Study On Dynamic Characteristics Of Liquid Film Seal Based On Cavitation Effect

The hydrodynamic mechanical seals represented by spiral groove liquid film seals have been widely used in various industries.However,with the increasing complexity of equipment operation conditions,the performance requirements of liquid film seals,such as high reliability and long life,are increasingly demanding.Limited by the working characteristics,the liquid film of end face will be cavitated in the gap divergence zone,which will inevitably affect the steady state and dynamic performance of the seal.In view of this,the effective control measures of cavitation,evolution process of cavitation zone,influence of cavitation on sealing steady state and dynamic performance parameters,and stability and tracking property of liquid film seal under cavitation conditions are discussed by numerical analysis and experimental research in this paper.The main work and conclusions obtained are as follows.A mathematical model of spiral groove liquid film seal based on mass conserving JFO cavitation boundary condition was established.The general equation was solved by SUPG finite element method and the dynamic characteristic parameters were calculated by small perturbation method.Meshing of the model,the solution domain with curve boundaries was transformed into a parallel quadrilateral region with straight line boundaries by coordinate transformation,and the boundaries among groove,land and dam were coincident with element boundaries respectively.Based on the downstream pumping spiral groove liquid film seal cavitation visualization test device,the cavitation photographs and related sealing performance parameters were collected,and the accuracy of program algorithm was verified by the comparison of theoretical and experimental results.Subsequently,the cavitation area distributions of spiral groove liquid film seal under different operating parameters were obtained on this test device.By analyzing the experiment results,the concepts of critical speed and pressure of cavitation were proposed.Then the effects of structural parameters of spiral groove,medium viscosity,balance radius,spring force and surface topography on cavitation rate were analyzed theoretically.The results show that the critical operating parameters of cavitation corresponding to different factors are different,and the changing trend of critical pressure of cavitation with each factor is opposite to that of critical speed of cavitation.The effective control of cavitation can be achieved by reasonable selection of various factors.According to the shapes of cavitation zone of spiral groove liquid film seal under different working conditions,three nodes for representing the boundary of cavitation zone were proposed: when the cavitation zone extended to the radius of groove root,the boundary points of cavitation can be taken as the maximum radius point with maximum circumferential angle,the maximum radius point with minimum circumferential angle and the minimum radius point.When the cavitation zone did not extend to the radius of groove root,the boundary points of cavitation can be taken as the maximum circumferential width point,the maximum radius point and the minimum radius point.After that,the effects of rotating speed,pressure,film thickness,medium viscosity,structural parameters and surface topography on cavitation zone were analyzed,and the mechanism of cavitation evolution was revealed.In order to explore the necessity of taking into account the cavitation effect under different factors,the effects of operating parameters and structural parameters on opening force,leakage,dynamic stiffness coefficients and dynamic damping coefficients of spiral groove liquid film seal under JFO boundary and Sommerfeld boundary were compared and analyzed.On this basis,the effects of simultaneous change of several structural parameters and surface topography on sealing performance were further studied.The results show that the variation trends of opening force and leakage with each factor under the two kinds of boundries are basically the same,but the optimal structural parameters may not be the same.The results of JFO boundary are smaller than those of Sommerfeld boundary when cavitation exists on the seal face,and the difference between the two is determined by the size of cavitation rate and the strength of hydrodynamic effect.The dynamic characteristic parameters under the two boundary conditions are related to the cavitation rate.The range analysis shows that the order of influence of spiral groove structural parameters on sealing performance is different.Besides,the taper has a great influence on sealing performance parameters,and the influence of waviness on sealing performance is determined by waviness number and waviness amplitude.When the film thickness is small,the opening force,dynamic stiffness coefficients and dynamic damping coefficients are slightly increased when considering the roughness of seal face,but the regions with the greatest influence are different.Also,the roughness of different regions has different influence on leakage.Nevertheless,generally speaking,the effect of roughness is limited.Considering the cavitation effect,support spring stiffness and O-ring damping,the three-degree-of-freedom stability equation of liquid film seal and the dynamic tracking property equation of stationary ring were established.Furthermore,the critical criterion of seal instability was given,and the amplitude ratio and phase difference between stationary ring response and rotating ring excitation were proposed to characterize the tracking property of stationary ring.Finally,the effects of different factors on seal stability and dynamic tracking property were analyzed.The results show that the spiral groove liquid film seal does not suffer axial instability,and the phenomenon of "half frequency oscillation" exists in angular direction.The zero phase difference tracking of stationary ring can be realized by appropriate selection of spring stiffness and O-ring damping.The influences of dynamic stiffness coefficients and dynamic damping coefficients on stationary ring tracking property are different under different factors.Adequate increase of absolute value of negative taper is helpful to improve the liquid film seal stability and stationary ring tracking property;when waviness number is different,the waviness amplitude has different influence on critical moment of inertia,amplitude ratio and phase difference;critical moment of inertia,amplitude ratio and phase difference change little with roughness.