Flow patterns appearance

Through visual identification, Alves (A2) has defined the different flow patterns that occur in horizontal gas-liquid systems Nicklin and Davidson (N2) have defined the different visual flow patterns appearing in vertical gas-liquid systems. These flow patterns are depicted in Figs. 1 and 2. The... 

At low liquid rates, the onset of instability occurs at a constant value of the total superficial velocity, and is predictable from holdup and flooding data for wetted wall columns. As liquid flow rates increase, Nicklin and Davidson predict that unstable flow begins at lower values of the gas flow rate. For high liquid flow rates, however, the slug length becomes important, and the unstable flow will begin at higher values of gas flow rate. Therefore, a definite liquid flow rate exists at which an unstable flow pattern appears with a minimum gas flow rate. 

With no outlet weir, the effects of liquid and gas flows on the flow pattern appear to be the converse of those described in items 1 and 2 above (161,162). 

In a horizontal, heated tube these flow patterns appear one after the other. For the same reasons as in a vertical evaporator tube, thermodynamic equilibrium is not achieved because of the radial temperature profile. Fig. 4.46 shows the different flow patterns in a horizontal evaporator tube, under the assumption that the liquid enters the tube at a sufficiently low velocity, below 1 m/s. It is clear... 

Figure 10 shows representative streamline patterns for oblates, prolates, and spheres in Newtonian and shear-thinning fluids similar results (not shown here) are obtained for dilatant fluids. The streamline patterns for sphere match with the literature predictions for example see Clift et al. (1978) for Newtonian fluids and Adachi et al. (1973) for power-law fluids (n< 1). The effect of the flow behavior index on streamline patterns for a sphere is found to be negligible, except the fact that the wake formation is somewhat delayed. For prolate spheroids (E = 5), no wake formation occurs even at Re = 100, whereas for oblates, a visible wake is formed even at Re= 10 for = 0.2. To recap, the flow patterns appear to be much more sensitive to the... 

FIGURE 6.5 Flow patterns appearing in a bubble column. 

Numerous studies for the discharge coefficient have been pubHshed to account for the effect of Hquid properties (12), operating conditions (13), atomizer geometry (14), vortex flow pattern (15), and conservation of axial momentum (16). From one analysis (17), the foUowiag empirical equation appears to correlate weU with the actual data obtained for swid atomizers over a wide range of parameters, where the discharge coefficient is defined as — QKA (2g/ P/) typical values of range between 0.3 and 0.5. 

Many of the by-products of microbial metaboHsm, including organic acids and hydrogen sulfide, are corrosive. These materials can concentrate in the biofilm, causing accelerated metal attack. Corrosion tends to be self-limiting due to the buildup of corrosion reaction products. However, microbes can absorb some of these materials in their metaboHsm, thereby removing them from the anodic or cathodic site. The removal of reaction products, termed depolari tion stimulates further corrosion. Figure 10 shows a typical result of microbial corrosion. The surface exhibits scattered areas of localized corrosion, unrelated to flow pattern. The corrosion appears to spread in a somewhat circular pattern from the site of initial colonization. 

The flow pattern efficiency depends solely upon the shape of the velocity profile in the circulating gas. In terms of the integrals appearing in the gradient equation, the flow pattern efficiency is given by equation 86. 

For fully developed incompressible cocurrent upflow of gases and liquids in vertical pipes, a variety of flow pattern terminologies and descriptions have appeared in the hterature some of these have been summarized and compared by Govier, Radford, and Dunn Can. J. Chem. Eng., 35, 58-70 [1957]). One reasonable classification of patterns is illustrated in Fig. 6-28. 

Relatively uncomphcated transparent tank studies with tracer fluids or particles can give a similar feel for the overall flow pattern. It is important that a careful balance be made between the time and expense of calculating these flow patterns with computational flirid dynamics compared to their apphcabihty to an actual industrial process. The future of computational fluid dynamics appears very encouraging and a reasonable amount of time and effort put forth in this regard can yield immediate results as well as potential (or future process evaluation. 

