| Water-lubricated hydrodynamic spiral-groove bearings(SGBs)are greatly promising in application of high-speed rotating machinery due to their advantages such as large load capacity,good stability,low friction and self-pumping action.However,when the water-lubricated hydrodynamic SGB operates at a high speed,the effects of cavitation,fluid inertia,turbulence and heat on the bearing performance are significant.Therefore,it is necessary to establish an applicable lubrication model considering these four effects for the water-lubricated hydrodynamic SGB.This paper is devoted to the lubrication modeling for the high-speed water-lubricated hydrodynamic SGB,and the theoretical analysis and experimental investigation on the static and dynamic characteristics of this type of bearing are carried out systematically.The main research contents and conclusions of this paper are as follows:(1)The establishment of lubrication model for the high-speed water-lubricated hydrodynamic SGBsBased on the fluid dynamics of gas-liquid two-phase flow,the transient generalized Reynolds equation of liquid-phase,the transport equation of bubble volume fraction and the motion equation of a bubble are derived,and then a cavitation modeling based on the two-phase,heterogeneous Reynolds equation is proposed to reveal the mass,momentum and energy transfer mechanism between the gas-liquid two phases.Considering the coupling effect of cavitation,fluid inertia,turbulence and heat,the energy equation of liquid phase and the static and dynamic generalized Reynolds equations of liquid phase are derived,and then the lubrication model for the high-speed water-lubricated hydrodynamic SGBs is established.The boundary-fitted coordinate system and the control-volume finite difference scheme are implemented to solve the bearing lubrication model,and a general method for calculating the static and dynamic characteristics of the three degrees-of-freedom spiral-groove thrust bearing(SGTB)and four degrees-of-freedom spiral-groove journal bearing(SGJB)is proposed.The theoretical results correlate well to the experimental data,which indicates that the proposed lubrication model can be used to analyze the static and dynamic characteristics of high-speed water-lubricated hydrodynamic SGBs.(2)The effects of cavitation and centrifugal force on the characteristics of the single-pad high-speed water-lubricated SGTBsAround the following four cases: a)without cavitation,without centrifugal force,b)with cavitation,without centrifugal force,c)without cavitation,with centrifugal force,d)with cavitation,with centrifugal force,under different rotational speed and film thickness,the influence of cavitation and centrifugal force on the static and dynamic characteristics of the single-pad high-speed inwardly pumping and outwardly pumping water-lubricated SGTBs is discussed.An experiment setup is developed to measure the film thickness,volume flow rate and axial stiffness coefficient of the single-pad high-speed water-lubricated SGTBs,the film cavitation photographs at different film thicknesses are captured by the industrial high-speed camera,and a quantitative description of the cavitation phenomenon is achieved.The results are as follows:When the modified Reynolds number exceeds 0.63,the coupling effect of cavitation and centrifugal force on the static and dynamic characteristics of two types of single-pad SGTBs must be considered.For the inwardly pumping SGTB,the reverse flow will appear when the modified Reynolds number reaches 1.2.Considering the coupling effect of cavitation and centrifugal force,the axial load capacity,stiffness and damping coefficients of two types of SGTBs reduce,the flow rate of the inwardly pumping SGTB decreases substantially,while the flow rate of the outwardly pumping version increases significantly.It is observed by the experiment that the distribution of caviation bubbles in the water film is discrete and transient.(3)Comparative study on characteristics of two types of typical double-pad high-speed water-lubricated SGTBsA comparative study on the static and dynamic characteristics is conducted between the double-pad high-speed inwardly pumping and outwardly pumping water-lubricated SGTBs.The effects of operating conditions(the tilt angle,rotational speed and eccentricity ratio)and configuration parameters(the radius ratio,spiral angle,radial groove extent and groove depth)on the static and dynamic performances of two types of SGTBs are analyzed.The results are as follows:Under a high speed condition,the double-pad outwardly pumping SGTB has a larger power loss than the inwardly pumping one.But when the radius ratio exceeds 0.7,the outwardly pumping SGTB can perform better than the inwardly pumping version,with good axial load capacity,higher resistance to tilting,larger stiffness and damping coefficients and always positive radial flow rate.The double-pad inwardly pumping and outwardly pumping SGTBs have the same configuration parameters corresponding to the maximum axial load capacity,but they have the different configuration parameters corresponding to the maximum axial stiffness coeffcient.The seal dam has significant influence on the axial load capacity and stiffness coefficient of two types of SGTBs.(4)Characteristic analysis of high-speed water-lubricated SGJBsThe variation of static and dynamic characteristics versus the operating conditions(rotational speed,eccentricity ratio and tilt angle)is analyzed for the high-speed water-lubricated asymmetrical partly grooved SGJB.The results show that the water-lubricated SGJB also has good stability under the high speed and small eccentricity conditions.The research of this paper further improves the two-phase flow lubrication theory for the hydrodynamic SGBs,and reveals the influence of cavitation and fluid inertia on the performance of high-speed water-lubricated hydrodynamic SGBs.The research results can provide an effective method for analyzing the static and dynamic characteristics of high-speed water-lubricated hydrodynamic SGBs,and lay a theoretical foundation for the engineering design and application of this type of bearing. |
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