Study On Cavitation Mechanism Of Liquid Film Seals And Its Effects On Sealing Performance

As a pivotal basic component of the rotating equipment,the sealing performance of spiral groove liquid film seals(SGLFS for short)directly determines the running state of the corollary equipment.With the development of process media delivery equipment to high temperature,high speed,low temperature,high viscosity and multiphase medium and so on,as well as the national laws and regulations on the safety advocates of environmental protection,the stability and reliability of liquid film seals need to be improved further.Due to the working principle and operating characteristics of the SGLFS,the lubricating film with micron thickness between the sealing surfaces ruptures partially and cavitation is generated.Expansion of cavitation in liquid film changes the lubrication state between the sealing surfaces as well as inevitably affects sealing performance,which can not be ingnored.In view of this,taking the SGLFS as object,cavitation induction mechanism and its effects on sealing performance were studied by the method of experimental and theoretical researches,and the applicabilities of different cavitation boundaries were defined.Based on the above researches,a groove structure for suppressing cavitation in the liquid film was proposed and optimization of the groove structure was carried out.Specific works and conclusions are described as follows.Effects of different spiral groove functions and structure parameters on cavitation distribution,evolution and cavitation boundary at different operation conditions were analyzed based on the cavitation visualization and data collection of SGLFS,and the relations between cavitation boundary and condition parameters of the liquid film were fitted.Experimental investigation shows that the cavitation characteristics are obviously different with different grooving position of spiral groove and different pumping functions,these are mainly due to the effect of the existing dam area in the middle groove type seal and the combined action of Couette flow and Poiseuile flow.The increasing of oil pressure helps to inhibit the cavitation occurrence,while the increasing of rotating speed is opposite.But the effects of oil pressure on the evolution of cavitation in the two types of inner groove seal and middle groove seal are different,especially for the inner groove seal.The increasing of groove depth or groove number and the decrease of groove angle are able to reduce cavitation area of SGLFS with different pumping funcitons,but the influencing mechanisms are not the same.A physical model of SGLFS was built with consideration of surface topography.The anomalous physical domain composed of spiral curves was transformed into an inerratic computational domain by coordinate transformation.The equivalent density and viscosity of cavitation were defined,and two variables including cavitation index and general variable were introduced.Governing equation unified in the new coordinate system was made being dimensionless and the cavitation mathematical model was established based on the JFO cavitation boundary.Values of theoretical calculation by the program algorithm of the JFO cavitation mathematical model were compared with the experimental values.The results show that the program algorithm has high accuracy,which lays foundation for the follw-up theoretical analysis.The cavitation characteristics under different cavitation boundaries were investigated based on the cavitation mathematical model of SGLFS,and the effects of multiple factors such as spiral groove structural parameters,operating parameters,property parameters of liquid film,liquid film thickness and microscopic surface topography on the cavitation induction were analyzed.Theoretical researches indicate that the root of cavitation induction is because of film thickness mutation at the boundary of groove and dam in the low pressure zone,and the liquid into grooves throttles and expands.Liquid film fractures initiation as the liquid film pressure lowers to cavitation pressure,causing the occurrence of cavitation.The parameters other than the micro surface topography change the two dimensional scale of cavitation along the helical and circumferential directions in different degrees respectively,and affect the cavitation degrees of the corresponding locations and the changing law varies according to the parameters.As the sealing face is non-slotted and the film thickness is smaller,the surface roughness promotes cavitation but the value is limited,and the situations of low wave amplitude,a certain range of high frequency wave number or larger negative taper promote cavitation easily but the positive taper effectively inhibits cavitation.As the end face is slotted and the film thickness is larger,the influence of roughness in cavitation is very smaller and can be ignored,the increasing of wave amplitude promotes cavitation obviously and changes the location of cavitation occurrence,but the impact of taper is limited.The theoretical values of liquid film thickness,friction torque and leakage calculated from the Reynolds and JFO cavitation boundaries were compared with experimental values measured from cavitation visualization experiment,and the effects of liquid film cavitation on sealing performances were analyzed and the applicabilities of different cavitation boundaries were defined.The results show that the occurrence of cavitation in the liquid film lowers load-carrying capacity,reduces leakage and has a smaller effect on liquid film friction torque but increases friction coefficient,compared with the SGLFS with full liquid film lubrication.When the cavitation area in the liquid film is smaller,the two cavitation boundaries can be able to predict load-carrying capacity and leakage.While the cavitation area is larger,the JFO cavitation boundary predicts the two parameters more accurately.To reduce the occurrence of cavitation in liquid film and improve the hydrodynamic performance,the structure of circumferential beveled-step was introduced into the rectangular section spiral groove and corresponding mathematical model was established,increasing liquid film pressure of groove-dam boundary and downstream local region.Based on the circumferential beveled-step structure,a mathematical model of structural optimization was established,with the spiral groove structural parameters as the optimization variables.And the optimum values of structural parameters were obtained based on the mathematical model.The results indicate that the structure of circumferential beveled-step decreases the probability of cavitation in liquid film,change the location of cavitation occurrence and the cavitation shape,and the hydrodynamic lubrication performance of liquid film is also greatly improved.The liquid film seals with the groove structure optimized have better hydrodynamic lubrication performance and lower occurrence of cavitation in liquid film.