Detachment, of bubbles

The subject of diffusion-controlled bubble growth is, of course, a rather small part of the large subject of bubble dynamics, whose scope is too broad to be included in this review. Specifically excluded are cavitation bubbles, whose collapse is inertia rather than diffusion controlled, the formation and detachment of bubbles from orifices, oscillations of bubbles in a pressure field, and the challenging subject of the mechanism of nucleate boiling heat transfer, in which bubble formation and detachment must certainly play a dominant role. 

The last two chapters cover the topics of the production of chlorine and caustic and the phenomena of electrolytic gas evolution. In Chapter 5, Hine et al. describe the engineering aspects of the three processes used in the chlor-alkali industry, and in Chapter 6, Sides reviews the macroscopic phenomena of nucleation, growth, and detachment of bubbles, and the effect of bubbles on the conductivity of and mass transfer in electrolytes. 

The refractory used for metal refining processes and continuous casting molds is commonly poorly wetted by the molten metal [39], Water model experiments have shown that bubbles ascending near a wall of poor wettability sometimes attach to the wall [32] which differs markedly from bubbles ascending near a wall of good wettability. Although the interaction between a wall and bubbles has been widely studied, focus has been on wetted surfaces [40-44], In contrast, there is little information on the attachment and detachment of bubbles from a wall of poor wettability [32]. 

The evolution of gases, such as in dre example given above of dre formation of CO(g) in dre U airsfer of sulphur between carbon-saturated iron and a silicate slag, requires dre nucleation of bubbles before dre gas can be eliminated from the melt. The possibility of homogeneous nucleation seems unlikely, and the more probable source of gas bubbles would either be at the container ceramic walls, or on detached solid particles of the containing material which are... 

Thorncroft GE, Klausner JF, Mei R (1998) An experimental investigation of bubble growth and detachment in vertical upflow and downflow boiling. Int J Heat Mass Transfer 41 3857-3871 Thorncroft GE, Klausner JF, Mei R (2001) Bubble forces and detachment models Multiphase Sci Technol 13 35-76... 

The removal rate of particles or the rate of flotation from pulp is essentially governed by (i) collision between particles and bubbles (ii) adhesion of particles to bubbles and (iii) detachment of particles from bubbles. Keeping these factors in mind, one can arrive at the following relationship ... 

Note that the local boiling void calculated this way is independent of channel length. The prediction of the point of bubble departure (void detachment) is, however, important in predicting the subcooled boiling void. Rouhani (1967) assumed... 

The bubble layer is assumed to have constant void fraction along the length before DNB, with a balanced rate of bubble detachment and bubble condensation in the layer. Hence, the average properties p, p, and c of the bubble layer are assumed to be independent of position. 

Janssen and Hoogland (J3, J4a) made an extensive study of mass transfer during gas evolution at vertical and horizontal electrodes. Hydrogen, oxygen, and chlorine evolution were visually recorded and mass-transfer rates measured. The mass-transfer rate and its dependence on the current density, that is, the gas evolution rate, were found to depend strongly on the nature of the gas evolved and the pH of the electrolytic solution, and only slightly on the position of the electrode. It was concluded that the rate of flow of solution in a thin layer near the electrode, much smaller than the bubble diameter, determines the mass-transfer rate. This flow is affected in turn by the incidence and frequency of bubble formation and detachment. However, in this study the mass-transfer rates could not be correlated with the square root of the free-bubble diameter as in the surface renewal theory proposed by Ibl (18). 

Steep temperature gradients inside the catalyst layer will enhance the bubble formation and bring about efficient product desorption and effective regeneration of vacant active sites consequently. There irreversible processes are followed by another irreversible act of bubble detachment from the surface. 

The bubble is now assumed to detach when its center has covered a distance equal to the sum of the radius of the final bubble arid the radius of the orifice. If the radius of the orifice is R and if it is assumed that V0 = (4tt/ 03/3), then the time of bubble formation can be obtained by plotting on the same axes Eq. (9) and (r + K) as a function of time from Eq. (8). 

The above equation is applicable when the bubble detaches at s = rF. But if the orifice diameter is large, the authors recommend the use of the relation s = (rF + 2 ) to obtain a better value of the time of bubble formation. Then Eq. (39) is modified to... 

The bubbles appear to be responsible for a large amount of mixing of the solids. A rising bubble draws up a spout of particles behind it and carries a wake of particles equal to about one-third of the volume of the bubble and wake together. This wake detaches itself at intervals. The pattern in a bed containing a large number of bubbles is, of course, very much more complex. 

Previous work on the application of the acoustic emission technique to corrosion processes has been carried out by Mansfeld and Stocker (i) who were able to show that the detachment of hydrogen bubbles from a polarised metal surface gave measurable signals. 

Impaction of water drops on solid surfaces has been studied (G3), and under some circumstances smaller drops are detached and leave the surface. Impingement of drops on thin liquid films may also cause breakup (K3, S5). Breakup of bubbles in fluidized beds due to impingement on fixed horizontal cylinders has also been observed (G4). Sound waves may lead to instability of bubbles in liquids (S2I). 

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