Research On Interfacial Leakage Mechanism Of Contact Mechanical Seal Based On Percolation Theory

The contact mechanical seal for preventing equipment leakage has become the focus of attention owing to the requirements of long-term safe operation,environmental protection and resource conservation.Since the mechanical seal theory is developed in 1885,people have never stopped on the investigation of leakage mechanism of contact mechanical seal,in order to reveal the reasons to make it better for the application of industrial equipment.Predecessors have proposed the“fluid exchange flow theory”,“wave effect”and“leakage channel fractal model”to explain the leakage mechanism of mechanical seal contact interface under certain working conditions,and contributed to the safe operation of the equipment.However,these theories can't fully explain the leakage phenomenon of the seal interface at rest,and also can't answer illustrate the leakage problem of the parallel end face when the“waviness”disappears after being worn out.Persson and Bottiglione answered these two questions based on the percolation leakage channel model.However,they ignored the multi-scale effect of surface morphology and its effect on the percolation properties of the seal interface.Not only that,these studies did not account for the capillary force of the leakage media,and assumed that there was a leakage fluid in the leakage channel,and the fluid flow was laminar flow,without considering the impact of driving forces on leakage.Therefore,the mechanical seal interface leakage mechanism needs to be deeply investigated.In this thesis,the percolation theory was introduced to investigate the percolation threshold of porous seal interface,and the percolation characteristics at different mesh layers of porous seal interface were determined accordingly.The contact model between the rotating and stationary rings of mechanical seal was equivalent to an ideal rigid smooth surface and a rough surface,while the variations of real contact area,the deformation,and the porosity after loading were analyzed.3D reconstruction and finite element method were performed on the rough surfaces of the rotating and stationary rings to verify the effectiveness of theoretical research on the porosity of the seal interface.Based on Knudsen number and capillary force,the macroscopic leakage criterion of porous seal interface was established under percolation conditions.The fluid flow resistance of seal interface under macroscopic leakage conditions was analyzed,and the formula to calculate the leakage was derived based on the size of pore throat.Besides,the leakage mechanism of contact mechanical seal was also explained.The main results and conclusions were summarized as follows:?1?Based on the percolation theory,the percolation characteristics of seal interface at different mesh layers were discussed,and the relationship between the porosity of seal interface and the pore throat size of percolation channel was analyzed.The results illsutrated that with the increase of the number of mesh layers n,the corresponding percolation threshold?_c gradually decreased from the single-layer mesh percolation threshold 0.593.When the number of mesh layers tended to infinity,the percolation threshold approached to a certain value of 0.316.When the porosity was greater than 0.316,the seal interface formed a leakage channel,and the relationship between the pore throat size and the porosity was approximately linear.?2?A contact mechanics model based on the new method for fractal characterization of rough surface contours was proposed,and a percolation leakage channel on the initial porosity of porous seal interface was established.To overcome the scale dependence of contact model based on the statistical parameters and the shortcomings that the initial profile of existing fractal contact model depended on contact area or sampling length,a new fractal contact model for rough surface was established based on fractal dimension D_s,fractal roughness G and the base size l of the largest asperity.The effects of face pressure and morphology parameters on the porosity of porous seal interface after loading were discussed.The results illsutrated that,the deformation of asperity initiated from elastic deformation to elastoplastic deformation,and then transformed into full plastic deformation with the increase of the loading.When D_s was small,the influence of G on the increase of real contact area and the deformation can be ignored,however,when D_s increased,the increase of G had an obvious effect on the real contact area after the deformation.The initial porosity?₀ of the seal interface increased with the fractal dimension D_s,irrespective of the fractal roughness G.The porosity?after loading decreased with the increasing of normal load p_c,and rapidly decreased with the increasing of D_s and the decreasing of G.According to the stationary ring fractal dimension 2.3?D_s?2.5,the fractal roughness 10^-11?G?10^-9 and the face surface p_c?0.5MPa,a single-layer grid micro-channel structure model could be established,because the porosity at the seal interface was always greater than 0.593.?3?Based on the measurement data of OLS4100 laser scanning microscope,the 3-D topography of rough surface of rotating and stationary rings was restructured.The Ansys software was used to simulate the contact model of reconstructed the rotating and stationary rings.The results showed that,the initial porosity of reconstructed seal interface was consistent with the initial theoretical porosity.As the load p_c increaseed,the deformation area of contact interface gradually diffused from a convex peak to the periphery.The results of theoretical calculations of porosity with face pressure were basically in accordance with that of finite element simulation.?4?Based on the capillary force and the Knudsen number,the conditions of macroscopic leakage of liquid or gas at the seal interface were analyzed,and the macroscopic leakage criterion of percolation channel was established.The resistance of fluid flow in the leakage micro-channel was deduced and numerically simulated by fluent software.The leakage rate based on the pore throat size was deduced using the simplified N-S equation and the continuity equation.The results showed that,macroscopic leakage in the seal interface could be attributed to the presence of leakage channels and large enough fluid driving force.When the pressure difference on both sides of seal interface was greater than the capillary force p_L,the liquid would flow through the percolation channel as a macroscopic leakage.When the gas Knudsen number K_n of permeation channel was less than 0.01,the gas would flow through the percolation channel as a macroscopic leakage.The flow resistance in the leakage channel was the sum of the frictional drag and the local resistance.As the inlet speed of leakage passage increased,the fluid resistance in the passage enhanced.The smaller the size of pore throat in the leakage channel,the higher flow resistance on the change in inlet velocity.The leakage mechanism of contact mechanical seal interface was explained at last.?5?The variations of topography parameters,medium pressure,spring pressure,and rotational speed on the leakage rate were experimentally studied to verify the effectiveness of percolation leakage model in the contact mechanical seal interface.The surface topography parameters of the rotating and stationary rings were measured by the contact profilometer.Based on the axial load,the size of pore throat and theoretical leakage rate of the leakage channel were calculated.By injecting 0.1⁰.5MPa compressed air and measuring the leakage with a glass rotameter,the variations of leakage with end profile parameters,medium pressure and axial load were obtained.The results showed that,the smoother the surface of the stationary ring?D_s was larger,and G was smaller?,the smaller the height of the percolation channel and the leakage rate.The greater the pressure of the medium,the greater the driving force of fluid flow and the leakage rate.When the spring pressure was low,the axial load had little effect on the leakage channel of seal interface.Owing to the influence of inertial force and viscosity,the speed was higher,while the leakage rate was lower.When the capillary force at pore throat of the leakage channel was greater than fluid driving forces,there was no macroscopic leakage at the seal interface.The study results on the leakage mechanism of seal interface can provide a theoretical basis for the optimal design and leakage control of contact mechanical seal through the engineering application.