液滴碰撞液膜润湿壁面空气夹带数值分析

采用复合水平集-流体体积法并综合考虑传热及接触热阻的作用, 对液滴碰撞液膜润湿壁面空气夹带现象进行了数值分析. 揭示了夹带空气形成机理, 探索了夹带空气特性参数随碰撞速度和液膜厚度的变化规律, 获得了夹带空气作用下液滴碰撞润湿壁面的传热机理. 研究结果表明: 撞壁前气液两相压力差是引起气液相界面拓扑结构变化以及夹带空气形成的主要原因; 液滴碰撞速度与压缩空气层内压力以及相界面形变高度密切相关; 液滴接触液膜时, 碰撞轴上液滴底部和液膜表面速度相等, 大约是碰撞速度的1/2; 碰撞速度对夹带空气层底部到破碎点的无量纲弧长和最大无量纲夹带空气直径均存在较大的影响; 液滴和液膜的无量纲形变高度与斯托克斯数密切相关; 液膜初始厚度对液滴和液膜的无量纲形变高度和最大无量纲夹带空气直径影响较大; 撞壁初始阶段, 碰撞中心区域夹带空气对壁面热流密度分布存在较大的影响.

Numerical analysis on air entrapment during droplet impacting on a wetted surface

A numerical model is developed using the coupled level set and volume of fluid method including heat transfer and contact resistance to simulate air entrapment during a droplet impacting on a wetted surface. The dynamic characteristics of the phase interface are analysed. The mechanisms of deformation of the phase interface and formation of entrapped air are explored. The effects of impacting velocity and thickness of liquid film on characteristics of entrapped air are studied. The mechanism of heat transfer is also obtained in this article. The obtained results are as follows. The pressure difference between liquid and gas before the droplet impacting is the main factor determining the deformation of phases interface and the formation of air entrapment. The larger the impacting velocity, the larger the pressure inside the compressed air film is. When the droplet contacts the liquid film, the velocities of the droplet and liquid film increase to their maximum values, and at the impacting axis, they are approximately the same, nearly half the impacting velocity. The velocity distributions of phase interface of the droplet and liquid film are nearly the same in the area of impacting center. The impacting velocity has important effects on the dimensionless arc from bottom to breaking point and the dimensionless diameter of the air. The dimensionless arc and dimensionless diameter decrease with increasing impacting velocity. The dimensionless deforming heights of the droplet and liquid film are closely related to Stokes number: the larger the Stokes number, the larger the dimensionless deforming heights are, and they can be expressed as a power function with Stokes number. The initial thickness of liquid film also affects dimensionless deforming heights of the droplet and liquid film and dimensionless diameter of the entrapped air: the larger the dimensionless thickness of the liquid film, the larger the dimensionless deforming heights are, and the dimensionless diameter decreases with increasing dimensionless thickness of the liquid film. At the very initial stage of the impact, the entrapped air is important for surface heat flux distribution. The entrapped air presents contraction, breakup and detachment. The surface heat flux distribution changes closely with evolution of the entrapped air and tends to be uniform. The effect of the entrapped air on the surface heat flux distribution decreases gradually.