Mechanisms of bubble

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... 

Zun, I., 1990, The Mechanism of Bubble Non-homogenous Distribution in 2-Phase Shear Flow, Nuclear Eng. Design 118. 155—162. (3)... 

The phenomenon of bubbling has attracted much attention from fluidization technologists to theorize on the origin and mechanics of bubbles and to elaborate on their mathematical modeling, but it has not been sufficiently recognized as indicative of the need for devising better modes of G/S contacting in which bubbles are suppressed or even totally eliminated. 

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. 

Determinations, Am. Inst. Chem. Engrs. Symposium on Mechanics of Bubbles and Drops, Detroit, Mich., Nov. 27-30, 1955. 

Nevertheless, in order to get some insight into the mechanism of bubble growth, and following the classic derivation of Scriven (34), we derive here the particular case for the rate of growth of a single bubble in a quiescent infinite liquid (Fig. 8.11), with the viscous forces acting as the rate-controlling step. 

Under the real operating conditions of a fluidized-bed reactor, a number of interacting bubbles occur in the interior of the fluidized bed. As a rule, the interaction leads to coalescence. As detailed studies have shown, this process is quite different from that between gas bubbles in liquids because of the absence of surface-tension effects in the fluidized bed [31, 32], Werther has derived a simple empirical correlation (based on the mechanism of bubble coalescence) for the growth of the mean bubble size dy (diameter of the sphere of equal volume) with increasing height h above the grid [33, 34] ... 

To close this Section we comment on two papers that do not fit under any neat heading. The first of these is by Xiao et al,261 who study the final stages of the collapse of an unstable bubble or cavity using MD simulations of an equilibrated Lennard-Jones fluid from which a sphere of molecules has been removed. They find that the temperature inside this bubble can reach up to an equivalent of 6000 K for water. It is at these temperatures that sonolumines-cence is observed experimentally. The mechanism of bubble collapse is found to be oscillatory in time, in agreement with classical hydrodynamics predictions and experimental observation. The second paper, by Lue,262 studies the collision statistics of hard hypersphere fluids by MD in 3, 4 and 5 dimensions. Equations of state, self-diffusion coefficients, shear viscosities and thermal conductivities are determined as functions of density. Exact expressions for the mean-free path in terms of the average collision time and the compressibility factor in terms of collision rate are also derived. Work such as this, abstract as it may appear, may be valuable in the development of microscopic theories of fluid transport as well as provide insight into transport processes in general. 

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. 

In the other extreme, when the exchange mechanism of bubble processes dominates (Vjnj = 0), and the bubble mechanism has the diffusion coefficient and solubility dependence prescribed for initial gas transfer across a clean bubble (m = 1 and n = 0.5), the gas supersaturation has no dependence on the physicochemical properties of the gases ... 

Increasing the pressure enhanced the stability at every temperature (Figures 40 and 41), and this effect can be explained by the mechanism of bubble disproportionation. During foam aging, the small bubbles shrink,... 

Silicas are the most widely used active particles in defoamer formulations. Precipitated silicas are used almost exclusively. To be effective, the silica must be reacted with an agent, typically polydimethylsUoxane, to render the surface hydrophobic. The mechanism of bubble breaking is the dewetting of the silica particle by the foam lamella, which creates a defect in the film that leads to its rupture. The criterion for dewetting is a three-phase contact angle of 90° or more (the three phases are the aqueous foam lamella, the silica particle, and the carrier oil in which the particle is dispersed). Patterson has identified the properties of silica that optimize performance in... 

Use STR when relatively small amounts of gas are needed, or low OTR is required. This is because the mechanism of bubble breakup is dictated by the efficiency with which the impeller can break up the gas stream. If the gas stream is too fast, then poor breakup occurs. Therefore, the mixing limits the gas input velocity to < 0.1 m/s. The tradeoff between mixing intensity and OTR is critical in selecting a configuration. See Table 6.10. Mixing ranges from as low as... 

---