Abstract This paper numerically investigates air entrapment behavior when a water droplet impacts on a solid substrate in the presence of ambient air. The dynamics of air entrapment is specifically described, in particular, retraction, contraction and toroid formation have been discussed. Volume-of-fluid method has been used in order to track the liquid-gas interface. The effect of surface wettability on the air entrapment and its evolution is studied. In line with experimental findings, we have observed a dimple formation on droplet surface at the impact point in the present study which has not been reported using a numerical study so far. It has been demonstrated using numerical study that the formation of dimple leads to the air entrapment. The surface wettability is demonstrated to have significant influence on the evolution of the air film into bubble. The processes during the evolution, such as inertial retraction, contraction and pinch-off are delayed on increasing the contact angle. Air bubble formed on the substrate gets detached from the substrate for low contact angle of 35 ° (hydrophilic surface) however, it remains attached to the substrate for higher contact angles: 90 ° and 120 ° (hydrophobic surface). Further, the influence of air entrapment on the heat transfer characteristics is studied. The deformation of the droplet as it approaches the substrate, influences the boundary layer on the substrate which eventually affects the wall heat flux. Post impact, the wall heat flux is primarily influenced by the liquid fraction and the air entrapped. In the region of only air, the wall heat flux is very low compared with that in the region of water or mixture of water and air. The present numerical results have been validated employing experimental results available in the literature.

中文翻译：

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液滴撞击基底期间空气滞留及其对对流传热影响的计算研究

摘要 本文数值研究了在环境空气存在的情况下，当水滴撞击固体基材时的空气滞留行为。具体描述了空气滞留的动力学，特别是讨论了回缩、收缩和环形形成。已使用流体体积法来跟踪液-气界面。研究了表面润湿性对空气滞留的影响及其演变。与实验结果一致，我们在本研究中观察到在撞击点的液滴表面形成了凹痕，迄今为止尚未使用数值研究报告。已经使用数值研究证明，凹坑的形成会导致空气滞留。表面润湿性被证明对空气膜向气泡的演变具有显着影响。进化过程中的惯性收缩、收缩和夹断等过程随着接触角的增加而延迟。在 35° 的低接触角（亲水表面）下，在基板上形成的气泡会从基板上分离，但在接触角为 90° 和 120°（疏水表面）的情况下，它仍然附着在基板上。此外，研究了空气滞留对传热特性的影响。液滴接近基板时的变形会影响基板上的边界层，最终影响壁面热通量。撞击后，壁面热通量主要受液体部分和夹带的空气的影响。在只有空气的区域，与水或水和空气的混合物区域相比，壁面热通量非常低。目前的数值结果已经使用文献中可用的实验结果进行了验证。