Taylor Bubble Flow

The liquid in the film alongside the Taylor bubble flows in the opposite direction, with negligible interfacial shear from the gas on the bubble. The average gradient due to friction and acceleration across a slug unit is... 

Bubble train flow Elongated bubble flow Intermittent flow Plug flow Segmented flow Slug flow Taylor bubble flow... 

Van Baten and Krishna [57] developed a model for rising Taylor bubble flow in circular capillaries by considering two contributions to mass transfer (1) the caps (assumed to be hemispherical) at either end of the bubble and (2) the liquid film surrounding the bubble. They put forward the following relationship for the overall volumetric mass transfer coefficient (see Chapter 11) ... 

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)... 

Kew P, Cornwell K. Confined bubble flow and boding in narrow spaces. Proceedings of the 10th International Heat Transfer Conference, paper 18-FB-12, New York Taylor and Francis, 1994 473 478. 

Slug flow-segmented flow transition. Taylor bubbles will be formed when the gas flow rate is increased to such an extent that it forces bubbles to become closely packed and to agglomerate into Taylor bubbles. 

Slug flow-chum flow transition. As the gas flow is increased even more, a transition to chum flow occurs. The subjective discrimination between slug flow and chum flow makes it difficult to identify the transition exactly. Taitel et al. [3] use the definition that is based on the behavior of the liquid film between the Taylor bubble and the wall. In this case the chum flow is characterized as the condition where oscillatory motion of the liquid is observed. 

In segmented flow in cylindrical capillaries, Taylor bubbles consist of a cylindrical part and two caps at the front and the rear menisci. The form of the caps may be axisymmetric or nonaxisymmetric. In a balance over a long inviscid bubble surrounded by a moving incompressible and viscous fluid, capillary, viscous, inertial, and gravity forces are taken into account. The latter three, relative to capillary force, are expressed in the following dimensionless numbers ... 

In segmented flow, the subsequent liquid slugs are separated from each other by the Taylor bubbles. Thus, each liquid slug is enclosed by the two ends of the adjacent Taylor bubbles. 

Figure 2 presents three typical flow patterns were observed at narrow annulus, which are the flow with small bubbles whose size is less than a channel width (see Fig. 2a), the flow with large Taylor bubbles (see Fig. 2b) and the flow with the cell structure of liquid plugs, (see Fig. 2c). The flow pattern map is presented on Fig. 3. The first type of the flow is observed at the superficial liquid velocities greater than 2 m/s when the flow becomes turbulent (point 1 and line A in Fig. 3). At such velocities the flow is turbulent and small bubbles...

Figure 3. Air-water flow pattern map in a narrow gap, 1 - small bubbles, 2-small and Taylor bubbles, 3- Taylor bubbles, 4-branched Taylor bubbles, 5-cell flow without ripple waves, 6- cell flow with ripple waves.

Data have been presented allowing identification of different flow patterns in a narrow annular channel with a gap less than capillary constant. For large superficial velocities the flow with Taylor bubbles and cell flow regime with liquid plugs are typical. 

Slug Flow most of the gas is located in large buUet-shaped bubbles, which have diameters almost equal to the tube diameter and are sometimes designated as Taylor bubbles. They move uniformly upward and are separated by liquid... 

For recirculation flow the Taylor dispersion mechanism was introduced by Shyu and Miyauchi (S13). Equation (4-12) is a revised result for it. For this flow regime, Ohki and Inoue (02) developed an expansion model with parameters adjusted to the data available, and also introduced the Taylor dispersion mechanism for the low-gas-velocity region of uniform bubble flow. 

Slug flow. Here the bubbles have coalesced to form large bubbles (sometimes called Taylor bubbles) that are separated by slugs of liquid, the latter often containing a dispersion of smaller bubbles. 

In order to overcome the coupling of power dissipation and mass transfer, we need to consider a different mechanism for gas-liquid contacting. If we turn to laminar flow, an external structure should be used to create or maintain the surface area. For example, in a falling-film reactor the gas /liquid interfacial area is roughly equal to the wall area. In capillaries at moderate velocities, the predominant flow pattern is called Taylor [29] flow, see Fig. 6.3. In Taylor flow, the gas bubbles are too large to retain their spherical shape and are stretched to fit inside the channel. Surface tension pushes the bubble towards the channel wall, and only a thin film remains between the bubble and the wall. 

In small channels, a number of flow patterns can be observed, and the same terminology and classifications as in large channels are commonly used. Because of the dominance of the surface tension forces, stratified flow is rarely observed in small channels. In general, bubble flow appears at low gas flow rates. As the gas flow rate increases, Taylor bubbles form. With further increase in the gas flow rate, annular flow appears with the liquid forming an annulus which wets the wall. At high gas and liquid flow rates, chum flow occurs where there is a liquid film at the wall and the gas flow in the center is interrupted by the firequent appearance of frothy bubbles and slugs. 

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