Exploration On The Performance Of A Controllable Stiffness Gas Bearing With Shape Memory Alloy Springs And Its Rotordynamic Response

A rotor system supported by Gas foil bearing(GFB)has tremendous advantages of high-speed,high efficiency,lightweight construction,environmentally friendly,since the atmosphere can be used in the system as the working medium to achieve oil-free lubrication operation in high-speed.The main application area,actually,is turbo machinery that ranges from micro gas turbine,compressors and expanders for air conditioning,to large aerospace equipment.There is no doubt that the system needs to operate continuously in a relatively stable state.It is well known that conventional gas foil bearings achieve constant performance once the design values of the rotor-bearing system are selected.However,the operating conditions and requirements of the system are constantly changing.This may cause a high level of sub-synchronous motion at some fxed speed,and intermittent contact and wear of the top foil and shaft surface during startup/stopdown.Moreover,different excitation sources can affect the working state of the rotor,which may lead to large vibration of the rotor,and even result in the failure of the system.Structure defect seriously restricts the development of the GFB and its application in some new equipment.To meet these practical requirements of the system,this paper proposes a new gas bearing which use shape memory alloy(SMA)as the flexible support structure of gas bearing,called shape memory alloy gas foil bearing(SMAGFB).Based on the reversible phase transition of shape memory alloy material,the performance of SMAGFB varies with changing the properties of the SMA material,and the operation state of the rotor is further adjusted.The main contents and the achievements of of this paper are as follow.Experiment facilities are built to measure the properties of SMA material according to the constitutive model of SMA material,then,the phase transition temperature of SMA material are obtained.Considering the phase transition property,the preload and friction between SMA springs,the stiffness matrix of nested SMA springs is established.The stiffness matrix of top foil is calculated by finite element method.So,the theoritical model of the total structural stiffness of SMA bearing can be obtained by superimposing above two stiffness matrices.The hysteresis curves of the bearing during phase transition are measured by static push-pull experiment,which can verify a rationality of SMA structure model and confirm the influence of SMA material phase transition on bearing structure performance,building a foundation of theoritical prediction of SMAGFB.The Reynolds equation of gas film pressure is derived based on the mass and force equilibrium condition.Then it combines with the equation of structure motion to establish the elastohydrodynamic model of SMAGFB.The continuous gas film pressure is discretized by the finite difference method and assigned to corresponding joints of bearing support structure.Finally,the Reynolds equation and the structural motion equation are calculated by newton iterative method,and the numerical solution is obtained under the steady state condition of SMAGFB.This paper discusses the variation law of film pressure,thickness distribution,rotor position and extremity bearing capacity under conditions of different loads,rotational speeds,spring arrangement and material conditions,which provide theoretical guidance for the structural design of SMAGFB.The mearused structure stiffness and damping coefficients of SMAGFB vary with excitation frequency by a certain excitation facilities.Lots of experimental results prove that the phase transition of SMA material obviously affects the dynamic stiffness and damping coefficients of SMAGFB.Based on a perturbation method,the transient Reynolds equation and the motion equation of bearing's supporting structure are linearized.The finite difference method is used to solve the theoretical model of dynamic coefficient of gas bearing,the influence of different structure parameters on dynamic stiffness and damping coefficient of SMA bearing is obtained.The nonlinear calculation model of SMA bearing is established to study the effect of unbalance mass and arrangement of SMA springs on the axis track of the rotor,so as to prove that the phase transition of SMA spring can change the performance of SMAGFB and the working state of the system.The equivalent heat transfer model of bearing support structure and rotor is deduced by analyzing the heat transfer path on SMA bearing structure,rotor and considering the effect of cooling flow and heating flow of SMA springs on the heat transfer path.Hence,a thermal elastohydrodynamic theoretical model included the interaction between gas film temperature and pressure is established by considering the effect of gas film end discharge on its temperature boundary,rotor internal space,the rotor section beyond the length of the bearing and temperature boundary of bearing sleeve.The finite difference method is used to solve the spatial distribution of gas film temperature.The experimental results show the validity of the theoretical model with setting up the bearing temperature measuring system to obtain the temperature distribution at different speeds.By analyzing the temperature of each structure part and heating flow of SMA spring and cooling flow under the cases of different loads and nominal clearances,the heat dissipation of each structure part is verified,and the rationality of the designed cooling method of bearing is proved.The nonlinear dynamic model including rotor motion equation,Reynolds equation and structural deformation equation is established by considering the influence of rotor unbalanced mass,gyro effect and gas flim force.The acceleration of the rotor at the next time point is obtained by solving the rotor motion equation with Wilson-? method,and then the motion law of the rotor and the force of the gas film on the rotor at the next time point are obtained again.A data acquisition system for rotor motion supported by a SMAGFB and a GFB is established to analyze the influence of different clearances on vibration and to verify the accuracy of calculated results with experimental data.The dynamic response and motion mechanism of rotor with different static loads,unbalance-masss and arrangement of SMA springs are studied.Predicted results show that the frequency of subsynchronous vibration can be changed by the change of elastic modulus and structure deformation caused by phase transition of SMA springs.This implies that the reasonable application of distributed SMA can improve the stability of the system.To sum up,an intelligent material namely SMA which can actively change its material properties is introduced into the gas bearing as the support structure of SMAGFB.Finally,the stability and bearing capacity of the rotor system supported by SMAGFB are improved.At the same time,measured and theoretical results explore the influence of different structure parameters and operation parameters on the static and dynamic characteristics,nonlinear motion law,temperature distribution and the rotor dynamic response.These data gives a conclusion that the phase transition of SMA material can effectively regulate the system performance and that the designed cooling system can reduce the temperature of the bearing and restore the initial state of SMA material.