The different flow patterns

The numerous forms of two-phase flow can be divided into certain basic types, between which transition and mixed states are possible. The basic types of flow pattern in upward two-phase flow in a vertical, unheated tube are shown in Fig. 4.44. 

In bubble flow, Fig. 4.44 a, the gas or vapour phase is uniformly dispersed in the continuous liquid phase. Only very small bubbles are spherical the larger ones are oblate. This type of flow pattern occurs when the gas fraction is small. In plug flow, Fig. 4.44 b, large bubbles (plugs) almost fill the entire tube cross section. Between the plugs, the liquid is interspersed with small bubbles. 

The wispy-annular flow, Fig. 4.44 d, consists of a relatively thick liquid film at the wall, even though the liquid fraction in the vapour or gas core of the flow is still large. The film is interspersed with small bubbles, and the liquid phase in the core flow is mainly made up of large drops that sometimes coalesce into liquid strands. This type of flow pattern is normally observed when the mass flux is large. 

A pattern that frequently appears is annular flow, Fig. 4.44 e. It is characterised by the fact that the main portion of the liquid mass is at the wall and the gas or vapour phase, that is interspersed with drops, flows in the core of the tube at a significantly higher velocity. 

As a result of evaporation and especially at high velocities of vapour or gas, the liquid film at the wall disintegrates and a spray or drop flow is formed, Fig. 4.44 f. This occurs particularly in evaporation at high pressure. 

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In air-water flow the different flow patterns occur simultaneously in different micro-channels. Although the gas core may occupy almost the entire cross-section of the triangular channel, making the side walls partially dry, the liquid phase always remained continuous due to the fact that the liquid was drawn into the triangular corners by surface tension. 

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... 

A one-parameter model, termed the bubbling-bed model, is described by Kunii and Levenspiel (1991, pp. 144-149,156-159). The one parameter is the size of bubbles. This model endeavors to account for different bubble velocities and the different flow patterns of fluid and solid that result. Compared with the two-region model, the Kunii-Levenspiel (KL) model introduces two additional regions. The model establishes expressions for the distribution of the fluidized bed and of the solid particles in the various regions. These, together with expressions for coefficients for the exchange of gas between pairs of regions, form the hydrodynamic + mass transfer basis for a reactor model. 

Note that if the liquid mass flow-rate is kept constant and the gas mass flow-rate is started, the different flow patterns are observed as the gas mass flow-rate increases (Fig. 5.2-2). 

In this section, only general findings will be discussed which are valid for all packings investigated. Differences between the monolith types or between distillation packings and monoliths, which can be explained by the different flow patterns, are discussed in Section 8.4. 

Different drying chamber forms and different methods of hot air introduction accompany the different flow pattern forms and are selected according to... 

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... 

As the different flow regimes are determined by the forces between the two phases, above all by the inertia and gravitational forces, it is appropriate to mark the dependence of the boundaries between the different flow patterns on these forces in diagrams, so-called flow maps. This type of flow map was first presented by Baker [4.80]. Therefore, we also speak of Baker-diagrams. These diagrams only provide a rough orientation because the decisive forces in the different flow regions are not known with sufficient certainty. In particular, for a two-phase flow with... 

Either the entire amount of the liquid on a tray or part of it may be taken out and returned to the tray directly below it. The combined liquid flow (the pumparound and the liquid flowing directly to the tray below) would be the same as the liquid flow down to the tray below had there been no pumparound. From the standpoint of equilibrium stages, the column would perform exactly as though the pumparound did not exist although the tray efficiency may be affected due to the different flow pattern. If the liquid is returned several trays below the draw tray, the trays between the draw tray and the return tray are bypassed by the pumparound liquid. The amount of fractionation in that column section is therefore reduced. This pumparound thus tends to lower the overall number of effective trays in the column. 

Figure 7.2 Standard and modified Cpeak vessel) USP II dissolution apparatuses including illustrations of the different flow patterns within the beakers and photographs taken at a paddle stirring rate of 100 rpm showing a heap of pellets beneath the paddle in the standard method compared to the desirable dispersion of pellets in the modified method.

A comparison of Fig. 12.23(a) and the results obtained by Lin et. al (Ref 8) for a flat-shoulder tool in aluminum 6111-T4, discussed earlier, shows that the stir zone for the concave tool (light-gray area around the probe and the shoulder) is much larger compared to that of the flat tool. Due to different flow patterns, the shapes of the interface between the upper and lower sheets under the shoulder indentation are quite different. The different flow patterns also result in different shapes of spot friction welds. 

Although the exact position of the transition boundaries between the flow patterns are inherently related to the liquid-liquid system studied, the arrangement of the regions of the different flow patterns are similar to a number of systems. Models based on the Capillary, Reynolds and Weber numbers have been developed in order to allow an a priori prediction of the flow patterns using fluid properties and flow velocities. A general criterion for flow pattern identification in a given micro-channel was given in terms of dimensionless ratio of Reynolds to Capillary (ReJ Ca< ) numbers as a function of the product of Reynolds number and hydraulic diameter (Readh/Cd) by Kashid and Kiwi-Minsker (2011) and was applied to different literature data which are summarised in Fig. 2.5. 

The increase in separation for the different flow patterns can be conpared for separation of oxygen and nitrogen. Geankoplis (20Q2) presents a problem with a feed that is 20.9% oxygen, Pj. = 190 and Pp... 

It is convenient to use staged models tFigure 17-191 to analyze the different flow patterns in permeation systems. Essentially, this represents a numerical integration of the differential equations representing the three configurations shown in Figure 17-18 fCoker et al 19981. 

Increasing flow velodties in the microreactor lead to pulsations and the formation of segmented flow. The different flow patterns observed in microstructured packed beds were studied in detail by van Herk et al. [98]. They confirmed the segregated flow... 

The different flow patterns largely resemble those known from flow in other continuous flow conduits as pipes, tubes, capillaries, and monoliths [29-40]. Bubbly flow, slug flow (Taylor flow), annular, and churn flow are found and a few more intermediate regimes between the ones mentioned. These comprise different gas-liquid configurations such as segmented flow (bubble-train), gas core with encompassing stable thin liquid film, which wets the channel wall, and dynamic wavy liquid films. In case of high gas contents, spray is created with small droplets in... 

Figure 5. Friedel ApJAp as a function of the vapor quality and for the different flow patterns.

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