Internal circulation patterns

Internal circulation patterns have been observed experimentally for drops by observing striae caused by the shearing of viscous solutions (S7) or by photographing non-surface-active aluminum particles or dyes dispersed in the drop fluid [e.g. (G2, G3, J2, L5, Ml, SI)]. A photograph of a fully circulating falling drop is shown in Fig. 3.5a. Since the internal flow pattern for the Hadamard-Rybczynski analysis satisfies the complete Navier-Stokes equation... 

Experimenters who have observed asymmetry of internal circulation patterns have generally attributed this to accumulation of surface-active materials at the rear, causing a stagnant cap (see Chapter 3). It seems likely that at least part of the asymmetry results from the forward shift of the internal vortex at nonzero Re, as predicted numerically. 

Surface-active contaminants play an important role in damping out internal circulation in deformed bubbles and drops, as in spherical fluid particles (see Chapters 3 and 5). No systematic visualization of internal motion in ellipsoidal bubbles and drops has been reported. However, there are indications that deformations tend to decrease internal circulation velocities significantly (MI2), while shape oscillations tend to disrupt the internal circulation pattern of droplets and promote rapid mixing (R3). No secondary vortex of opposite sense to the prime internal vortex has been observed, even when the external boundary layer was found to separate (Sll). 

Internal circulation patterns and turbulence. Sasakura et al. (138) and Rachez et al. (139) investigated internal age distributions by means of tracers and proposed representative models of the circulation pattern. Bryant and coworkers (135, 140, 141) and Reuss studied more specifically circulation times in large stirred tanks by using radio flow-followers (aiming at applications for fermenters > 0.5 m ). They showed that circulation times tQ were log-normally distributed ... 

FIGURE 5.36. Schematic comparison of key features of the five cell technologies. A Cell inlet and outlet location B Internal circulation pattern C Gasket design D Membrane disposition. 

A Vs - scale containment was constructed. The experimental model took into consideration the internal containment structures and volumes also. This was mainly done to establish internal circulation patterns for air and steam mixture. Numerous instrumentations were used to obtain temperature, pressure velocities, evaporation and condensation rates. 

To summarize, even though some Fe- and Mn-oxides may dissolve in the anoxic sediments of estuaries and thereby release trace metals, reoxidation of the Fe and Mn in the upper layers of the sediments, coagulation of Fe-Mn-humic-trace metal mixtures in the water column, and circulation patterns within the estuary all combine to return trace metals to the upstream particulate fraction. Turekian has suggested that while these processes allow significant internal cycling of trace metals in... 

As pointed out, common modes of interface in the case of liquid-liquid two-phase flow are slug flow and parallel flow (Figure 15.9). In the case of slug flow, two mechanisms are known to be responsible for the mass transfer between the two liquids (a) internal circulation [66, 75, 84] takes place within each slug and (b) the concentration gradients between adjacent slugs lead to the diffusion between the phases. In the case of the parallel pattern, the flow is laminar and the transfer of molecules between the two phases is supposed to occur only by diffusion. 

In the case of airlift reactors, the flow pattern may be similar to that in bubble columns or closer to that two-phase flow in pipes (when the internal circulation is good), in which case the use of suitable correlations developed for pipes may be justified [55]. Blakebrough et al. studied the heat transfer characteristics of systems with microorganisms in an external loop airlift reactor and reported an increase in the rate of heat transfer [56], In an analytical study, Kawase and Kumagai [57] invoked the similarity between gas sparged pneumatic bioreactors and turbulent natural convection to develop a semi-theoretical framework for the prediction of Nusselt number in bubble columns and airlift reactors the predictions were in fair agreement with the limited experimental results [7,58] for polymer solutions and particulate slurries. 

Fig. 4.40A reveals that the overall solid circulation pattern does not deviate significantly from a fluidized bed without internals (see also De Jong et al., 2012a) solids rise in the center of the bed and are recirculated via both sides. In the bottom of the bed, as well as in between the upward and... 

Figure 10 Schematic of the internal circulation model for bubble breakup (a) internal and external flow fields (b) circulation velocity on surface S (c) force balance and 3-D view of surface S and the flow pattern on S. (From Luo et al., 1999.)...

In every reactor there are flow patterns that are related to the reactor dimensions, such as axial mixing, radial mixing, macro mixing and internal circulation. In continuous flow reactors these cause a certain residence time distribution. The characteristic dimensions of concentration differences c sed by these phenomena are on the order of the reactor dimensions, from say 10 m to 10 m. In fact, these phenomena are the only ones that are strongly dependent on scale. Scale effects can be investigate in the absence of chemical reactions, in so called "cold models". 

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