Abstract When a gas phase is injected into a liquid phase, it gives rise to a rich, fascinating and mysterious fluid dynamic phenomenology. The lack of knowledge regarding this phenomenology, a shortcoming in the design and operation of multi-phase reactors, is related to the absence of an unique definition of the flow regimes. To date, different studies gave different definitions of the flow patterns and, subsequently they experimentally obtained some global and local flow properties, with no physical-based description of the flow patterns. Is there a theory able to determine a-priori the boundaries of different flow regimes (and, thus, the flow regime for a given set of boundary conditions, given the phases and the system design)? Answering this question requires changing the present point of view in defining and describing the flow regimes and it is the primary motivation of this paper. To achieve this goal, a new theory has been formulated, which changes the present way of approaching bubble columns and which is based on the following statement: the fluid dynamics in gas-liquid bubble columns is interpreted by means of a general relationship―built upon five flow regime transitions―between two global fluid dynamic parameters. The strategic path to formulate this theory is, first, to go back to the pioneering studies and formulate basic relationships on the basis of a general principle. Subsequently, in order to support and verify the theory a comprehensive and multi-scale experimental investigation has been performed and coupled with previous experimental studies. The different experimental studies have been conducted in a gas-liquid large-scale bubble column (height of 5.3 m; inner diameter of 0.24 m) operated in the batch and in counter-current modes; to study all flow regimes, the bubble column was tested with five gas spargers (viz., pipe sparger in open tube and annular gap configuration, spider sparger, two different perforated plates and needle spargers) with different values of the aspect ratios and different liquid phases. It was found that, in an air-water bubble column, the gas velocity is approximately 0.03 m/s either in the case of the destabilization of the mono-dispersed homogeneous flow regimes or in the case of the destabilization of the pseudo-homogeneous flow regimes. Increasing the gas sparger opening induces a narrowing of the boundaries between the transitional flow regimes; conversely, increasing the bubble column aspect ratio destabilizes the existing flow regimes up to some critical values. Also, increasing the superficial liquid velocity in the counter-current mode destabilizes the homogeneous flow regime. Finally, it has been observed that the prevailing effect of the liquid phase, in a bubble column operated with a “coarse” gas sparger, is to change the boundary of the homogenous flow regime. It has also been discussed how the change in coordinates of the flow regime transition between the homogeneous and the heterogeneous flow regimes is caused by the changes in the size distribution. This study is intended to outline a precise definition of the flow regimes in bubble columns and it poses a rational basis for future studies.

中文翻译：

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气泡柱流体动力学：流动状态和综合实验研究的新视角

摘要 当气相注入液相时，就会产生丰富、迷人、神秘的流体动力学现象学。缺乏关于这种现象学的知识，这是多相反应器设计和操作中的一个缺点，与缺乏对流态的独特定义有关。迄今为止，不同的研究给出了流动模式的不同定义，随后他们通过实验获得了一些全局和局部流动特性，而没有基于物理的流动模式描述。是否有一种理论能够先验地确定不同流态的边界（因此，对于给定的一组边界条件，流态，给定阶段和系统设计）？回答这个问题需要改变当前定义和描述流态的观点，这是本文的主要动机。为了实现这一目标，已经制定了一个新的理论，它改变了目前接近泡罩塔的方式，它基于以下陈述：气液泡罩塔中的流体动力学是通过一般关系来解释的——建立在五个流态转换——在两个全局流体动力学参数之间。形成这一理论的战略路径是，一是回到开创性的研究，在一个总的原则的基础上制定基本的关系。随后，为了支持和验证该理论，已经进行了全面和多尺度的实验研究，并结合以前的实验研究。不同的实验研究已经在气液大型气泡塔（高度5.3 m；内径0.24 m）中以间歇和逆流模式运行；为了研究所有流态，泡罩塔用五个气体分布器（即开口管和环形间隙配置的管道分布器、蜘蛛形分布器、两种不同的多孔板和针形分布器）进行了测试，这些气体分布器具有不同的纵横比值和不同的液体阶段。发现，在空气-水泡柱中，气体速度约为 0。03 m/s 无论是在单分散均匀流态不稳定的情况下，还是在伪均匀流态不稳定的情况下。增加气体分布器开口导致过渡流态之间的边界变窄；相反，增加气泡塔纵横比会使现有流态不稳定，达到某些临界值。此外，在逆流模式下增加表观液体速度会破坏均匀流动状态。最后，已经观察到，在使用“粗”气体分布器操作的泡罩塔中，液相的主要作用是改变均匀流动状态的边界。还讨论了均匀流态和异质流态之间流态过渡坐标的变化是如何由尺寸分布的变化引起的。本研究旨在概述气泡塔中流态的精确定义，并为未来的研究奠定合理的基础。