Mechanisms of bubble formation

Tray efficiency is as high as for bubble caps and almost as high as sieve trays. It is higher than bubble caps in some systems. Performance indicates a close similarity to sieve trays, since the mechanism of bubble formation is almost identical. The real point of concern is that the efficiency falls off quickly as the flow rate of vapor through the holes is reduced close to the minimum values represented by the dump point, or point of plate initial activation. Efficiency increases as the tray spacing increases for a given throughput. 

The wall superheat that corresponds to bubble formation in liquid flow can be estimated using an approach that is not connected to the mechanism of bubble formation. Such tentative estimation makes it possible to consider only the low level of wall superheat. According to Kays and Krawford (1993) the temperature distribution in turbulent flow and Pr 1 is... 

Kurana and Kumar (K20) have written the general equations taking the variation of flow into consideration for the two step mechanism of bubble formation. The fluctuations of pressure and the corresponding variations in flow are treated by an electrical analog. 

In both the gassed (aerated) stirred tank and in the bubble column, the gas bubbles rise through a liquid, despite the mechanisms of bubble formation in the two types of apparatus being different. In this section, we shall consider some common aspects of the gas bubble - liquid systems in these two types of reactors. 

The mechanism of bubble formation by nucleation requires supersaturation of the dissolved gas [11-13] and a nucleus radius greater than the critical [7], The main sources of heterogeneous nucleation are usually surface irregularities capable of containing entrapped gas, e.g. pits and scratches. The bubbles typically develop over the electrode surface, grow in size until they reach a break-off diameter and subsequently detach into the electrolyte. After detachment, some residual gas remains at the nucleation site and another bubble will form at the same place [2,13,14], In most two-phase flow simulations [15-19], it is assumed that bubbles detach with a constant diameter, although from experiments [20,21] it is know that electrochemically formed bubbles show a size distribution. 

Pourdarvish, R., Danner, R. P., Duda, J. L. (2009). Mechanism of bubble formation in the drying of polymer films. Journal of Applied Polymer Science, 111(1), 417-428. 

Heat transfer by nucleate boiling is an important mechanism in the vaporization of liqmds. It occurs in the vaporization of liquids in kettle-type and natural-circulation reboilers commonly usea in the process industries. High rates of heat transfer per unit of area (heat flux) are obtained as a result of bubble formation at the liquid-solid interface rather than from mechanical devices external to the heat exchanger. There are available several expressions from which reasonable values of the film coefficients may be obtained. 

McDonough and Hemmingsen (ref. 419) confirm that for bubbles to develop in vertebrates from such low gas supersaturations, some mechanism or structure must promote the initial in vivo bubble nucleations. They cite, as one initial possibility, the popular, general hypothesis that animals contain a reservoir of microscopic gaseous nuclei in the body fluids or tissues, which expand into bubbles when the organism is decompressed (ref. 2). These authors point out that results consistent with this hypothesis have been obtained with shrimp (ref. 429) and rats (ref. 430), where the application of relatively high hydrostatic pressure before decompression apparently reduced the incidence of bubble formation, presumably by forcing potential gas nuclei into solution before they could serve as bubble precursors (ref. 419). 

The exchange processes taking place at the atmosphere-ocean border were experimentally studied by Kiseleva (1990) and Zaitsev (1988), among others. They showed that at high wind speeds the rate of gas exchange sharply increases. This is connected with the mechanism of foam formation on wave crests as well as the intense activity of air bubbles being trapped and held beneath the water surface. The dependence of the amount of spray Q on height over the water surface can be approximated well by a linear function. For instance, at a wind speed of V = 11.1 m/s this approximation is (Kiseleva, 1990) ... 

