Wakes of slugs

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. 

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. 

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 ... 

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. 

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... 

A Cross-sectional area C Average tracer concentration in the liquid leaving the column C Initial concentration of tracer D Internal diameter of the tube D Axial dispersion coefficient f Frequency of slug formation g Gravitional acceleration H Height of liquid in the column in the absence of gas bubbles Height of the column i Wake length... 

Length of column equivalent to fully mixed wake L Slug length... 

The New Model, Mechanistic Cyclic Form In this form we follow each slug, the wake of the slug, and the piston-flow region from formation at the bottom of the fluidized bed to the top of the bed where the slug bursts. Thus the steady-state is approached from an initial state in which the bed is fluidized with non-reacting gas and then reacting gas is admitted at zero time. Only the final steady-state solutions are presented here for comparison with the time-averaged version, the two-phase model of Hovmand and Davidson and the experimental data. 

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... 

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],... 

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