液滴撞击平面液膜的流动特性和换热性能数值模拟

发布时间：2019-04-26 12:21

【摘要】：在我们的生活和工业生产中随时随处都可以看到液滴撞击液膜这种现象,研究这种现象的内部机理可以为实际工程应用带来重要的理论依据。本论文采用一种新的算法CLSVOF,对单液滴撞击水平液膜后产生水花现象、双液滴撞击水平液膜后流体的动力学特性及双液滴撞击热壁面液膜后换热性能及其影响因素进行了数值模拟和分析。主要结论如下： (1)单液滴撞击水平液膜的数值模拟结果表明,撞击后之所以产生飞溅是因为流体内界面的不稳定造成的,模拟结果还发现了气泡夹带现象；当液滴直径较小时,没有小气泡产生,液滴直径不断增大产生的小气泡数目逐渐增多,并且气泡消失的时间越长。 (2)不同水平间距的两液滴同时撞击液膜的数值结果表明,双液滴以不同速度撞击液膜后会形成表面波动、皇冠状水花、飞溅产生二次液滴,而且两表面波相向运动时碰撞会产生中心射流,其在向上运动时会断裂产生飞溅液滴；we数决定了水花边缘是否产生飞溅,we较小时不产生二次液滴,当we数逐渐增大,水花的厚度变薄,且产生的二次液滴数目增多；we数的增大,水花高度越大,射流高度越大,而水花高度、射流高度开始回落的时间也随we数的增大而提前；液滴间距的变化对刚开始水花高度几乎没有影响,一段时间后,液滴间距越大水花高度反而越小,在这之后,液滴水平间距越小水花高度越小；而对射流高度,在撞击刚开始,液滴水平间距越大射流高度越小；一段时间后,液滴间距越大射流高度越大。 (3)双液滴先后撞击液膜的计算结果显示,液滴间距不同,撞击后流动形态出现很大差异,液膜厚度对液膜及水花形态影响不大,但对气泡消失影响很大。液膜厚度越大,气泡消失时间越长；液滴间距较小时,水花底部直径随时间逐渐增大,且液膜厚度越大,水花底部直径越小；当液滴间距较大时,水花底部直径先增大后减小在增大。 (4)不同垂直间距的两液滴先后撞击热壁面液膜的数值结果表明,液滴撞击速度越大,壁面平均热流密度也就越大,液滴直径、双液滴间垂直间距和液膜厚度对平均热流密度的影响较小；撞击区域的热流密度随液膜厚度、垂直间距和液滴直径的增大而减小,交界区的热流密度随液膜厚度和双液滴垂直间距的增大而增大;液滴直径不同,交界区的热流密度先随液滴直径的增大而减小,后随液滴直径的增大而增大,交界区热流密度的波动幅度随液膜厚度、垂直间距和液滴直径的增大而减小。
[Abstract]:The phenomenon of droplet impinging on liquid film can be seen everywhere in our daily life and industrial production. The study of the internal mechanism of this phenomenon can bring important theoretical basis for practical engineering application. In this paper, a new algorithm, CLSVOF, is used to produce water blossoms after a single droplet impinges on a horizontal liquid film. Numerical simulation and analysis are carried out on the dynamic characteristics of the fluid after the double droplets impact on the horizontal liquid film and the heat transfer performance of the liquid film after the double droplet impinges on the hot wall and the influencing factors are simulated and analyzed. The main conclusions are as follows: (1) the numerical simulation results of the impact of a single droplet on a horizontal liquid film show that the spatter is caused by the instability of the internal interface of the fluid, and the phenomenon of bubble entrainment is also found in the simulation results; When the droplet diameter is small, no small bubbles occur, and the number of small bubbles produced by increasing the droplet diameter increases gradually, and the longer the bubble disappears, the longer the bubble disappears. (2) the numerical results of the simultaneous impact of two droplets with different horizontal spacing on the liquid membrane show that after the two droplets hit the liquid membrane at different speeds, there will be surface fluctuations, crown-shaped water blossoms and spatter to produce secondary droplets. Moreover, when the two surface waves move in opposite direction, the central jet will be generated, and the spatter droplets will break when the two waves move upward. The we number determines whether spatter occurs at the edge of the flower, and the secondary droplets are not produced when we is small. When the number of we increases gradually, the thickness of the blossoms becomes thinner, and the number of secondary droplets increases. With the increase of we number, the higher the water blooming height, the greater the jet height, and the time when the water blooming height and jet height began to fall back also advance with the increase of we number. After a period of time, the larger the droplet spacing is, the smaller the flower height is, and after that, the smaller the horizontal distance between the droplets is, the smaller the flower height is. For the jet height, the larger the droplet spacing is, the smaller the jet height is at the beginning of the impact, and after a period of time, the larger the droplet spacing is, the greater the jet height is. (3) the calculation results of the double droplets impinging on the liquid membrane show that there is a great difference in the flow morphology after the impact with different droplet spacing. The thickness of the liquid film has little effect on the liquid film and water flower morphology, but it has a great effect on the disappearance of bubbles. The larger the film thickness is, the longer the bubble disappearance time is, and the smaller the droplet spacing is, the larger the thickness of liquid film is, the smaller the diameter of the bottom of water flower is, the larger the thickness of liquid film is, the more the diameter of the bottom of water flower increases with time. When the droplet spacing is large, the diameter of the bottom of the water floss increases first and then decreases. (4) the numerical results show that the larger the velocity of liquid drop impact, the greater the mean heat flux and the diameter of the liquid film on the hot wall with two droplets with different vertical spacing. The vertical spacing between two liquid droplets and the thickness of liquid film have little effect on the mean heat flux. The heat flux density in the impact region decreases with the increase of film thickness, vertical spacing and droplet diameter. The heat flux density in the boundary region increases with the increase of liquid film thickness and the vertical distance between two droplets. When the droplet diameter is different, the heat flux density in the boundary region decreases first with the increase of the droplet diameter, and then increases with the increase of the droplet diameter. The fluctuation amplitude of the heat flux density in the boundary region decreases with the increase of the film thickness, the vertical spacing and the droplet diameter.
【学位授予单位】：大连理工大学
【学位级别】：硕士
【学位授予年份】：2014
【分类号】：TB611

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