Dispersions at micron scale are usually made by merging gas and liquid streams in a mixing element and subsequent decay of the gas stream to a dispersion [251-262]. Mixing elements often have simple shapes such as a mixing tee (dual-feed gas-liquid) or triple-feed (liquid-gas-liquid) arrangements. The dispersion is passed either in a microchannel (or many of these) or in a larger environment such as a chamber, which, for example, provides volume to fill in porous materials such as catalyst particle beds, foams or artificial structures (microcolumn array). The mechanisms for bubble formation have not been investigated for all of the devices... 

Observation of the mechanisms of lamella formation in single capillaries, etched media, and bead packs, followed by development of pore-level theory for the formation, flow, and disappearance of lamellae ("bubbles") and experimental tests of the theories (36-41). 

McLaren, A. C., Cbok, R. F., Hyde, S. T., Tobin, R. C. (1983). The mechanisms of the formation and growth of water bubbles and associated dislocation loops in synthetic quartz. Phys. Chem. Minerals, 9, 79-94. 

Bubbles have long been thought to play an important role in air-sea gas exchange. The main mechanism behind bubble formation is the entrainment of air in breaking waves. There are at least three ways in which bubble formation may enhance air-sea gas transfer rates above those predicted by the gas transfer theories already... 

Newitt, M.D., Dombrowski, N. and Knelman, F.H. (1954), Liquid entrainment 1. mechanism of drop formation from gas or vapor bubbles, Trans. Inst. Chem. Eng., 32, 244. 

The mechanism of droplet formation from ultrasonic excitation of the liquid depends on the frequency and intensity applied [21]. At an intensity of 10 W/cm2, capillary surface waves are formed from cavitation at frequencies less than 100 kHz. Cavitation can occur at lower-power intensities when dissolved gases are present. Droplets are thrown outward by the rupture of the cavitation bubbles or the wave crests. For such conditions, the droplet diameter can be related to the capillary wavelength [21,22],... 

Circular volcano -shaped deposits can occur as a result of bubble formation in connection with boiling heat transfer and these deposits can act as nucleation points for further evaporation and deposition. As the process of deposition continues, the surface conditions will be modified and the scale may contain fissures, so that steam formation may occur within the deposit. The process is generally referred to as wick boiling that may influence the mechanism of heat transfer. The process of scale formation under boiling conditions is complex. 

Spence, A. Analysis of Bubble Formation and Removal in Rotationally Moulded Products. PhD thesis. Department of Mechanical and Manufacturing Engineering, Queen s University in Belfast, Belfast, Northern Ireland, 1994. 

It is pointed out in [252] that more than 10% of the world oil recovery is accomplished by injecting steam or CO2 for the production of oil by EOR. Here, the foam bubble size is comparable or slightly more than the pore diameter. To increase oil production, highly disperse steam foam is required, which pushes the oil out of the reservoir while moving both as a continuous and discontinuous flow. It has to be emphasized that the choice of surfactant is important to accomplish the process. Typically, the surfactant concentration is much higher than CMC, which predetermines the expensiveness of the process. The foam is destroyed when coming in contact with oil and rock, which requires its increased consumption. Three mechanisms of foam formation under reservoir conditions are proposed in [253], determined by the liquid flow rate and the throat radius between the pores. When the foam flows in a reservoir, its aggregative stability is disturbed due to coalescence. Hence, the choice of... 

Gas phase properties As stated before, all the model equations involve parameters that are determined by the behavior of bubbles, either alone or in groupings, and the analysis becomes more of an exercise in bubble fluid mechanics than in reactor design. For plug-flow gas phase reactors there are a number of correlations that relate in-reactor bubble properties as a function of the inlet conditions. These are available for the bubble volume Vb, the bubble rise velocity Vb, the surface to volume ratio a, and the number of bubbles per unit volume N. In addition, if bubbles are spherical (or approximately so), information on db allows determination of a and Vb- However, these correlations are subdivided by the gross characteristics of bubble formation, namely whether there is a gas phase consisting of discrete bubbles, or whether there is interaction among bubbles with some coalescence, commonly termed a swarm bubble phase. 

---