Coalescence of bubbles

In the riser, baffles are placed at intervals to break up bubbles by increasing turbulence and shear. At the top erf the riser the expanded section decreases the upward flow rate of the medium and this, together with the lack of baffles, decreases turbulence and shear, which in turn promotes coalescence of bubbles. Larger bubbles form which have increased slip velocity, so they more easily disengage from the medium. 

It should be pointed out that Eq. (10) can correctly represent experimental results (cited papers recorded reasonable agreement between theory and experiment) only when diffusion of gas from bubble to bubble can be ignored. When the bubble is small, surface Laplace pressure PL P, and coalescence of bubbles occurs in such a way as to make the volume of the resul tant bubble greater than the sum of the original bubble volumes [27]. 

R. J. Mannheimer. Factors that influence the coalescence of bubbles in oils that contain silicone antifoamants. Chem Eng Commun, 113 183-196, March 1992. 

There are two mechanisms in growth of a bubble in acoustic cavitation [14], One is coalescence of bubbles. The other is the gas diffusion into a bubble due to the area and shell effects described before. This is called rectified diffusion. 

The coalescence of bubbles is driven by the two mechanisms. One is the attractive radiation force between bubbles called secondary Bjerknes force. The other is the other radiation force called the primary Bjerknes force which drives active bubbles to the pressure antinode of a standing wave field, ft should be noted, however, too strong acoustic wave repels bubbles from the pressure antinode as described in the next section [29, 30]. 

Relative importance of coalescence and rectified diffusion in the bubble growth is still under debate. After acoustic cavitation is fully started, coalescence of bubbles may be the main mechanism of the bubble growth [16, 34], On the other hand, at the initial development of acoustic cavitation, rectified diffusion may be the main mechanism as the rate of coalescence is proportional to the square of the number density of bubbles which should be small at the initial stage of acoustic cavitation. Further studies are required on this subject. 

Marcelja S (2006) Selective coalescence of bubbles in simple electrolytes. J Phys Chem B 110 13062-13067... 

Harrison, D. and Leung, L. S. Third Congress of the European Federation of Chemical Engineering (1962). The Interaction between Fluids and Particles 127. The coalescence of bubbles in fluidised beds. 

Figure 1.6 Coalescence of bubbles. Reprinted from Davidson, J.F. and Harrison, D., Fluidization, Academic Press, 1971, with permission from Elsevier.

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. 

Although the above derivation is for crystals, the theory is also applicable to bubble size distribution. In addition to the above four assumptions, the other conditions for its application include (v) no Ostwald ripening, which would modify CSD, and (vi) no coalescence of bubbles. 

The works of Fukuma et al. [23], Morooka et al. [24] and Clark [25] has indicated that the presence of solid particles in the reactor favours the transition to the heterogeneous flow regime. A critical solid hold-up exists, beyond which the coalescence of bubbles is more frequent. This critical value is higher with smaller particles. Clark [25], studied operation at pressure above 100 bar, and inferred that... 

While flowing through the internal, bubbles rise along the undersurface of the baffles and collide with the tongue-like bars, and are broken up into smaller bubbles as shown in Fig. 10. The difference in the direction of adjacent baffles increases the liquid turbulent intensity, which is beneficial to the breakup of large bubbles. With increasing distance from the internal, the turbulent intensity decreases and coalescence becomes dominant until a new equilibrium between the breakup and coalescence of bubbles is reached. 

The micromixing state in an agitated vessel is connected with coalescence of bubbles. Coalescence occurs mainly in the stirrer zone, where the recirculated gas bubbles partially mix with fresh gas in the cavities. It also occurs to a lesser extent in the highly turbulent stream leaving the stirrer, but it is virtually absent in other parts of the vessel because the low kinetic energy of the bubbles cannot stretch out the liquid film between a pair of bubbles to reach the coalescence thickness. 

Circumstantial evidence was observed in the post-bombardment surface morphology of gallium samples that were exposed to deuterium plasma and then rapidly cooled [20]. Small voids were observed in the sample surface that may have resulted from the coalescence of bubbles during the sample cool down period. Consistent with this view was the fact that the amount of deuterium retained in the samples was independent of the sample exposure conditions. Follow-up experiments are being performed to determine whether gallium samples exposed to deuterium plasma at elevated temperature (around... 

Thus, on the one hand, the foam film type (and therefore the type of stabilisation due to long-range and short-range forces) is the determining factor for the course of tp(Ap0) dependences. On the other hand, in some systems an avalanche-like destruction occurs at A/ cr. A reasonable question arises as to why a foam built up of different types of foam films destructs before reaching Aplr. If the foam is considered as a system built up of equilibrium foam films, then they should be infinitely stable. However, the foam is a more complex system, built up of foam films and borders, subjected to the effect of several other factors (gas diffusion transfer, coalescence of bubbles and changes in the foam film size, external actions, local stretching, collective effects of destruction, etc.) which can lead to its destruction. 

In such concentrated disperse systems three types of liquid films form foam films (G/L/G), water-emulsion films (O/W/O) and non-symmetric films (O/W/G). The kinetics of thinning of these films, their permeability as well as the energy barrier impeding the film rupture determine the stability of these systems. They might be subjected to internal collapse, i.e. coalescence of bubbles or droplets and increase in their average size, or to destruction as a whole, i.e. separation into their initial phases - gas, oil and water. 

Imafuku et al.46 measured the gas holdup in a batch (i.e., no liquid flow) three-phase fluidized-bed column. They found that the presence of solids caused significant coalescence of bubbles. They correlated the gas holdup with the slip velocity between the gas and liquid. They found that the gas holdup does not depend upon the type of gas distributor or the shape of the bottom of the column when solid particles are completely suspended. Kato et al.53 found that the gas holdup in an air-water-glass sphere system was somewhat less than that of the air-water system and that the larger solid particles showed a somewhat smaller... 

There are three main mechanisms which determine the bubble sizes coalescence of bubbles, splitting of a bubble under a given disturbance, and the occurrence of disturbances in the bubble-flow equipment. The latter two will be discussed in what follows. (Coales cence is discussed in C7, MIO, T16, and WIO). 

In addition, as distinct from the approach of Prince and Blanch [92], Kolev [46] (p 174) argued that the frequency of coalescence of bubbles should rather be determined having individual efficiencies ... 

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