The hydrodynamics of microlayer formation beneath vapour bubbles
The hydrodynamics of microlayer formation beneath vapour bubbles
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DOI：
10.1016/j.ijheatmasstransfer.2016.07.026
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发表时间：
2016-11
影响因子：
5.2
通讯作者：
S. Hänsch;S. Walker
中科院分区：
文献类型：
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作者：
S. Hänsch;S. Walker
“微层”，即在加热壁处快速增长的蒸汽气泡下方留下的液体薄膜（小于10 μm），在这些气泡离开壁时，可能是这些气泡中蒸汽的一个大的，甚至是主要的来源。考虑到它们的细长（与气泡的1-10 mm直径相比）和它们的高纵横比（径向范围可能大于它们厚度的100倍），这种微层相对简单地结合在气泡生长的微观CFD分析中。然而，它们的作用是特别重要的，因为微层的蒸发迅速产生蒸汽，蒸汽本身使气泡膨胀并产生更多的微层以蒸发。显然，需要对微层形成过程有很好的理解。在本文中，我们提出了第一性原理计算的流体动力学的形成这样的微层。这些似乎压倒性地表明，微层的存在和径向范围的决定因素是气泡的生长速率，更高的生长速率导致更扁平和更少的球形气泡，允许更大的微层被困在它们下面。当它们形成时，微层厚度在一定程度上取决于流体表面张力和液体粘度。值得注意的是，需要扩展这些流体动力学研究，以包括微层蒸发损耗的机械自洽模型。
‘Microlayers’, the thin (less than 10 μm) films of liquid left behind beneath rapidly-growing steam bubbles at a heated wall, can be a large, and even the dominant, source of the vapour in such bubbles by the time they depart the wall. Given their slenderness (compared to the 1–10 mm diameter of the bubble), and their high aspect ratio, (with radial extents of perhaps order >100 times their thickness), such microlayers are incorporated relatively simplistically in microscopic CFD analyses of bubble growth. However, their role is particularly important because the evaporation of the microlayer generates vapour rapidly, which itself expands the bubble and generates even more microlayer to evaporate. Plainly, a good understanding of the microlayer formation process is desirable. In this paper we present first-principles calculations of the hydrodynamics of the formation of such microlayers. These seem to show overwhelmingly that the determinant of the existence and radial extent of a microlayer is the bubble growth rate, with higher growth rates leading to more flattened and less spherical bubbles, allowing larger microlayers being trapped beneath them. When theyareformed, microlayer thickness is then to a degree dependent on the fluid surface tension and liquid viscosity. The need for an extension of these hydrodynamic studies to include a mechanistic self-consistent model of the evaporative depletion of the microlayer is noted.