Study On Phase Change Characteristics And Dynamic Behavior Of Liquid Film Lubrication Mechanical Seals

The dynamic pressure liquid film seal is widely used in aerospace and petrochemical industry because of its excellent lubricity and operation stability.However,with the complex changes of operating conditions,the sealed media tend to be diversified.When sealing some special media,under the interaction of temperature and pressure of liquid film,phase transition will occur,resulting in a significant change in seal lubrication performance.The existing phase change models are one-dimensional models based on axial cross-section and most of the theoretical research is focused on flat face mechanical seals,the influence of liquid film phase transition on the sealing performance of dynamic pressure liquid film seal has not been fully studied.In view of this,the phase change phenomena of dynamic pressure liquid film seals are experimentally studied and a phase change model suitable for dynamic pressure liquid film seals is established,the influence of liquid film phase change on the steady-state characteristics of the seal is analyzed,and the influence law and reason of each factor on the seal performance are analyzed and discussed in detail.the transient response of the seal ring at the moment of phase change and the stability of the seal under impact condition after phase change are discussed,finally,based on the influence of liquid film phase change on the stability and dynamic performance of the seal,the groove combination and structural parameters of the seal end face are optimized to improve the practicability of the seal under two-phase operation conditions,the main research work and research results are as follows.The liquid film phase transition phenomena of downstream pumping spiral groove seals were observed and data collected based on the liquid film phase transition visualization experimental device,the influence of medium temperature,rotation speed,medium pressure and spiral groove structure on the liquid film phase transition phenomena were analyzed.Research shows,when the spiral groove is inner groove type,there is no phase transition in the liquid film,but when the spiral groove is middle groove type,there is obvious phase transition in the liquid film,the phase transition region increases with the increase of medium temperature and rotation speed,and decreases with the increase of medium pressure,the change of spiral groove structure parameters will also affect the size of phase transition region.It can be concluded that the dynamic pressure effect has a significant effect on the liquid film phase transition phenomenon.Based on the pseudo-single-phase theory and two-fluid theory,the mathematical models of liquid film phase transition for flat face mechanical seals and dynamic pressure liquid film seals were established respectively,the accuracy of the model was verified by comparing with the experimental results.The phase change phenomena of dynamic pressure liquid film seals with different functions were simulated by using the phase change model,the sealing performance,flow field distribution,pressure field distribution and phase field distribution were analyzed and discussed.Research shows,the liquid film phase transition can increase the pressure in the low pressure region and change the distribution of flow field,and the influence degree increases with the increase of the liquid film temperature,the high pressure region formed by the hydrodynamic pressure effect inhibits the phase transition and the low pressure region formed by the hydrodynamic pressure effect promotes the phase transition,which results in the change of the shape of the phase transition region.By analyzing the change trend of seal performance when different grooves are set on the end face,it can be seen that the inner groove type downstream pumping spiral groove liquid film seal of has a good ability of restraining phase change.Taking downstream pumping spiral groove mechanical seal as the research object,the effects of various structural parameters,operating conditions,medium physical parameters and rotating shaft whirling on the phase change rate and sealing performance of fluid film are discussed and analyzed.Research shows,the change of structural parameters will obviously affect the ability of the spiral groove to produce hydrodynamic pressure effect,thus affecting the phase change distribution of the fluid film,but the reasons are quite different.The negative taper of the end face caused by the mechanical deformation and thermal deformation can form a convergent gap which has a good effect on restraining phase transition,but the effect is opposite when the taper is positive.The misalignment of the end face will change the circumferential distribution of the film thickness,resulting in a non-axisymmetric distribution of the phase transition region.The hydrodynamic pressure effect of different sealing media is different under the same structure and working condition parameters,although the sealing performance has the same change trend,the influence of liquid hydrogen is the greatest,the deduced formula can be used to describe the influence of different sealing media.Aiming at the problem that the steady-state phase change model can’t predict the different dynamic behavior of the seal caused by the phase change,the phase change dynamic model of spiral groove liquid film seal is established by combining the phase change model with the dynamic equation.The dynamic response of the seal performance at the moment of phase change and the influence of external shock on the seal performance after phase change are calculated,and the influence of various parameters on the dynamic response is analyzed.Research shows,phase change phenomena will cause axial vibration of the seal ring,and excessive phase change area will cause instantaneous opening failure of the seal,choosing appropriate parameters can reduce vibration amplitude and stability time.In order to improve the practicability of spiral groove liquid film seals under two-phase operation,based on the steady-state phase change numerical model of dynamic pressure liquid film seals,taking the groove combination and groove parameters as optimization variables,an optimization model was established to optimize the spiral groove liquid film seal.Research shows,the optimized groove can significantly reduce the leakage and effectively improve the sealing performance and practicability on the premise of guaranteeing the ability to restrain phase change.The optimization of the inner spiral groove profile based on the helix parameter equation can further reduce the leakage of the seal but also reduce the opening force of the end face,which has strong practicability in specific temperature range.