Slug wake

The main limitations of the approach adopted should be emphasized so that equation 4 is not misused. The theory developed is applicable only if the frequency of slugging is constant along the column, if the Reynolds number for the liquid plugs is below the critical value of 2,100, and if the slug wakes may be treated as... 

When the slugs are not well spaced, the slug following in the wake of another slug rises more quickly and after some distance the two slugs will coalesce. Moissis and Griffith (1962) have given a correlation to account for this ... 

A quantity Pr is defined as the pressure recovery in the wake due to the change in momentum of liquid between sections 2 and 3. This pressure recovery can be calculated from a momentum balance if the liquid velocity at section 2 is known. Applying Bernoulli s equation again at sections 1 and 3, remembering that the pressure around the slug must be constant, we have... 

Bubble induced secondary flow Moving bubbles generate secondary flows and wakes which promote local mixing near the membrane surface. Slug flow also results in an annular falling film as displaced liquid flows downward between the slug and the tube wall. 

For natural, dry agglomeration of very fine particles as described above, destructive forces, which are, for example, caused by bubbling or slugging, must be avoided. Therefore, the gas distribution plate is a finely pored diaphragm such as a sintered glass frit or a cloth which is supported by screens (see Fig. 7.85). Rewet agglomeration in fluidized beds, on the other hand, often requires the turbulent movement and wakes caused by the rising gas bubbles [B.42],... 

Slug flow Most of the gas is located in large buUet-shaped bubbles that have diameters almost equal to the tube diameter and are sometimes designated as Taylor bubbles. They move uniformly upward and are separated by liquid slugs that may contain small gas bubbles. Around the Taylor bubbles, there is a thin liquid faUing film that causes turbulence in the wake of the Taylor bubble. 

In this chapter, the work published by the authors is reviewed. The importance of the wakes of slugs as the primary agent in promoting mixing is considered first. Next, the problem of Taylor dispersion for laminar liquid flow along a tube is briefly outlined, and, finally, the analysis of dispersion in co-current flow of gas slugs and liquid in vertical tubes is presented. 

To better understand the whole process it is necessary to have detailed information about the pattern of flow in the wake of slugs this is reviewed in the following. 

In order to better characterize the laminar wakes, Campos and Guedes de Carvalho [4] performed a visual study inspired on the work of Maxworthy [II], The liquid in the lower half of the column, where the slug was injected, was made dark by means of a soluble dye. As the slug rose into the clear liquid, in the upper half of the column, the dark wake could be seen clearly in photographs like those shown in Figure 2, obtained with a still camera. 

Vertical and horizontal line segments marked on the column test section made it possible to scale down the prints and their analysis gave values of wake length 1, wake volume v, slug length L and slug volume V. The data obtained are shown in Figures 3 and 4. 

Some measure of the exchange of liquid between wake and surrounding fluid also was obtained, allowing the slugs to rise for 1 m in clear liquid and measuring the... 

Figure 3. Dependence of wake volume on slug length for 19 mm i.d. column.

A significant aspect of the model presented by Campos and Guedes de Carvalho is that it leads to the determination of a single parameter, the size of the fully mixed wake, which quantifies the extent of mixing induced by the rise of an individual slug [5]. If a column of liquid has an initial distribution of tracer in the form of a... 

Campos and Guedes de Carvalho also performed experiments on mixing of liquid with a pulsated gas feed [5]. They observed that an increased frequency of slug injection leads to decreased mixing of liquid, and they related this behavior to the detachment of the wakes of slugs in the pulsating liquid. 

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