液滴冲击不同浸润性壁面的数值分析

Numerical Analysis of Droplet Impact on Surfaces with Different Wettabilities

采用复合水平集和流体体积法并综合考虑传热和接触热阻的作用,建立了液滴碰撞水平壁面数值模型,并实验验证了模型的准确性。通过分析计算结果,探索了壁面浸润性对液滴撞壁过程的影响,揭示了液滴撞壁流动传热及飞溅机制,并建立了液滴飞溅临界条件理论判据。数值结果表明:液滴铺展系数的碰撞速度效应明显,碰撞速度越大,液滴的铺展系数越大,但液滴达到最大铺展系数所需无量纲时间与碰撞速度不相关;碰撞速度越大,液滴撞壁收缩幅度越大,壁面浸润性对铺展系数的影响越小。液体内部压力梯度是液滴铺展边缘产生射流和断裂的主要原因;Rayleigh-Plateau不稳定性和毛细波是射流颈部收缩和破碎飞溅的关键因素。

In order to explore the mechanism of fuel droplet impact on surface in diesel engine small-size combustor,a numerical model was developed using coupled level set and volume of fluid method including heat transfer and contact resistance. This model was verified by using experiments. The effect of wettabilities and the mechanism of droplet fluid and heat transfer as well as splashing during spreading were obtained according to results analysis. The theoretical condition of splashing is developed based on energy conservation equation. The results show that when the droplet contacts with the surface,it presents surface oscillation,spread and splash as the impact velocity increases. Spreading factor is closely related to impact velocity,the higher the impact velocity is,the larger the spreading factor is. The dimensionless time of maximum spreading factor is uncorrelated to impact velocity,the higher impact velocity is,the larger contraction is. The effect of wettabilities on spreading factor decreases with increasing impact velocity. The pressure gradient inside droplet is the main factor resulting in droplet spreading,breakup and splashing. Rayleigh-Plateau instability and capillary wave results in the shrink,breakup and splash from the liquid sheet. The conclusions are significant to spray control technology.