Abstract The role of surface tension in the mechanism of bubble growth and detachment for a co-flowing air-water two-phase flow in a micro-tube is addressed. A numerical investigation for a horizontal axisymmetric flow with the assumption of zero gravity and an upward flow accounting for gravity contribution is carried out. The continuous liquid phase is flowing in a tube of 500 μm inner diameter and the gas phase is axially injected through a nozzle of 110 μm inner diameter and 210 μm outer diameter. A single-fluid model is used to determine the flow field, solving the continuity and momentum equations associated with the volume of fluid method for interface tracking. An open source software, OpenFOAM, is utilized for solving numerically this problem. The prediction results show that the surface tension plays a double role. First, it keeps the bubble attached to the injection nozzle during bubble growth and neck formation. Then, it destabilizes the interface by pinching off the neck in the immediate vicinity of the nozzle at about a distance of 0.5 the nozzle diameter rather than right at the nozzle exit. In-depth analysis of the mechanism of bubble formation induced by the effect of surface tension is carried out. It is highlighted that this latter acts as an attachment force at the injection nozzle during the bubble growth and it acts over the entire interface of the bubble yielding the formation of a neck. Later, the capillary effects reduce the diameter of the neck until it breaks and yields the detachment of the bubble. Further investigation at the nozzle wall allows depicting the motion of the contact line during the process of bubble growth and its significant effect on the bubble formation.

中文翻译：

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表面张力在微管中气泡生长和脱离中的作用

摘要 讨论了表面张力在微管中并流气水两相流气泡生长和分离机制中的作用。对假设重力为零的水平轴对称流动和考虑重力贡献的向上流动进行了数值研究。连续液相在500μm内径的管中流动，气相通过内径110μm和外径210μm的喷嘴轴向注入。单流体模型用于确定流场，求解与用于界面跟踪的流体体积方法相关的连续性和动量方程。一个开源软件 OpenFOAM 被用于数值解决这个问题。预测结果表明，表面张力起双重作用。第一的，它使气泡在气泡增长和颈部形成期间附着在注射喷嘴上。然后，它通过在喷嘴直径约 0.5 倍的距离处而不是在喷嘴出口处夹断喷嘴附近的颈部来破坏界面的稳定性。对表面张力作用诱导气泡形成的机理进行了深入分析。值得强调的是，后者在气泡生长过程中充当喷嘴处的附着力，它作用于气泡的整个界面，从而形成颈部。后来，毛细管效应减小了颈部的直径，直到它破裂并产生气泡的分离。