剪切变稀液滴撞击不同浸润性壁面的数值模拟研究

Numerical simulation of shear-thinning droplet impact on surfaces with different wettability

基于有限元法,采用水平集方法捕捉相界面的移动,构建了液滴撞击固体壁面的数值模型.通过修正的幂律模型描述流体的非牛顿剪切变稀特性,探讨了剪切变稀特性对液滴撞击固体壁面后铺展行为的影响,分析了撞击不同浸润性壁面时剪切变稀特性对液滴撞击壁面行为的影响差异.研究结果表明:随着幂律指数m的减小,液滴撞击过程中的黏性耗散减小,液滴的形貌变化及无量纲参数变化更为显著.接触角为55°的情况下:当m降低至0.85时,液滴铺展过程中开始出现显著区别于牛顿流体液滴的振荡现象;当m降低至0.80时,液滴在回缩过程中会出现中心液膜断裂的情况.接触角为100°时,剪切变稀液滴均会出现振荡行为,振荡幅度随着m的减小而增大.接触角为160°时,牛顿流体液滴与剪切变稀液滴均会在回缩过程中弹起,但剪切变稀液滴的弹起速度更快.此外,基于数值计算结果,本文提出了接触角为55°情况下剪切变稀液滴撞击壁面后的最大无量纲铺展直径预测模型.

Controlling impact dynamics of droplets on solid surfaces is a significant problem in a variety of applications,such as inkjet printing,spray cooling and coating and so on.Most of fluids used in industries always contain various kinds of additives such as surfactants,polymers and particles.Therefore,these fluids exhibit non-Newtonian behaviors,for instance,yield-stress,viscoelastic,shear-thickening and shear-thinning.The impact dynamics of Newtonian droplets on solid surfaces has been extensively investigated.However,the number of researches about fluids with non-Newtonian properties is comparatively very small.In this work,we employ the finite element scheme coupled with level set method to simulate the impact process of droplets on solid surfaces.The numerical simulation models the presence of shear-thinning viscosity by using the truncated power-law rheological model.We first conduct a mesh convergence study and verify the numerical model.The simulation results are found to be in good agreement with experimental data in the literature.By performing extensive numerical simulations and varying the rheological parameters and surface wettabilities,the influences of these parameters on the impact dynamics are evaluated,and the dominant effects that govern the spreading and receding process are determined.The simulation results show that for the case of droplet impacting on surface with contact angle q=55°,the spreading is stronger with power-law index decreasing as evidenced by larger shape deformation and faster interface moving speed.As power-law index decreases,we expect the maximum dimensionless diameter to increase and the minimum dimensionless height to decrease during inertial spreading.For the case of droplet with lower power-law index(m=0.85 and 0.80),which indicates lower viscous dissipation during impact,the dimensionless parameters have significant differences.After first receding,the impacting droplet is not balanced any more and it starts to spread again until its kinetic energy is completely damped by fluid viscous dissipation.For the case of droplet(m=0.80)impacting on surface,the center breakage can be observed during droplet spreading,which results from the effect of strong shear-thinning property.When a shear-thinning droplet impacts on a surface with contact angle q=100°,the oscillation behavior can be observed and the oscillation amplitude increases as power law index decreases.Bouncing phenomenon can be observed when a droplet impacts on surface with contact angle q=160°,regardless of rheological property.Finally,we propose an empirical model to predict the maximum dimensionless diameter of shear-thinning droplet impacting on the surface with contact angle q=55°as a function of non-Newtonian Reynolds number Ren.