Bubbles behavior

The modeling of fluidized beds remains a difficult problem since the usual assumptions made for the heat and mass transfer processes in coal combustion in stagnant air are no longer vaUd. Furthermore, the prediction of bubble behavior, generation, growth, coalescence, stabiUty, and interaction with heat exchange tubes, as well as attrition and elutriation of particles, are not well understood and much more research needs to be done. Good reviews on various aspects of fluidized-bed combustion appear in References 121 and 122 (Table 2). 

The difference between the curves for pure water and seawater again illustrates the significance of small concentrations of solute with respecl to bubble behavior. In commercial bubble columns and agitated vessels coalescence and breakup are so rapid and violent that the rise velocity of a single bubble is meaningless. The average rise velocity can, however, be readily calculated from holdup correlations that will be given later. 

For fluidized beds, part of the gas flows through the emulsion at minimum fluidization velocity Uo, leaving U - Ug to influence bubble behavior. Then equation (4) is modified to read ... 

The second method to calculate of void fraction is based on gas velocity measurement. A high-speed motion analyzer was employed by Zhao and Bi (2001b) to visualize bubble behavior in the test sections. 

Visual observation in the studies by Hetsroni et al. (2002a,b, 2003), Qu and Mudawar (2002) proved bubble behavior at incipient boiling in micro-channels (d i < 1 mm) and concluded that it was quite different from that in larger channels. After nucleation, bubbles first grew to detachment size before departing into the liquid flow. The detachment size was comparable to that of the micro-channel... 

Because of the damagingly high temperature of the heater surface at DNB in a water flow, most studies of bubble behavior near the boiling crisis have been conducted on a Freon flow, where the surface temperature is much lower than in a water flow. The validity of the simulations of boiling crisis has been established in many studies, such as those of Stevens Kirby (1964), Cumo et al. (1969), Tong et al. (1970), Mayinger (1981), and Celata et al. (1985). 

The bubble behavior near the boiling crisis is three-dimensional. It is hard to show a three-dimensional view in side-view photography, because the camera is focused only on a lamination of the bubbly flow. Any bubbles behind this lamination will be fussy or even invisible on the photograph, but they can be seen by the naked eye and recorded in sketches as shown in Section 5.2.3. For further visual studies, the details inside bubble layers (such as the bubble layer in the vicinity of the CHF) would be required. Therefore, close-up photography normal and parallel to the heated surf ace is highly recommended. 

These different approaches are complementary to each other in basic concept. However, these analyses have not provided clear insight information of the bubble layer at the CHF about the bubble shape (spherical or flat elliptical), bubble population and its effect on turbulent mixing, and bubble behavior. The bubble behavior in a bubble layer could involve bubble rotation caused by flow shear, normal bubble velocity fluctuation, and bubble condensation in the bubble layer caused by the subcooled water coming from the core. Further visual study and measurements in this area may be desired. 

In the sonochemical reactors, selection of suitable operating parameters such as the intensity and the frequency of ultrasound and the vapor pressure of the cavitating media is an essential factor as the bubble behavior and hence the yields of sonochemical transformation are significantly altered due to these parameters. It is necessary that both the frequency and intensity of irradiation should not be increased beyond an optimum value, which is also a function of the type of the application and the equipment under consideration. The liquid phase physicochemical properties should be adjusted in such a way that generation of cavitation events is eased and also large number of smaller size cavities are formed in the system. 

Acoustic cavitation is as a result of the passage of ultrasound through the medium, while hydrodynamic cavitation occurs as the result of the velocity variation in the flow due to the changing geometry of the path of fluid flow. In spite of this difference in the mechanisms of generation of two types of cavitation, bubble behavior shows similar trends with the variation of parameters in both these types of cavitation. The two main aspects of bubble behavior in cavitation phenomena are ... 

Moholkar VS, Pandit AB (1997) Bubble behavior in hydrodynamic cavitation Effect of turbulence. AIChE J 43 1641-1648... 

In order to verify the simulation results, experiments on bubble behavior in bubble columns are carried out under conditions similar to the simulations. A 3-D rectangular bubble column with the dimension of 8 x 8 x 20 cm3 is used for the experiments. Four nozzles with 0.4 cm I.D. and a displacement of 2.4 cm are designed in the experiments. For single-nozzle experiments, air is injected into the liquid bed through one of the orifices while the others are shut off. The outlet air velocity from the nozzle is approximated using the measured bubbling... 

