temperature condition are studied.By analyzing the effects of boundary slip flow on static and dynamic characteristics of novel journal foil bearing,the more prominent rarefied gas boundary slip flow impact is verified in novel journal foil bearing.Compared with traditional journal foil bearing,it indicates that this novel journal foil bearing with micro groove structure can decrease the side leakage and increase pressure around load domain efficiently.The load capacity and dynamic properties are improved.Moreover,with the increment of micro groove depth,the bearing performance is reinforced further.A novel bump-type thrust foil bearing with surface micro structure is put forward,in which a micro pit in several microns depth is constructed in the middle of every sector top foil.Considering the boundary slip flow,the rarefied gas pressure governing equation of thrust foil bearing is established with the film thickness expression under tilting condition of thrust runner.The influences of pit depth on static and dynamic characteristics of the novel thrust foil bearing are discussed with the boundary slip flow effect into consideration.The results show the surface micro pit on top foil is beneficial to form an extra convergent wedge and improve gas pressure,yielding enhanced axial load capacity and dynamic proprieties.As the side leakage of novel journal foil bearing is decreased,a more evident thermal problem may suffer.Taking the boundary slip flow and temperature jump of rarefied gas into account,the energy equation and its boundary condition are built.Combined with the pressure governing equation and foil deflection equation,the flow-thermal-elastic coupled model of journal foil bearing is established.By analyzing the journal foil bearing temperature under different micro groove depth of top foil,it is observed that the gas film temperature is higher for a larger groove depth.Compared with the static characteristics at room temperature,the coupled thermal effect would expand gas and raise its viscosity,and then increase the load capacity and viscous friction torque of journal foil bearing.What's more,the static performance increases more evidently for higher gas film temperature level.In the case of nonlinear dynamic analysis of foil bearing-rotor system,the traditional weak coupling algorithm solves transient bearing and rotor equations asynchronously,which results in a poor computational efficiency.From the aspect of time series statistics,this dissertation proposes an efficient time series forecasting algorithm.Compared with weak coupling algorithm,the time series forecasting algorithm is demonstrated a faster convergence rate and higher efficiency.Then,the nonlinear dynamic response of foil bearing-rotor system is discussed and the sub-synchronous vibration is analyzed.The results indicate the novel journal foil bearing with surface micro structure can delay the onset speed of sub-synchronous vibration.The linear analysis point out the rotational stiffness of thrust foil bearing would increase the lower-order critical speeds of rotor system.The dynamic characteristics experiment of gas foil bearing-rotor system is proceeded on a turbocharger rotor system test rig supported by gas foil bearings.By comparing with simulation results,the accuracy of prediction for rotor dynamic analysis is verified.By adjusting the nominal radial clearance of journal foil bearing,a high speed and stable oil-free turbocharger rotor system is obtained.During the test,the effect of journal bearing's micro groove depth on rotor dynamic stability is investigated,and a consistent result with simulation is obtained.Therefore,the novel journal foil bearing is beneficial to improve dynamic stability of rotor system.