Flow separation in falling liquid films
Flow separation in falling liquid films
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下降液膜中的流动分离
DOI：
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发表时间：
2011
期刊：
影响因子：
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通讯作者：
R. Kneer
中科院分区：
文献类型：
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作者：
G. Dietze;R. Kneer
尽管液体膜在涉及传热和传质的各种技术应用（例如核反应堆、冷却塔和燃气轮机）中广泛使用，它们经常发挥重要作用，但这些薄液体层内潜在的动量和热传递过程仍有待充分阐明。特别地，这适用于由于薄膜的自然不稳定性而形成的表面波对上述过程施加的影响。在这种情况下，一些实验和数值观测表明，毛细波区域（在大表面波之前）的动量和传热经历了剧烈的变化。事实上，一些结果表明，在这个区域出现了向上流动（即与重力加速度相反）。此外，还注意到壁面和界面传递系数大幅增加的证据。最近，作者建立了在二维和三维层流下降液膜的毛细波区发生流动分离，部分解释了上述观察结果。他们表明，该区域液气界面曲率的强烈变化导致不利的压力梯度（由于表面张力的作用），足以诱导流动从壁面分离。本文对这一现象在两种不同的二维流动条件下的运动学和控制动力学及其对传热的影响进行了深入的实验和数值研究。实验中，速度测量（使用激光多普勒测速仪和粒子图像测速仪）和薄膜厚度测量（使用共聚焦色差成像技术）在专门设计的光学测试装置中进行。在数值方面，采用流体体积法（VOF）对全Navier-Stokes方程和能量方程进行了模拟。除了这些研究之外，还进行了三维垂直下降水膜的数值模拟，以研究先前文献中研究过的流动条件。在此基础上，研究了三维表面波存在下毛细管流动分离的特性。结果表明，流动分离发生在复杂的三维毛细波区域的多个区域，形成了旋涡管形状的多个分离区。此外，该地区还存在由相同控制机制引起的展向流动和相关涡。这可以解释向三维液体膜转移的强烈强化。
Despite the use of liquid films in a wide variety of technical applications involving heat and mass transfer (e.g. nuclear reactors, cooling towers and gas turbines), where they often play an important role, the underlying momentum and heat transport processes within these thin liquid layers remain to be fully elucidated. In particular, this applies to the influence that surface waves, developing due to the film’s natural instability, exert on the mentioned processes. In this context, it has been suggested by several experimental and numerical observations that momentum and heat transfer in the capillary wave region (which precedes large surface waves) undergo drastic variations. Indeed, some results have indicated the occurrence of upward flow (i.e. opposed to the gravitational acceleration) in this region. Moreover, evidence of a large increase in wall-side and interfacial transfer coefficients has also been noted. Recently, the authors have established that flow separation takes place in the capillary wave region of 2and 3-dimensional laminar falling liquid films, partially explaining the above mentioned observations. They showed that the strong change in curvature of the liquid-gas interface in this region causes an adverse pressure gradient (due to the action of surface tension forces) sufficiently large to induce flow detachment from the wall. In the present paper, an in-depth experimental and numerical investigation of this phenomenon in terms of its kinematics and governing dynamics as well as its effect on heat transfer for two different 2-dimensional flow conditions is presented. Experimentally, velocity measurements (using Laser Doppler Velocimetry and Particle Image Velocimetry) and film thickness measurements (using a Confocal Chromatic Imaging technique) were performed in a specifically designed optical test setup. On the numerical side, simulations of the full Navier-Stokes equations as well as the energy equation using the Volume of Fluid (VOF) method were performed. In addition to these investigations, the numerical simulation of a 3-dimensional vertically falling water film, for flow conditions studied in a previous experimental contribution to the literature, was performed. Based on these data, the characteristics of capillary flow separation in the presence of 3-dimensional surface waves were studied. Results show that flow separation takes place in several areas of the resulting complex 3-dimensional capillary wave region, developing multiple separation zones in the shape of vortex tubes. In addition, spanwise flow and an associated eddy induced by the same governing mechanism are shown to occur in this region. This could explain the strong intensification of transfer to 3-dimensional liquid films.