Theoretical Study On Characteristics Of Compliant Foil Bearings Lubricated With Supercritical Carbon Dioxide

Due to the excellent capacity of heat transfer and compression characteristics,Supercritical carbon dioxide?S-CO₂?is very suitable as a refrigerant for thermodynamic cycles.In recent years,the study of S-CO₂ closed Brayton cycles?CBCs?has become a hot topic because of the theory of sustainable development.However,blame for high speed and poor operate temperature and pressure,the oil-free rotor system of CBCs also faces a series of problems.In this thesis,journal and thrust compliant foil bearings?CFBs?where S-CO₂ work as the lubricant medium are analyzed based on numerical simulation after theoretical derivation work was completed,author's work can be divided into three part.Part I,begin with continuous equation and momentum equation,two Reynolds equation that suit for condition where the density and viscous may change,which correspond to journal and thrust bearing respectively are obtained.Refer to the modification method of oillubricated bearing in turbulence condition,semi-empirical constants were added into the equation to consider the effect of high Reynolds Number.Use proper foil deformation model,coupled with dimensionless steady-state Reynolds equation,explicit iterative solution format with relaxation factor based on finite difference method were formulated.Part II,by using MATLAB codes to simulate the pressure and film distribution,the load capacity and deflection angle result of the air CFBs were obtained and compared with the data from references,which confirmed the correctness of the solver.Then use the same solver with turbulence modification were used to calculate the S-CO₂ CFBs' static parameter by combining with a fluid property look–up subroutine.So the distribution of pressure,lubrication film thickness and modification constants for turbulence as well as the pressure peak,deflection angle,load capacity and friction torque results of S-CO₂ CFBs are obtained and compared the capacity with the traditional CFBs later,the optimized pad number of a thrust S-CO₂ CFB also was provide under specific condition.Part III,in transient modified Reynolds equation,a linear perturbation method was used and coupled with the structure deformation equation to get the dynamic characteristics of S-CO₂ CFBs near the equilibrium position.After that the effect of dynamic characteristic coefficients on eccentricity,clearance,bearing number,rotational speed,structure flexibility,and excitation frequency were investigated.In this research,a new idea for solving the Reynolds equation in an explicit iterative way is proposed for the special formulation characteristics of the two Reynolds Equations.Transform the Reynolds equation into a standard type to establish a general iterative solution module,thus different Reynolds equation for different bearings could be solved by simplifying different Reynolds equations into the standard type and calling the solve module,numerical programming process become easier and more reliable.To calculate the property according to the local pressure of S-CO₂,a look-up table approach is implemented in the coding process based on data generated with NIST REFPROP.Furthermore,inspired from the perturbation method,a new and original method which based upon the static characteristic calculating codes is presented for dynamic characteristics simulation.When excitation frequency equal to zero,the method works well so it's suitable enough for rigid bearings.These ideas and techniques in this thesis can be used for reference in other bearing simulation research,and the conclusions are valuable for S-CO₂ CFBs' design,analysis,and optimize work,too.