Bubble slug flow, coalescent

Bubbly/slug flow. Bubbles both longer and shorter than the channel diameter (Fig. 2.30b). The bubble frequencies increase rapidly with the heat flux in the evaporator, reached a peak 900 Hz and then decreased due to coalescence. 

Since sparger hole diameter in some sense relates to bubble diameter, this correlation suggests mixing increases with increased bubble diameter. These workers also suggest that dispersion coefficient for the coalescent bubble-slug flow conditions follows ... 

The first flow pattern zone corresponds to the isolated bubble (IB) regime where the bubble generation rate is much higher than the coalescence rate. It includes both bubbly flow and/or slug flows and is present up to the onset of coalescence process domination. The second zone is the coalescing bubble (CB) regime, which is... 

Wallis points out that, from continuity considerations and bubble dynamics, the cocurrent flow of uniformly dispersed bubbles as a discontinuous phase in a liquid can always be made to occur in any system and for any void volume. (This is not true for countercurrent flow.) Coalescence of bubbles may occur, of course, and if this coalescence is sufiiciently rapid, a developing type of flow is observed, usually from bubble to slug flow. Because of this behavior, the particular flow pattern observed in bubble flow is quite dependent on the previous history of the two-phase mixture. This would be true for both horizontal and vertical flow. 

Bubble flow-slug flow transition. Transition from dispersed bubbles to slugs requires a process of coalescence. As the gas flow rate is increased, the bubble density increases. This closer bubble spacing results in an increase in agglomeration. However, increased liquid flow rate can cause a breakup of larger bubbles, and this might be sufficient to prevent the transition. The maximum bubble void fraction at which the transition happens is around 0.25 (see Refs. 5 and 3). 

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. 

Slug flow Where the bubbles coalesce to form slugs of vapor, which may occupy the cross section of the tube. Slug flow is similar to the flow pattern in a percolating coffee pot and is the predominant flow pattern in vertical thermosiphon reboilers. 

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