Research On Flow Characteristics Of Aerodynamic Bearings With Variable Cross Sectional Clearance

Aerodynamic bearings have the characteristics of high-limit speed、strong bearing capacity and self-adaptation stability,and have been widely used in various oil-free supporting rotating machinery of national defense industry.Under high-speed rotation conditions,and in the small gap of the circumferential variable cross section of the aerodynamic bearing,when the shear stress between the solid and the lubricating gas film exceeds the limit value,the flow will produce interface slip.The performance of the bearing is significantly different under slip and no slip conditions.Therefore,studying the influence of slip on the performance of aerodynamic bearings plays an important role in improving the accuracy of air bearings design and promoting the development of air bearings.Firstly,this paper takes the multi-leaf bump foil aerodynamic bearing as the research object,and the fluid-solid coupling model between the flow and multi-leaf superposition/top foil and bump foil composite elastic boundary is established.On the basis of this,the ultimate shear stress of the wall is derived by the Van der Waals force.Considering the slip conditions that can occur on the shaft side and the foil side,the compressible flow Reynolds equation of aerodynamic bearing based on ultimate shear stress under four kinds of slip conditions is obtained.The flow and performance calculation method of aerodynamic bearing based on the comparison of ultimate shear stress cycle is established,and its effectiveness is verified by literature experimental data.It provides a reference for the performance analysis of aerodynamic bearings under interface slip conditions.Secondly,the numerical simulation model of multi-leaf bump foil aerodynamic bearing（radial）is established.The influence of different bearing parameters and different slip states on the flow and bearing static characteristics in the variable cross-section gap of dynamic pressure gas bearing is analyzed.In addition,the influence of the elastic foil on the slip state is analyzed.The research shows that with the increase of eccentricity and rotor speed,the flow in the bearing gap is more likely to produce interface slip,and the static characteristics of the bearing are different under different slip conditions.Compared with no slip condition,slip on the shaft side will reduce the film pressure and reduce the bearing capacity,and the law of slip on the foil side is reversed;the frictional resistance of the bearing is reduced when the slip occurs,and the frictional resistance decreases by an average of 4%when the shaft side slips,and the slip friction resistance of decreases by 15%when the foil side slips.Furthermore,the study also finds that the effect of interface slip on rigid bearings is larger than that of elastic foil bearings,the bearing capacity of rigid bearings decreases by 36%on average,and the average of elastic foil bearings decreases by 28%.This shows that the elastic foil is advantageous in suppressing the negative influence of the slip on the bearing capacity of the bearing.Then,based on the compressive flow Reynolds equation of the aerodynamic bearing with ultimate shear stress,the calculation model of bearing dynamic characteristics in four slip states is obtained and the calculation method is established by using the small perturbation method.The numerical analysis of the influence of different bearing parameters and different slip states on the dynamic characteristics of the bearing is carried out.The results show that the stiffness coefficient and damping coefficient of the bearing will increase significantly when slip occurs,and the increase will go up with the increase of eccentricity and aerodynamic viscosity.When the eccentricity increases from 0.1 to 0.5,K_xy of slip on both sides increases from 1.6 times to 2.2 times,andD_xy increases from 1.7 times to 2.3 times.This shows that slip can improve the stability and anti-interference ability of the bearing,and the effect is more obvious when the eccentricity and the aerodynamic viscosity are large.Finally,a aerodynamic bearing（radical）performance test bench is built to test the takeoff speed、ultimate bearing capacity and the change of bearing capacity with the eccentricity,and the experimental results is compared with the numerical calculations.The results show that:the numerical calculation is consistent with the experimental results,and the maximum error is 15%,which further verifies the feasibility of the above calculation method.In addition,the experiment also finds that the floating speed is between4.5?10³ r/min and 5.0?10³ r/min under the bearing parameters studied in this paper,and the frictional resistance drops by about 77%after lifting;when the speed increases from5.0?10³ r/min to1.2?10⁴ r/min,the ultimate bearing capacity increases from 25N to 85N.In this paper,the variation of static and dynamic characteristics of bearings under different bearing parameters and different slip states is studied,which provides a useful reference for the design and engineering application of this type of bearing.