Bubble properties

To provide the pr equisite knowledge for designing the three-phase fluidized-bed reactors with new modes, the hydrodynamics such as phase holdup, mixing and bubble properties and heat and mass transfer characteristics in the reactors have to be determined. Thus, in this study, the hydrodynamics and heat and mass transfer characteristics in the inverse and circulating three-phase fluidized-bed reactors for wastewater treatment in the present and previous studies have been summarized. Correlations for the hydrod3aiamics as well as mass and heat transfer coefficients are proposed. The areas wherein future research should be undertaken to improve... 

Gas bubbles in liquid metals and in fluidized beds have been the subject of special studies because of their practical importance and because of the experimental difficulties associated with studying bubble properties in opaque media. Much of the work has been carried out in so-called two-dimensional columns, where a sheet of liquid or fluidized particles, typically 1 cm thick, is confined between two parallel transparent walls. Bubbles span the gap between the front and rear faces and can be observed with backlighting. 

Liger-Belair, G. (2005). The physics and chemistry behind the bubbling properties of champagne and sparkling wines A state-of-the-art review. /. Agric. Food Chem. 53, 2788-2802. 

Bubble properties Maximum bubble No hmit to size, No bubbles, No limit to size,... 

TABLE 4 - AIR BUBBLE PROPERTIES AS NaCI CONCENTRATION VARIES... 

Following the principles of the Petrie model, and recalling that the film thickness <5 is much smaller than the radius S/R thin-film approximation, which implies that field equations are averaged over the thickness and that there are no shear stresses and moments in the film. The film is regarded, in fact, as a thin shell in tension, which is supported by the longitudinal force Fz in the bubble and by the pressure difference between the inner and outer surfaces, AP. We further assume steady state, a clearly defined sharp freeze line above which no more deformation takes place and an axisymmetric bubble. Bubble properties can therefore be expressed in terms of a single independent spatial variable, the (upward) axial position from the die exit,2 z. The object... 

The foam films at the upper bubble layer are in contact with the atmosphere and, therefore, the forces that act on them are less balanced compared to those acting on the bubble in the foam bulk. That is why the bubble properties (respectively, the foam film properties) at the upper layer of the foam column are the most important since they provide the stability of the whole foam under steady-state conditions. 

The electroresistivity probe, recently proposed by Burgess and Calder-bank (B32, B33) for the measurement of bubble properties in bubble dispersions, is a very promising apparatus. A three-dimensional resistivity probe with five channels was designed in order to sense the bubble approach angle, as well as to measure bubble size and velocity in sieve tray froths. This probe system accepts only bubbles whose location and direction coincide with the vertical probe axis, the discrimination function being achieved with the aid of an on-line computer which receives signals from five channels communicating with the probe array. Gas holdup, gas-flow specific interfacial area, and even gas and liquid-side mass-transfer efficiencies have been calculated directly from the local measured distributions of bubble size and velocity. The derived values of the disper-... 

Drops and bubbles are indeed the same mathematical object. However, in marine water studies, the profile analysis of captive (or emerging) bubbles is preferable in respect to the analysis of drops. Actually, from the physical point of view, bubbles exhibit some differences in respect to drops a) diffusion to the air-water interface from a semi-infinite medium (rather than from the small volume confined by the drop) b) limited evaporation c) possibility of observing bubble properties both in quiescent hydrodynamic conditions or in laminar flow regime. Moreover, a captive bubble can be expanded to very large dimensions. 

INFL UENCE OF PARTICLE AND BUBBLE PROPERTIES ON COLLISION EFFICIENCY... 

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. 

For the discussion of the effect of gas permeation on bed hydrodynamics, we first focus on the solids circulation patterns, followed by the bubble properties. 

The findings described above can be schematically summarized as depicted in Fig. 4.21. In all cases, the magnitude of the effects depends on the background fluidization velocity and amount of gas extraction or gas addition. It can be expected that the change in particle behavior has a pronounced effect on the bubble properties and bubble size distribution, which is discussed next. 

The most striking aspect is the fact that the average equivalent bubble diameter in a system with internals is much smaller compared to the system without internals, despite the fact that the other bubble properties (bubble aspect ratio and number of bubbles) are relatively similar. Compared to this difference in bubble size, the influence of the permeation of gas through the membrane tubes is of minor importance. The increased bubble breakup due to the presence of the membrane tubes can clearly be distinguished from the local decrease in bubble size. 

The mixing in gas-solid fluidized beds is primarily due to two processes created by the passage of bubbles, bubble wake and bubble drift, as diseussed earlier. The convective solids transport and mixing can be estimated based on the bubble properties, as suggested by Geldart (1986). The solid cireulation flux in a fluidized bed can be ealeulated from the equation... 

The bubble properties have also been measured by Werther (1976b) in large-diameter fluidized beds, 1 m in diameter and an approach for calculating the... 

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