| During operation of the oil–gas micro-lubrication system,the lubricant is driven by high-speed air flow and sprayed on the surface of the component part.The wall-attached oil film is formed with time due to the continuous impact of oil droplets on the wall.Then subsequent oil droplets impact on the oil film.Relevant studies have shown that oil droplets are easily squeezed by lubricants to form tiny bubbles.The bubbles will destroy the integrity of the oil film and accelerate the oxidation deterioration of lubricating oil,resulting in poor lubrication and significantly affecting the service life of equipment.In this paper,the basic research on the effect of micro-bubble on the dynamics of the oil droplet impinging on the wall is carried out.The main contents and conclusions of this paper are as follows.1)The Coupled level set and volume of fluid method(CLSVOF)is used to simulate the evolution process of the hollow droplet impacting on the solid wall.The feasibility of numerical model and calculation method is verified by comparing with experimental data of predecessors.2)Based on the developing background of oil-gas micro-lubrication,the behavior of the hollow oil droplet impacting on a rectangular groove surface is numerically studied.The morphological evolution process of the hollow oil droplet is investigated.The formation mechanism of the central jet and the distribution of air entrainment are analyzed.And the effects of grooved width,grooved depth and impact position on the oil droplet spreading process are discussed.The results show that the central jet and air entrainment can be formed after the hollow oil droplet impacting on the rectangular groove surface.The formation of the central jet originates from the velocity vortex at the bottom of the bubble.The position of air entrainment in the grooves is influenced by the spreading velocity of the hollow oil droplet.When the liquid viscosity increases,the spreading length of the hollow oil droplet decreases in all directions.The width of the groove has great influence on the spreading length of the hollow oil droplet in the vertical and parallel groove directions,but has little influence on the spreading height.When the dimensionless grooved width is equal to 0.3,the neck jet is formed and the spreading length of the vertical groove direction increases rapidly in the later stage of motion.In addition,the grooved depth also has an important influence on the spreading of the hollow oil droplet in all directions.With the increase of grooved depth,the central jet becomesmore difficult to form.The location of impact has little influence on the evolution of the hollow oil droplet,but it has some influence on the air entrainment in the groove.3)A two-dimensional axisymmetric model is used to simulate the behavior of the hollow oil droplet impacting on the heated wall.The morphological evolution process of oil droplets impacting on the heated wall is investigated.The rupture mechanism of the liquid shell when the central jet is discussed.And the flow and heat transfer characteristics of droplets after impacting are analyzed.The results show that with the increase of impact velocity,there are three different evolutionary processes,i.e.spreading,transition and central jet,after the hollow oil droplet impacts on the heated wall.Pressure and velocity distribution indicate that the shell rupture at the central jet originates from the combined effect of inertial force and surface tension.The variation in spreading length of hollow oil droplets is similar in different evolutionary processes,but the variation in the center height of hollow oil droplets is different.The wall heat flux and the position of the maximum heat flux increase with impact velocity.In addition,the wall temperature influences the flow and heat transfer characteristics of the hollow oil droplet impingement.Considering the viscosity-temperature characteristics of the lubricating oil,the spreading length of the hollow oil droplet increases with the wall temperature,but the central height of the hollow oil droplet is unaffected by the wall surface temperature.The wall heat flux and the position of the maximum heat flux also increase with the impact velocity.4)A numerical study on the impact of the hollow oil droplet on the oil film is carried out.The deformation and movement of the bubble when the hollow oil droplet impacts on the oil film are investigated.The dynamic mechanism of bubble rupture is discussed.And the influence of bubble size,impact velocity and liquid viscosity on the characteristic parameters of bubble deformation in the process of impact is analyzed.The results show that the bubble will deform and break up to form film droplet after the hollow droplet impacting on the oil film.The change of pressure and velocity gradient inside and outside the bubble are the main cause of bubble rupture.The bubble diameter has a great influence on the bubble rupture mode,single-point rupture occurs when the bubble is small,larger bubbles are more likely to cause multiple ruptures.The difference of force between different sizes of bubbles is larger,and there is no obvious correlation between the size of the bubble and the moment of rupture.The impact velocity and the viscosity of the liquid have a certain influence on the deformation,rupture and rupture time of the bubble.The larger the impactvelocity,the greater the kinetic energy of the oil droplet,and the more likely the bubble deformation and rupture.When the viscosity of the liquid increases,the bubble deformation is promoted at the early stage of the movement of the oil droplet,and the rupture behavior of the bubble can be delayed in the later period of the movement.5)The behavior of the oil droplet impacting on oil film which contains a bubble is numerically simulated.The deformation and movement of the bubble in the oil film are investigated.The dynamic mechanism of bubble rupture is discussed,and the influence of bubble size,bubble position and film thickness on the characteristic parameters of bubble deformation in the process of impact is also analyzed.It is found that as the oil droplet impacts on the oil film,the bubble may rupture on the free surface,presenting a stable deformation,or rupture in the oil film,which is greatly influenced by the bubble size.The bubble rupture is caused by the instability of the gas-liquid interface and the surface tension.The viscous shear force also plays an important role when the bubble rupture takes place in the oil film.The diameter of20?m is a critical value for a bubble to rupture on a free surface or inside an oil film,in which a bubble can keep stable in an oil film layer.As the bubble position and oil film thickness changes,the bubble deformation process changes correspondingly.Under the same impact conditions,bubbles at the top of the oil film are more likely to deform than those in the center of the oil film.Bubbles at the bottom of the oil film have the smallest total deformation and finally attach to the wall.As the film thickness decreases,the deformation quantity of bubbles increases.Thus,bubbles are easily broken.To this day,the basic research at oil-gas micro-lubrication is not yet mature,resulting in the lack of corresponding theoretical guidance in engineering applications.The research work in this paper can provide a theoretical reference for exploring the influence of micro-bubbles on lubrication process,and is of great significance for deepening the understanding of bubble dynamics in the process of droplet impingement on the wall. |
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