First we run the model so as to study the influenee of sereen resistanee on the overall flow patterns and on the maldistribution. The resulting profiles of inward radial veloeity at the inner sereen aeross the eatalyst bed appear in Figure 10-13 for different sereen resistanees. It ean be seen that higher sereen resistanee leads to more-uniform flow, as one would expeet. The existing sereens (with resistanee eoeffieients C2 of 2 X 10 /m) appear to be satisfaetory, sinee the deviations experieneed are less than 10%. 

The extension of generic CA systems to two dimensions is significant for two reasons first, the extension brings with it the appearance of many new phenomena involving behaviors of the boundaries of, and interfaces between, two-dimensional patterns that have no simple analogs in one-dimension. Secondly, two-dimensional dynamics permits easier (sometimes direct) comparison to real physical systems. As we shall see in later sections, models for dendritic crystal growth, chemical reaction-diffusion systems and a direct simulation of turbulent fluid flow patterns are in fact specific instances of 2D CA rules and lattices. 

The annular flow pattern discussed above shows a definite connection with burn-out, and enables a simple burn-out mechanism to be set forth. There are many other flow patterns referred to in the literature, however, and we will consider here what can be said about any connection they may have with burn-out. It does not follow that there must be a connection, as a flow pattern is essentially a description of the bulk conditions in a channel and depends upon the none-too-reliable results of visual observation, which is often impeded by optical distortion. Thus, although gross conditions may appear to change and one pattern give way to another, the hydrodynamic state prevailing close to the heated surface may remain practically unaffected and the burn-out mechanisms unaltered. 

Dispersed bubbles are observed (Fig. 5.6a) when the gas flow rate is very small such as [/gs = 0.0083 m/s. Two kinds of bubbles are observed one type is finely dispersed with a size smaller than the tube diameter, and the other type has a length of near to or a little larger than the mbe diameter with spherical cap and tail. The distance between two consecutive bubbles may be longer than ten times the tube diameter. This flow pattern is also considered as a dispersed bubbly flow. Often in air-water flow two kinds of bubbles appear together as pairs of bubbles in which the small-sized bubbles follow the larger ones. 

The objectives of the study by Kawahara et al. (2002) were to experimentally investigate the probability of appearance of different flow patterns in a circular micro-channel. The test section was a circular transparent channel made of fused silica with an internal diameter of 100 pm and length of 64.5 mm, providing an L/d ratio of 645. 

Fig. 5.11 Probability of appearance of different two-phase flow patterns at low liquid flow rates. Reprinted from Kawahara et al. (2002) with permission...

The liquid alone pattern showed no entrained bubbles or gas-liquid interface in the field of view. The capillary bubbly flow, in the upper part of Fig. 5.14a, is characterized by the appearance of distinct non-spherical bubbles, generally smaller in the streamwise direction than at the base of the triangular channel. This flow pattern was also observed by Triplett et al. (1999a) in the 1.097 mm diameter circular tube, and by Zhao and Bi (2001a) in the triangular channel of hydraulic diameter of 0.866 mm. This flow, referred to by Zhao and Bi (2001a) as capillary bubbly... 

Droplets appeared on the surface of the pipe (Fig. 5.33b) after increasing the water flow rate up to I/ls = 0.007 m/s. Spedding et al. (1998) referred to this regime as film plus droplet pattern. When the water flow rate increased and superficial liquid velocity was Gls = 0.03 m/s (Fig. 5.33c) droplets began to roll back into the liquid film. Kokal and Stanislav (1989) identified such a regime as annular plus roll wave flow pattern. The experimental facility used in the present study allowed us to achieve values of superficial gas velocities up to 20 m/s in the 49.2 mm pipe. 

Two-phase flow in parallel micro-channels, feeding from a common manifold shows that different flow patterns occur simultaneously in different microchannels. The probability of appearance of different flow patterns should be taken into account for developing flow pattern maps. 

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