Periodic bubble emission and appearance of an ordered bubble sequence (train) during condensation in a single microchannel
Periodic bubble emission and appearance of an ordered bubble sequence (train) during condensation in a single microchannel
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DOI：
10.1016/j.ijheatmasstransfer.2007.11.025
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发表时间：
2008-07
影响因子：
5.2
通讯作者：
Wei Zhang;Jinliang Xu;J. Thome
中科院分区：
文献类型：
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作者：
Wei Zhang;Jinliang Xu;J. Thome
在低流量条件下，对单微通道硅片内蒸汽的冷凝现象进行了直观的研究。微通道的横截面为矩形，深度为30μm，宽度为800μm，长度为5.0mm，覆盖有Pyrex玻璃，以便可视化气泡形成过程。通过改变饱和水蒸气凝结过程中的冷却速率，就有可能控制气泡形成的形状、大小和频率。在低冷却速率下，仅使用来自周围环境的自然空气对流，微通道中的流动模式由一个几乎稳定的细长气泡组成，该气泡附着在上游（靠近入口），并在细长气泡的下游挤压成一列椭圆气泡。观察到这些椭圆气泡以高频周期性地从细长气泡的尖端发射，其尺寸小于通道宽度。发出的气泡的形状在产生后不久就发生了变化，直到最终成为一个稳定的垂直椭圆，当它以恒定的速度向下游流动时，它的形状和大小保持不变。这些周期性发射的椭圆气泡形成了有序的气泡序列（序列）。在较高的冷却速率下，气泡形成频率显著增加，而气泡尺寸减小，同时在微通道下游形成完美的气泡列。在这种情况下，发射的气泡在与上游的细长气泡分离后立即形成圆形，没有任何进一步的修饰。本研究表明，可以开发一种控制微气泡大小和产生频率的方法，这可能对微流体的应用有兴趣。细长泡破裂的原因是细长泡内部微通道中心线处蒸汽速度最大，而细长泡尖端处水的表面张力较小，从而导致细长泡尖端处韦伯数较大。
Condensation of steam in a single microchannel, silicon test section was investigated visually at low flow rates. The microchannel was rectangular in cross-section with a depth of 30μm, a width of 800μm and a length of 5.0mm, covered with a Pyrex glass to allow for visualization of the bubble formation process. By varying the cooling rate during condensation of the saturated water vapor, it was possible to control the shape, size and frequency of the bubbles formed. At low cooling rates using only natural air convection from the ambient environment, the flow pattern in the microchannel consisted of a nearly stable elongated bubble attached upstream (near the inlet) that pinched off into a train of elliptical bubbles downstream of the elongated bubble. It was observed that these elliptical bubbles were emitted periodically from the tip of the elongated bubble at a high frequency, with smaller size than the channel width. The shape of the emitted bubbles underwent modifications shortly after their generation until finally becoming a stable vertical ellipse, maintaining its shape and size as it flowed downstream at a constant speed. These periodically emitted elliptical bubbles thus formed an ordered bubble sequence (train). At higher cooling rates using chilled water in a copper heat sink attached to the test section, the bubble formation frequency increased significantly while the bubble size decreased, all the while forming a perfect bubble train flowing downstream of the microchannel. The emitted bubbles in this case immediately formed into a circular shape without any further modification after their separation from the elongated bubble upstream. The present study suggests that a method for controlling the size and generation frequency of microbubbles could be so developed, which may be of interest for microfluidic applications. The breakup of the elongated bubble is caused by the large Weber number at the tip of the elongated bubble induced by the maximum vapor velocity at the centerline of the microchannel inside the elongated bubble and the smaller surface tension force of water at the tip of the elongated bubble.