Bubble formation from a downward‐pointing capillary nozzle was investigated in this study. The experiments were conducted at gas flow rate of 40–5,400 ml/h and inner nozzle radius of 0.030–0.255 mm. Experimental results show that microbubbles are formed continuously at moderate Weber number, which was not reported in pervious investigations with injecting gas through an upward‐pointing capillary nozzle. High‐speed visualization indicates that the formation of microbubbles arises from the convergence of the capillary waves induced by the partial coalescence of larger bubbles. A bubbling regime map is given to identify the critical conditions for the formation of microbubbles. In the present air‐water experiments, the generated microbubbles are 20–170 μm in diameter. From experimental data, a scaling law for microbubble size is proposed as a function of Weber and Bond numbers.

中文翻译：

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来自浸没式毛细管喷嘴的气泡部分聚结期间连续形成微气泡

在这项研究中研究了从向下指向的毛细管喷嘴形成的气泡。实验在40–5,400 ml / h的气体流量和0.030–0.255 mm的内部喷嘴半径下进行。实验结果表明，微气泡以适中的韦伯数连续形成，这在以前的研究中没有报告，即通过向上指向的毛细管喷嘴注入气体。高速可视化表明，微气泡的形成是由较大气泡的部分合并引起的毛细管波的收敛引起的。给出了起泡状态图以识别形成微泡的关键条件。在目前的空气水实验中，产生的微气泡直径为20–170μm。根据实验数据，