Experimental study of the onset of downstream motion of adhering droplets in turbulent shear flows
Experimental study of the onset of downstream motion of adhering droplets in turbulent shear flows
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DOI：
10.1016/j.expthermflusci.2019.109843
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发表时间：
2019-12
影响因子：
3.2
通讯作者：
Beawer Barwari;S. Burgmann;Artur Bechtold;M. Rohde;U. Janoske
中科院分区：
文献类型：
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作者：
Beawer Barwari;S. Burgmann;Artur Bechtold;M. Rohde;U. Janoske
在这项研究中，粘附在各种固体表面上的剪切流，这是由一个控制的空气流驱动的液滴的动力学进行了实验研究。进行了一系列的实验，以了解流体性质，润湿特性，液滴尺寸和流速的影响。实验中使用了矩形Plexiglas通道。液滴被放置在该通道的底壁上。通过透射光技术测量液滴的形状和轮廓位置，并通过图像处理进一步分析。剪切流导致液滴的变形和振荡，并最终导致向下游移动。确定了三种制度：状态I对应于变形和振荡，而在状态II中，变形且仍振荡的液滴向下游移动。第三种机制的特点是以滑动的方式持续向下游运动。状态II的开始，即下游运动的开始由临界空气速度限定。结果表明，接触角滞后和表面能对液滴运动开始的临界风速以及润湿特性和液滴体积有很大的影响。为了寻找临界速度的全局经验规律，基于液滴雷诺数和修正的拉普拉斯数推导了无量纲方法。经验定律与实验数据吻合得很好，可以作为液滴运动开始的临界速度的估计。
In this study, the dynamics of adhering liquid droplets on various solid surfaces in shear flow, which is driven by a controlled air flow, are investigated experimentally. A series of experiments are carried out to understand the effects of fluid properties, wetting characteristics, droplet sizes and flow velocities. A rectangular Plexiglas-channel is used for the experiments. Droplets are placed on the bottom wall of that channel. The droplet shape and contour position are measured by the transmitted light technique and are further analyzed by using image processing. The shear flow leads to a deformation and oscillation of the droplet and eventually to a movement downstream. Three regimes are identified: regime I corresponds to a deformation and oscillation, whereas in regime II the deformed and still oscillating droplets moves downstream. Regime III characterizes a continuous downstream movement in a gliding manner. The start of regime II, i.e. the onset of downstream motion is defined by a critical air velocity. Results show that the contact angle hysteresis and surface energy have a strong influence on the critical air velocity for the onset of droplet motion, as well as the wetting characteristics and droplet volume. To find a global empirical law for the critical velocity, a dimensionless approach is derived based on the droplet Reynolds number and a modified Laplace number. The empirical law is in good agreement with experimental data and may serve as an estimator of the critical velocity for the onset of droplet motion.