液滴撞击超亲水表面的最大铺展直径预测模型

Theoretical model of maximum spreading diameter on superhydrophilic surfaces

液滴撞击超亲水表面铺展之后形成的薄液膜铺展直径是喷雾冷却、降膜蒸发等传热传质过程的一项关键控制参数.以往模型在预测超亲水表面惯性力驱动下的最大铺展直径时,存在低韦伯数下呈反常趋势、高韦伯数下预测值偏低等问题.针对上述问题,本文采用高速摄像技术研究液滴撞击过程中的铺展水力学特性,发现了以往模型未完全考虑超亲水表面的铺展特性:球冠状液膜、高黏性阻力及重力势能做功.本文考虑了液膜球冠形态、重力势能、辅助耗散,修正了以往最大铺展直径的预测模型,并建立了适用于超亲水表面最大铺展直径的预测模型.通过对铺展过程中各能量成分分析发现,在超亲水表面上动能、表面能、重力势能均转化为黏性耗散能,其中:在低韦伯数下,表面能转化为黏性耗散能占主要作用;在高韦伯数下,动能转化为黏性耗散能占主要作用.并且,在低韦伯数下,重力势能和辅助耗散的引入对于准确预测超亲水表面最大铺展直径具有重要作用.将模型预测结果与实验结果比较发现,本模型成功消除了以往模型在低韦伯数下的反常趋势,且能较好预测宽韦伯数范围下超亲水表面最大铺展直径.同时,本模型可以预测亲水和疏水固体表面的液滴最大铺展直径.超亲水表面最大铺展直径的准确预测模型的提出对喷雾冷却,降膜蒸发中提高和控制流体铺展距离和传热效率具有重要意义.

Liquid droplets impacting on the solid surface is an ubiquitous phenomenon in natural,agricultural,and industrial processes.The maximum spreading diameter of a liquid droplet impacting on a solid surface is a significant parameter in the industrial applications such as inkjet printing,spray coating,and spray cooling.However,former models cannot accurately predict the maximum spreading diameter on a superhydrophilic surface,especially under low Weber number(We).In this work,the spreading characteristics of a water droplet impacting on a superhydrophilic surface are explored by high-speed technique.The spherical cap of the spreading droplet,gravitational potential energy,and auxiliary dissipation are introduced into the modified theoretical model based on the energy balance.The model includes two viscous dissipation terms:the viscous dissipation of the initial kinetic energy and the auxiliary dissipation in spontaneous spreading.The energy component analysis in the spreading process shows that the kinetic energy,surface energy,and gravitational potential energy are all transformed into the viscous dissipation on the superhydrophilic surface.The transformation of surface energy into viscous dissipation is dominant at lower We while the transformation of kinetic energy into viscous dissipation is dominant at higher We.It is found that the gravitational potential energy and auxiliary dissipation play a significant role in spreading performance at low We according to the energy component analysis.Moreover,the energy components predicted by the modified model accord well with the experimental data.As a result,the proposed model can predict the maximum spreading diameter of a droplet impacting on the superhydrophilic surface accurately.Furthermore,the model proposed in this work can predict the maximum spreading diameter of the droplet impacting on the hydrophilic surface and hydrophobic surface.The results of this work are of great significance for controlling droplet spreading diameter in spray cooling and falling film evaporation.