H. Yang, T. S. Zhao, and Q. Ye. In situ visualization study of CO2 gas bubble behavior in DMFC anode flow fields. Journal of Power Sources 139 (2005) 79-90. 

Sillen CWMP, Barendrecht E, Janssen LJJ, van Stralen SJD (1982) Gas bubble behavior during electrolysis. Int J Hydrogen Energy 7 577-587... 

Factors that are affected by the size of the bed, mainly those that are connected to the bubble behavior. The size of the bubbles has an impact on various physical properties such as the bed density, whereas it also influences the gas-solids contact and reactor performance (Matsen, 1996). For example, in small equipment, the bubbles may have dimensions approaching those of the bed, whereas it is not the case in large beds and a scale-up effect will surely exist. 

The local gas holdup and bubble behavior were measured by a reflective optic fiber probe developed by Wang and co-workers [21,22]. It can be known whether the probe is im-merging in the gas. The rate of the time that probe immerg-ing in the gas and the total sample time is gas holdup. Gas velocity can be got by the time difference that one bubble touch two probes and the distance between two probes. Chord length can be obtained from one bubble velocity and the time that the probe stays in the bubble. Bubble size distribution is got from the probability density of the chord length based on some numerical method. The local liquid velocity in the riser was measured by a backward scattering LDA system (system 9100-8, model TSI). Details have been given by Lin et al. [23]. 

Mass transfer is essential in EL-ALRs. Smaller bubbles and a uniform gas holdup radial distribution increase the interfacial area and improve mass transfer. Intensified turbulence increases the surface renewal frequency and decreases bubble size. A novel internal to improve mass transfer and the hydrodynamic behavior in a gas-liquid system is reported. Experiments were carried out to study the effect of the internal on the bubble behavior and liquid velocity in an EL-ALR. 

T.F. Wang, J.F. Wang, W.G. Yang, Y. Jin, Bubble behavior in gas-liquid-solid three-phase circulating fluidized beds, Chem. Eng. J. 84 (2001a) 397-404. 

The mechanism underlying air bubble behavior in the upper layer of the ocean has been poorly studied. The available theoretical results are based, as a rule, on a number of suppositions, which in many cases can drastically distort ideas about the real processes of gas exchange between the atmosphere and the ocean. Among these suppositions the following have been used most often ... 

Dense-phase fluidized beds with bubbles represent the majority of the operating interests although the beds may also be operated without bubbles. The bubbling dense-phase fluidized bed behavior is fluidlike. The analogy between the bubble behavior in gas-solid fluidized beds and that in gas-liquid bubble columns is often applied. Dense-phase fluidized beds generally possess the following characteristics, which promote their use in reactor applications ... 

In the experimental study of single-bubble behavior, the gas-solid fluidized bed to which a single bubble is introduced is usually maintained at the minimum fluidization condition. Characteristics of a single gas bubble in a gas-solid fluidized bed are similar to those in a liquid medium [Clift and Grace, 1985 Fan and Tsuchiya, 1990 Krishna, 1993]. 

Examining the variation of bubble behavior across Uc, a criterion for the bubbling-turbulent regime transition is proposed by Cai et al. (1992) as... 

To be able to scale-up a fluidized bed granulation process, the process has to be developed in a unit that is big enough to minimize issues like wall effect and slugging. The typical size should be at least 0.3 m in diameter. Two units should geometrically be similar. The state of fluidization should be kept the same. In fluidized bed operation, bubble behavior plays a very important role. Although bubbles tend to be bigger in larger units, fluidization air should be distributed in the unit in a similar way. 

Obviously, bed temperature should be kept the same during scale-up as well. Bed moisture content needs to be similar. Because of the difference in gas-solids flow pattern, including bubble behavior, the time for each granulation phase may vary. 

The bubbling behavior of a bed is also determined by the fluidizing velocity uQ. According to the two-phase theory of fluidization (29), the excess velocity over that required for minimum fluidization passes through the bed in bubbles, which provides a useful, although oversimplified (30, 31) view of bubble flow. 

Bubble Behavior in a Slurry Bubble Column Reactor Model... 

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