Formation of confined free film in the window on a vertical perforated plate
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摘要: 液膜是常见的气液接触方式之一，依据单或双自由面可分为壁面膜和自由膜。不同于传统液膜研究，板上开窗或开孔后会同时存在这2种液膜。实验采用了6种液体（Ka数52~3000）和10余种不同尺寸的矩形窗（9~1152mm2），观察了垂直板上下降液膜通过开窗区产生的自由膜流动行为。结果表明，在表面张力、惯性力、重力及黏性力的共同作用下，液体通过开窗区的方式有绕流、过流、偏流和"背流"4种主要形式。随着流量的增加，过流又可以有液滴、液柱、液膜以及它们的组合等丰富流型，同时窗内自由膜在表面张力主导下易受扰动而聚并、破碎，流型呈现多样性和动态性。当流量达到临界值时，窗口内会形成完整稳定的受限自由液膜，呈现板上壁面膜与窗口内受限自由膜交替共存的特殊液膜——孪生液膜。本文关联了临界Re数、Ka数和无量纲几何参数Nx，给出了成膜条件的经验判别式。孪生液膜具有的特殊波列结构、成膜过程的迟滞现象以及2种液膜和固体壁间相互作用引起的多重干扰等，都体现出与传统的壁面液膜和自由液膜不同的流动性质。实验结果将有助于工业过程中开窗和开孔的几何优化，强化传递过程，并进一步丰富传统的液膜研究领域。Abstract: Liquid film is a common contact way between gas/liquid and can be divided into wall-bounded film and free film according to the film formation process and the number of free surfaces. When a plate is perforated, both kinds of films exist. This paper describes some experimental observations of free-surface flows arising when a thin liquid film flows through the window on a vertical perforated plate. Dozens of rectangular windows (9~1152mm2) and six fluids (Ka from 52 to 3000) were used to investigate the flow mechanisms. Several typical flow patterns including pass-around flow, pass-through flow, bias flow and back-side flow are well defined under such a flow condition. Here a special focus is given to the window region, where various free-surface flow patterns composed of droplets, columns, sheets and their combinations were observed with increasing flow rate. Meantime, free film in the window is surface tension dominated and susceptible to disturbance, resulting in coalescence or break of liquid columns and films. At a critical flow rate, liquid film is able to full fill the window, forming a stable complete confined free film. Mutual influence between confined free film in the window region and wall-bounded film around shows special wavy trains, which is also called "twin liquid film". Based on experimental data and scaling analysis, an empirical equation which relates Reynolds number Re, Kapitza number Ka and a dimensionless length Nx is proposed to characterize the film formation conditions. It is found that critical film formation Reynolds number increases with Kapitz number and window size. And hysteresis phenomenon is manifested by obviously different flow transition Re for confined free film formation and breaking. The results can help window geometry optimization industrial processes to improve local heat and mass transfer. It can also enrich the traditional film flow investigations.
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Key words:
- twin-liquid film /
- flow transition /
- film formation /
- perforated plate
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图 4 壁面(a)和窗口区(b)移动接触线示意图
Figure 4. Sketch of moving contact line on the wall (a) and up the window (b)
图 5 不同流量下窗口区的部分流型图(介质为水)
Figure 5. Flow patterns in the window (L=48mm, W=12mm) with different Re (water)
图 6 不同矩形窗和物性下的成膜Re数
Figure 6. Critical film formation Re with different windows and fluids
表 1 实验流体的物理性质
Table 1. Physical properties of the liquids used
Ka 密度
ρ/(kg·m-3)黏度
μ/(N·s·m-2)表面张力
σ/(N·m-1)68%甘油水溶液 52.7 1160 0.0175 0.058 63%甘油水溶液 127 1150 0.0090 0.057 55%甘油水溶液 295 1110 0.0046 0.055 63%酒精溶液 406 880 0.0014 0.037 无水乙醇 670 790 0.0027 0.030 去离子水 2.93×103 1000 0.00098 0.072 表中： 
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