Numerical Investigation On Flow Performance Of Droplets Impinge Upon A Liquid Film

In internal combustion engine, droplets impact on a solid wall is a common phenomenon which has significant effect on the fuel evaporation rate. Rational use of the law of droplets collision with a wall and to find an effective way to promote fuel rapid evaporation and uniformly mixed is an important means to achieve fuel completely combustion and lower emissions. This thesis is based on the research background of internal combustion engine. Two oil droplets collision with static oil film and a single droplet collision with fuel film which contain fuel bubbles are studied by using numerical simulation coupled with interface tracking technology. The impact evolution process is researched from the interface scale, and the evolution laws and quantitative description method of fuel impact on a wall is obtained. The effect and rules of droplets moving characteristics, the droplets properties and other factors on the secondary atomization process of droplets hitting wall are studied. The main contributions are as follows:(1) The research on evolution process of double-droplet impinging upon a liquid film. Applying VOF method and VOF coupled with Level set method, the tracking of the evolution process of double-droplet vertically impinge upon a liquid film and two droplets on the same vertical impinge upon a liquid film along the different angles is achieved, and the flow phenomena of multiple droplets impact on liquid film and the evolution of crown are observed. The conclusion is:Compared with the single droplet impinge upon a liquid film, multiple droplets impinge upon a liquid film is not only restricted by its own movement, but also affected by mutual effect among multiple droplets. After double droplets vertically impinge upon a liquid film, a central jet is formed at the centerline of pairs of droplets. Two droplets vertical collision with liquid film is influenced by Reynolds number, Weber number and the space of droplets. The spray is unsymmetrical when droplets impinge upon a wall film using an angle, and the impact angle inhibits the splash of spray.(2) The research on evolution process of a single droplet impinging upon a liquid film which contains a fuel bubble. By changing the droplet’s initial velocity, bobbles movement, deformation and broken, bubbles merger and other phenomenon are investigated; the effect of bubbles movement on space liquid membrane development has also been studied too. The results showed that, when droplet impact liquid film using a low velocity, the phenomenon of bubble broken and the merger of broken bubbles was seen, when speed increases to a certain degree there is no bubble broken. When bubbles come into air, a crown or a jet will be formed near the position of gas-liquid interface broken, and with the increasing of impact velocity, this crown or jet will break up more intensely. Compared with other interface tracking method, VOF method has a great advantage on handling the issue of fusion and fragmentation of the phase interface, so in this part, VOF method is used to tracking gas-liquid interface.