Study On Steady-State Performance And Dynamic Behavior Of Two-Phase Mechanical Seals

For mechanical seals operating under high-speed and low-temperature conditions,due to the low boiling point of the sealing liquid medium,the liquid film of sealing end-face is prone to phase change under the action of viscous heat dissipation and low pressure on the end-face,so that the end-face fluid film is at gas-liquid miscible state,namely two-phase mechanical seals.Since the occurrence of phase change will significantly alter the mechanical seals performance,and the current model studies on two-phase mechanical seals are mostly hydrostatic type based on axisymmetric assumptions or hydrodynamic type based on isothermal assumptions,in order to be more accurate to study and predict the performance of two-phase mechanical seals,this paper establishes a mechanical seals phase change model that takes into account the thermal effect of the fluid film and the influence of the actual gas in the phase change gas phase.Taking the hydrostatic and hydrodynamic mechanical seals as the research objects respectively,the influence law of the phase change on the sealing performance is deeply studied.The main work and conclusions are as follows.Firstly,based on the theory of homogeneous fluid of multiphase flow,the phase change gas phase is described by the Villi equation,the latent heat of phase change absorption is characterized by the thermodynamic internal energy equation,and further based on the basic assumptions of the fluid lubrication theory,the thermal phase change model equation of the two-phase mechanical seals is derived.The numerical model was solved by finite element analysis using the commercial software COMSOL Multiphysics.By comparing with the axisymmetric continuous boiling model and the experimental measurement results,the correctness of the model is verified.Afterwards,for the hydrostatic mechanical seals,the influence of centrifugal inertia term on the sealing performance under high speed and low temperature conditions was discussed,and the influence of the film thickness and rotational speed,and the film thickness and medium temperature on the sealing performance are studied respectively.For the hydrodynamic mechanical seals,the double-row spiral groove is used as the research object.The phase change flow phenomenon and flow mechanism of two-phase mechanical seals are discussed.The effects of rotational speed,medium pressure and temperature on the physical field variables and sealing performance of hydrodynamic mechanical seals are explored,and the influence of main structural parameters on sealing performance is further studied.Finally,taking the hydrodynamic mechanical seals as the research object,the influence of the compensation ring mass,system stiffness,system damping and disturbance conditions parameters on the sealing performance and the dynamic response of the compensation ring under variable conditions is analyzed.The research results show that the inclusion of the centrifugal inertia term causes the film pressure of the internal flow hydrostatic pressure mechanical seals to decrease,the phase change rate increases,and the leakage decreases significantly.The transformation of medium viscosity gradient caused by temperature rise is the main reason for the appearance of double-balanced film thickness.For the hydrodynamic mechanical seals,the flow space diverges,the pressure drops through the sealing end-face and the formation of the viscous heat dissipation are the inducing factors of the liquid phase change.The accounting of thermal effects makes the physical parameters of homogeneous fluids change obviously,the phase change can be aggravated by the increase of rotational speed and temperature,the increase of pressure has a weak effect on the hydrodynamic pressure,and the suppression of the phase change is limited.In variable working conditions,larger system damping and smaller compensation ring mass can reduce the disturbance to the recovery stability time,and the large sudden drop of pressure may cause the compensation ring to oscillate and unstable.Although the larger temperature and rotational speed disturbance can return to the new balance,the large fluctuation of working condition should be avoided in practice to ensure the stable operation of seals.