Lattice Boltzmann Simulation And Experimental Study On Clearance Microflow In Gas-lubricated Journal Bearings

Gas-lubricated journal bearings have the advantages of high limit speed,long service life,high machining accuracy and good stability,compared with traditional rolling bearings.They have been widely used in oil-free high-speed rotating machinery such as aerospace,precision manufacturing and cryogenic refrigeration.At present,the application field of gas-lubricated journal bearings is constantly expanding,and it is developing in the direction of high speed and miniaturization.The lubricating fluid characteristics and microfluidic effect in bearing clearance have more prominent influence on bearing performance.It is very important to carry out related theoretical research.The macroscopic Reynolds equation based on the continuum assumption is no longer applicable when the fluid is in the transition regime due to the extremely small gas film thickness.Therefore,the flow and thermal properties in the bearing clearance are investigated from the mesoscopic scale in this paper,and the visualization experiments are also performed.The main researches are as follows.For the simplified gas foil journal bearinga,a lattice Boltzmann model is firstly established to study the clearance flow characteristics and bearing performance.The multi-relaxation time(MRT)lattice Boltzmann model of D2Q9 is used,the Langmuir velocity slip boundary conditions are introduced through the non-equilibrium extrapolation scheme to realize the micro-flow simulation in the slip and transition regime.The model is also extended to an arbitrary form of body-fitted grid and solved by the generalized form of interpolationsupplemented lattice Boltzmann method(GILBM).The effects of different bearing parameters and velocity slip on the clearance flow characteristics and bearing performance are investigated using the present model.The research shows that the velocity distribution curve at the maximum clearance bends downward,and the gas backflow phenomenon occurs when the eccentricity ratio is greater than 0.73,the speed is lower than 30 krpm or the clearance is less than 40μm,and the boundary velocity slip will accelerate the appearance of backflow.The velocity distribution at the minimum clearance is curved upward,and the degree of curvature is increased at low speed or large clearance.The slip velocity on the journal side has a sine-like distribution law,while the slip velocity on the bearing sleeve side has a parabolic distribution.With the increase of eccentricity ratio or rotational speed,or the decrease of clearance size,the velocity slip effect of the boundary will be strengthened.The extreme pressure of gas film,load carrying capacity and frictional torque show an increasing trend,and the increase of velocity slip degree will cause more influence on the static characteristics of the gas bearing.A model for the investigation of viscous thermal dissipation of gas film in gas-lubricated journal bearing is established.The double-distribution-function(DDF)lattice Boltzmann model in the form of total energy is applied to the body-fitted grid through finite difference discretization,and the Langmuir temperature jump boundary condition is introduced.After the accuracy of model is verified,the temperature distribution of gas film under different parameters was simulated.The results show that when the temperature difference between the journal and bearing sleeve is large,the radial temperature distribution is linear,and heat conduction is dominant.When the temperature difference between the two sides is small,the radial temperature is parabolic and viscous heat dissipation dominates.As the eccentricity ratio increases,the maximum gas film temperature increases approximately linearly,and the temperature increase caused by temperature jump at an eccentricity ratio of 0.8 is approximately6.9 times greater than that in the non-eccentric case.As the rotational speed increases,the maximum gas film temperature increases faster and faster,and the degree of viscous heat dissipation enhancement due to the temperature jump is not much different.As the clearance size decreases,the maximum gas film temperature increases and the rate of change accelerates,and the effect of temperature jump increases gradually.The temperature increase caused by the temperature jump at 25μm is about 7.5 times that at 75μm.A test bench for the study of clearance flow characteristics of the gas-lubricated journal bearing is developed.The effects of different eccentricity ratio and rotational speed on the velocity and pressure fields of the clearance fluid are tested and compared with the simulation results.The study shows that with the increase of eccentricity ratio,the velocity at the middle position of maximum clearance increases linearly,and the velocity at the middle position of minimum clearance and the maximum gas film pressure show different trends of increase.With the increase of rotational speed,the velocity at the middle position of maximum clearance and minimum gap as well as the maximum gas film pressure all increase linearly.The experimental results show similar variation patterns to the simulation results and the maximum error is14.73%,indicating the reliability of numerical model.