Slug flow geometry

Figure 3.34 Slug-flow geometry. (From Taitel and Barnea, 1990. Copyright 1990 by Academic Press, Orlando, FL. Reprinted with permission.)...

The basic mechanism for transition from bubble to slug flow appears to be the same as in vertical pipe flow. That is, as the gas flow rate is increased for a given liquid flow rate, the bubble density increases, many collisions occur and cell-type Taylor bubbles are formed, and the transition to slug flow takes place. As shown in the case of vertical pipe upflow, Taitel et al. (1980) assumed that this transition takes place when ac = 0.25. This criterion is also applicable here. However, because of the preferable geometry in the rod bundle, where the bubbles are observed to exist, instead of in the space between any two rods, this void fraction of 0.25 applies to the local preferable area only, a.L. The local voids, aL, can be related to the average void by (Venkateswararao et al., 1982)... 

In gas-liquid two-phase flow, several flow patterns exist such as bubbly, slug, plug, and annular flow depending on the pipe configuration, geometry, and flow conditions. Of these types of flows, slug flow is one of the most complex, owing to its intermittent and transient nature. Despite its complexity, industrial processes often require an online, accurate, and noninvasive estimation of such flow. If this is accomplished, industrial processes can be kept within acceptable quality limits and additionally, financial losses may be reduced. 

A relatively simple model to describe the gas-liquid mass transfer in circular channels with slug flow pattern was proposed by van Eaten and Krishna [47]. For their fundamental model the authors considered an idealized geometry of the Taylor bubbles as shown in Figure 7.12. The bubbles consist of two hemispherical caps and a cylindrical body. The Higbie penetration model was applied to describe the mass transfer process of a compound from the gas phase to the liquid (Equation 7.8). For a rising bubble, the liquid will flow along the bubble surface of the cap. The average distance... 

In microfluidic devices, multiphase flows are created when two (or more) immiscible fluids come into contact. Depending on the interaction between the mterfacial and viscous forces, the resulting multiphase flow can take different forms, such as suspended droplets, slugs (droplets occupying the whole channel) or stratified flow (parallel) [141, 142]. In addition to the forces exerted between the two liquids, the channel geometry and physical characteristics also play an important role in the process [143]. In this respect, the use of hydrophobic channels is suitable for the formation of water-m-oil emulsions, whereas hydrophilic channels favor the creation of oil-m-water emulsions [144]. 

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