Flow And Heat Transfer Characteristics Of Successive/Synchronous Oil Droplets Impinging The Wall Under The Influence Of Micro-cavitation

Droplet impingement phenomenon is widespread in nature and industrial production.Relevant research has made significant progress in many fields such as inkjet printing and function table interface design.In the process of oil-gas micro-lubrication,the film flow and heat exchange process of oil droplet impingement have always been a hot issue in academic circles.When the oil-gas micro-lubrication system is working,the high-speed airflow transports the lubricating oil to the lubrication area in the form of micro-oil droplets,forming a lubricating oil film layer on the surface of the friction pair,and the airflow plays a role of flow and heat dissipation while transporting the oil droplets.The water splash and secondary splash caused by the impingement of oil droplets and the oil film disturb the spatial flow field near the lubrication point,which in turn affects the microscopic formation process of the oil film and the heat transfer characteristics of this area.The oil droplet ejection and its impingement process are complicated,and there are successive/synchronous impingements during the oil droplet interaction.In the process of oil and gas transportation,oil is easily mixed into cavitation under the influence of factors such as high pressure and temperature.The existence of cavitation will accelerate the oxidation and deterioration of lubricating oil,and will also have a certain impinge on the integrity of the oil film,which will ultimately affect the lubrication effect and equipment service life.At present,the theoretical research on the flow and heat transfer of multi-droplet impingement is still in the perfect stage,and the understanding of the droplet impingement behavior under the influence of micro-cavitation is still not deep.This paper conducts a basic study on the behavior of continuous/synchronous oil droplets impinging the oil film under the influence of micro-cavitation,which can provide a theoretical reference for exploring the influence of micro-cavitation on the process of multi-droplet impingement.At the same time,it is of great significance to deeply understand the theory of two-phase flow and heat transfer at the free interface and to improve the effect of lubrication design.The main contents and conclusions of the paper are as follows:1)Study on the flow characteristics of successive oil droplets impinging the wall under the influence of micro-cavitation.Using the coupled level set-volume fraction method(CLSVOF),a two-dimensional rotational axisymmetric numerical model of double oil droplets successively impinging the oil film was established to study the behavior of the double oil droplets successively and positively impinge the oil film under the influence of micro-cavitation.The focus is on the influence of micro-cavitation on the dynamics of the double oil droplets successively impinging the oil film.The formation mechanism of the jet during the impingement is discussed,and the influence of micro-cavitation on the formation process and morphology of the central jet and the neck jet is analyzed.Studies have shown that when the cavitation oil droplets participate in the impingement,a central jet is generated,and the formation mechanism of the central jet generated by the cavitation in the leading or trailing oil droplets is different.The solid oil droplets participate in the impingement without the central jet phenomenon,the leading hollow oil droplets are not.Leading hollow oil droplet is not conducive to the growth of crown water splash;when the leading oil droplet is hollow,the impinge of the impingement process between the bottom oil film of the impingement pit and the subsequent oil droplet on the size of the crown splash is less than that when the leading oil droplet is solid.2)Study on the heat transfer characteristics of successive oil droplets impinging the wall under the influence of micro-cavitation.Focus on the influence of different cavitation distribution forms on the heat transfer characteristics of double oil droplets successively impinging the oil film,discuss the formation mechanism of special heat transfer phenomena during the impingement.The influence of different bubble distribution forms on the geometric size of the temperature wake and the cooling effect of the collision area is analyzed.Studies have shown that: during the falling process of high-temperature oil droplets,a "cicada wing-like" temperature wake appears;the difference in peripheral velocity gradients during the falling and spreading process of solid and hollow oil droplets affects the geometric size of the wake;In the late stage of the impingement,under the combined action of the space flow field and the pressure field,a vortex is formed in the collision pit,and the vortex has the effect of increasing the heat flow density in the impingement area;Different cavitation distribution forms have different effects on the cooling and heat dissipation effect during the impingement,and the participation of hollow oil droplets is not conducive to the cooling and heat dissipation during the impingement.3)Study on the flow and heat transfer characteristics of synchronous oil droplets impinging the wall under the influence of micro-cavitation.In this paper,the established three-dimensional numerical model of the simultaneous impinging of two oil droplets on the oil film is numerically simulated and analyzed.The focus is on the effect of micro-cavitation on the flow dynamics and heat transfer characteristics of the simultaneous impinge of two oil droplets on the oil film.The formation mechanism of the central film jet during the impingement and the heat flux distribution characteristics during the heat transfer process are discussed.The influence of micro-cavitation on the formation process,morphology and heat transfer performance of the impingement zone of the central film jet is analyzed.Studies have shown that during the process of simultaneous oil droplets impinging the oil film,the participation of hollow oil droplets in impingement will reduce the splash size of the central film jet,and at the same time easily cause the central film jet to deflect;In the process of synchronized oil droplets impinging the oil film,the hollow oil droplets have the effect of increasing or suppressing the heat flux at the center of the impingement area under different conditions.