Dispersed-droplet flows

Different authors have identified various flow regimes in large channels. In both vertical and horizontal configurations these include bubbly, dispersed bubbly, slug, pseudo-slug, churn, annular, annular mist and dispersed droplet flows. An important difference in minichannels is that the liquid flow is preferentially laminar. Surface tension effects have more and more influence as the hydraulic diameter is reduced. Gravity becomes negligible compared to surface tension so that the orientation is less influential. 

Dispersed droplet flow. In this regime, which can only normally exist in heated systems, the liquid phase is completely dispersed as droplets in the vapor. 

A correlation for the dispersed droplet flow regime (see Fig. 15.137) was developed by Groeneveld [337] and, for tubes, the correlation is as follows ... 

The value of the impingement Weber numb far exceeds the spreading limit of 80. Thus both the coherent jet and droplets spread out upon inpingement to the floor and form a coiium liquid film rather than bouncing back and form a dispersed droplet flow. This indicates that most of the mass that is discharged as a jet and mostly disintegrate into droplets reforms a coherent liquid film upon impingement to the floor. Therefore, for the coiium dispersion in the cavity, the liquid film entrainment becomes the most important mechanism. The duration of tiie entrainment depends on the liquid film residence time in the cavity. Hence flie liquid film motion and transport out of the cavity is also important... 

The influence of the vi.scosity ratio 8 on the flow behavior in a capillary was discussed by Rumscheidt and Mason [lOj. They pointed out that when the viscosity ratio is small, the dispersed droplets are drawn out to great lengths but do not burst, and when the viscosity ratio is of the order of unity, the extended droplets break up into smaller droplets. At very high viscosity ratios, the droplets undergo only very limited deformations. This mechanism can explain our observations and supports our theoretical analysis assumptions, summarized previously as points 2, 3, and 4. 

In electrophoresis an electric field is applied to a sample causing charged dispersed droplets, bubbles, or particles, and any attached material or liquid to move towards the oppositely charged electrode. Their electrophoretic velocity is measured at a location in the sample cell where the electric field gradient is known. This has to be done at carefully selected planes within the cell because the cell walls become charged as well, causing electro-osmotic flow of the bulk liquid inside the cell. From hydrodynamics it is found that there are planes in the cell where the net flow of bulk liquid is zero, the stationary levels, at which the true electrophoretic velocity of the particles can be measured. 

If an extensional force is also applied in addition to the pure shear force (for types of flow, see Fig. 9.2), the critical value of the Weber number is considerably lower. It is possible to break up droplets in an extensional flow even when the viscosity ratios are very high. Thus, extensional flow is significantly more effective than pure shear flow when attempting to disperse droplets and break up high-viscosity gels or polymer particles. 

To break up agglomerates or disperse liquid droplets, flow forces that exceed a certain minimum value are required. In addition, the type of flow is crucial for the dispersion result, namely, the respective ratio of shear and extension. The shear and extension rates are not constant over the cross-section in an extruder. There are zones with more or less loading. In addition, there are zones with almost pure shear flow and zones where extensional flow dominates. The different zones in the cross-section of a twin screw extruder are shown in Fig. 9.15. 

FIGURE 3 (a) Vertical droplet flow created by controlled particle dispersion used in ViaNase ID (Kurve Technology, Bothell, WA). (b) Deposition pattern produced by controlled particle dispersion. (Reproduced from ref. 42 with permission from Drug Delivery Technology.)... 

Unlike a solid-in-liquid suspension, the viscosity of an emulsion may depend upon the viscosity of the dispersed phase. This dependence is especially true when internal circulation occurs within the dispersed droplets. The presence of internal circulation reduces the distortion of the flow field around the droplets (26), and consequently the overall viscosity of an emulsion is lower than that of a suspension at the same volume fraction. With the... 

To develop an understanding of the emulsion flow in porous media, it is useful to consider differences and similarities between the flow of an OAV emulsion and simultaneous flow of oil and water in a porous medium. As discussed in the preceding section, in simultaneous flow of oil and water, both fluid phases are likely to occupy continuous, and to a large extent, separate networks of flow channels. Assuming the porous medium to be water-wet, the oil phase becomes discontinuous only at the residual saturation of oil, where the oil ceases to flow. Even at its residual saturation, the oil may remain continuous on a scale much larger than pores. In the flow of an OAV emulsion, the oil exists as tiny dispersed droplets that are comparable in size to pore sizes. Therefore, the oil and water are much more likely to occupy the same flow channels. Consequently, at the same water saturation the relative permeabilities to water and oil are likely to be quite different in emulsion flow. In normal flow of oil and water, oil droplets or ganglia become trapped in the porous medium by the process of snap-off of oil filament at pore throats (8). In the flow of an OAV emulsion, an oil droplet is likely to become trapped by the mechanism of straining capture at a pore throat smaller than the drop. 

The viscosity of microemulsions has been studied several times in order to determine hydration and interactions between the dispersed droplets. It was found that an increase in hydration of the surfactant molecules resulted in rheological behavior more similar to that of suspensions containing solid particles in low concentrations. In any case, the microemulsions showed Newtonian flow characteristics. 

The problem of mean value with regard to the dispersed droplet EPR (3.115) sounds as the following to find three coefficients Amean, Bmea , and crmean which determine the monodisperse EPR flow (3.85) and provide the velocity distributions U(x, z) and V(x,z) as close as possible to those of the multidisperse flow (3.115), for which the spectral coefficients A(r), B(r), and cr(r) are known. 

Furtheron, the dispersed droplets are the smaller the closer to unity the viscosity ratio of the components is (62-64). Their sizes decrease also if the first normal stress difference of the dispersed phase becomes smaller than that of the matrix (61). The droplet size, moreover, is influenced by the tendency to further break down of elongated particles due to capillary instabilities (61) as well as by coalescence via an interfacial energy driven viscous flow mechanism. All these procedures and dependences affect the structure formation within their typical time scales (61,62). 

Behzadi A, Issa RI, Rusche H (2004) Modelling of dispersed bubble and droplet flow at high phase fractions. Chem Eng Sci 59(4) 759-770. 

In absorption, instead of a solid surface dissolving into a liquid, we have a gas dissolving into a liquid. The liquid may exist as dispersed droplets or as a flowing film like that shown in Fig. 16.2. Since gas and liquid layers are free to move, two boundary layers can arise at the interface. 

Large shear rates enhance deformation capabilities of the dispersed phase domains generally as droplets, flowing with the matrix during the mixing and further... 

The transfer of analyte from one phase to another in liquid-liquid extraction is achieved at the interface between the two immiscible phases. With appropriate agitation of the phases, mass transport within the two phases is of minor importance for the transfer efficiency. Maximum exposure of the phase interface is therefore pursued in batch extractions by vigorous shaking, resulting in the formation of highly dispersed droplets of one phase in the other. However, such conditions are not achievable in continuous flow systems such as FIA. Nevertheless, in most cases phase transfer factors of... 

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