凹槽结构强化液滴合并弹跳的数值研究

Numerical Study of Enhanced Droplet Merging and Bouncing by Groove Structure

目的 为研究凹槽结构对液滴合并弹跳现象的影响,提高液滴自弹跳的速度和能量转化率。方法 通过有限元方法模拟,研究了液滴在凹槽结构上的动态行为和槽深H、槽宽W及液滴尺寸对V型槽和矩形槽上液滴的弹跳速度、无量纲速度、表面能转化率、动量和总动能的影响。结果 凹槽内与槽外液滴间的合并弹跳包含液滴接触、产生液桥、三相线收缩和液滴弹离表面4个过程;宽高比相同时,液滴弹跳速度和表面能转化率均随液滴半径先增大后减小,凹槽宽高比为1.5,液滴合并弹跳半径为0.25 mm工况下表面能转化效率最高;矩形槽与V型槽,宽高比为1.5,液滴合并弹跳半径为0.25 mm时,液滴弹跳速度与表面能转化率随半径比先增大后减小,在半径比为1时达到峰值,二者表面能转化率最大值为27.87%和30.66%。槽宽恒定时,凹槽结构强化液滴合并弹跳存在最佳凹槽宽高比和最优弹跳半径,V型槽提升液滴弹跳效率比矩形槽高约12%。结论 合并后液滴撞击V型槽侧壁的反作用力使液滴旁瓣较矩形槽更早受到侧壁面抑制而发生回流,从而提升了其能量转化效率,减少了液滴的振荡损失与黏性耗散。研究结果在设计高效的自清洁功能表面、提高冷凝表面换热效率、预防和抑制换热器表面结冰、成霜等工业领域有重要的应用意义。

In recent years,the phenomenon of droplet agglomeration and bouncing,which is a characteristic of superhydrophobic surfaces,has received close attention from scholars.The results of the study have important industrial applications in enhancing heat transfer,anti-icing and frost protection and surface self-cleaning.The construction of various groove structures on superhydrophobic surfaces is highly effective in enhancing droplet merging and bouncing and improving surface energy conversion efficiency.In order to improve the bouncing speed and energy conversion rate of droplet self-bounce,the effect of groove structure on droplet merging bounce phenomenon was studied.The effects of droplet size on the droplet momentum,total droplet kinetic energy,dimensionless bounce velocity,bounce velocity and surface energy conversion rate in different directions of droplet merging,and bouncing on V-shaped and rectangular troughs were investigated by numerical simulations with deionized water as the working medium for the same trough width W and the ratio of trough depth to trough width H/W=1,1.5 and 2.The simulations were based on a two-dimensional model of two-phase laminar flow with a pair of spherical droplets of equal diameter bouncing together on a superhydrophobic surface with a fluted structure.The phase field method was used to calculate the problem.The simulation results showed that the combined bouncing between droplets inside and outside the groove consisted of four processes:droplet contact,liquid bridge generation,three-phase line contraction and droplet bouncing off the surface.When the ratio of groove width to groove depth was the same,the droplet bounce speed and surface energy conversion rate increased and then decreased with the droplet radius.At this point,the combined bounce velocity of the rectangular trough droplets was 0.318 m/s and the dimensionless bounce velocity was 0.562.The combined bounce velocity of the V-shaped trough droplets was 0.355 m/s and the dimensionless bounce velocity was 0.627.Both rectangular and V-shaped grooves broke the capillary-inertial scalar law for super-sparse horizontal surfaces.And the optimal droplet combined bounce radius was 0.25 mm when the width to height ratio was 1.5.For rectangular grooves,when the ratio of groove width to groove depth was 1 and 2,the optimal droplet merging bounce radius was 0.2 mm.At this point the droplet bounces was 0.329 m/s and 0.301 m/s respectively,with dimensionless velocities of 0.52 and 0.47.The V-shaped groove was better than the rectangular groove in enhancing droplet bounce.For rectangular grooves and V-shaped grooves,the surface energy conversion rate reached a maximum of 27.87% and 30.66% for both with a width to height ratio of 1.5.The notch structure enhanced the droplet merging and bouncing with optimal bounce radius and notch aspect ratio.The droplet bounce speed and surface energy conversion rate increased and then decreased with the radius ratio,reaching a peak at a radius ratio of 1.V-shaped grooves improved droplet bounce efficiency by approximately 12% compared with rectangular grooves.In conclusion,the reaction force of the merged droplet hitting the side wall of the V-shaped groove makes the droplet side flap be inhibited by the side wall surface earlier than that of the rectangular groove and backflow occurs,thus enhancing its energy conversion efficiency and reducing the oscillation loss and viscous dissipation of the droplet.