Patterns stratified flow

Flow Regimes in Multiphase Reactors. Reactant contacting, product separations, rates of mass and heat transport, and ultimately reaction conversion and product yields are strong functions of the gas and Hquid flow patterns within the reactors. The nomenclature of commonly observed flow patterns or flow regimes reflects observed flow characteristics, ie, armular, bubbly, plug, slug, spray, stratified, and wavy. 

For all flow conditions tested in that study, a bubbly flow pattern with bubbles much smaller than the channel diameter (100 pm) was never observed. While liquid-only flows (or liquid slugs) containing small spherical bubbles were not observed, small droplets were observed inside gas core flows. Furthermore, no stratified flow occurred in the micro-channel as reported in previous studies of two-phase flow patterns in channels with a diameter close to 1 mm (Damianides and Westwater 1988 Fukano and Kariyasaki 1993 Triplett et al. 1999a Zhao and Bi 2001a). 

The purpose of these experiments was to characterize different flow details under conditions when the superficial gas velocity is constant and the superficial liquid velocity increases. The upward flow regimes are presented in Fig. 5.33. Figure 5.33a shows the stratified flow pattern at [/gs = 20 m/s and J/ls = 0.005 m/s. In the region of pure stratified flow the liquid layer is drawn upward by the gas via the interfacial shear stress. No droplets could be observed at the interface. Such a regime was also observed by Taitel and Dukler (1976), and Spedding et al. (1998). 

In large tubes, as well as in tubes of a few millimeters in diameter, two-phase flow patterns are dominated in general by gravity with minor surface tension effects. In micro-channels with the diameter on the order of a few microns to a few hundred microns, two-phase flow is influenced mainly by surface tension, viscosity and inertia forces. The stratified flow patterns commonly encountered in single macro-channels were not observed in single micro-channels. 

Figure 12.45. Flow patterns in condensation, (a) Stratified flow (b) Annular flow...

Figures 3.1a and 3.16 show the flow patterns in vertical and horizontal pipes, respectively. Obviously, stratified flow does not exist in vertical flow, because of the relative direction of the flow and gravitational force, and a more symmetrical flow pattern is possible in vertical flow than in horizontal flow. Flow patterns identified in the figure can be described as follows.

The pattern of annular flow tends to form at higher gas velocities the substantial amount of work done on this topic is reviewed by Hewitt (1982). A procedure for stratified flow is given by Cheremi-sinoff and Davis [AIChE J. 25, 1 (1979)]. 

The mechanics and applications of multiphase flow has been an area of continuing interest to chemical, environmental, and civil engineers (23,77). The multiphase flow patterns may be classified as bubble flow, plug flow, stratified flow, wave flow, slug flow, annular flow, spray flow, and froth flow. Typical sketches of these various flow patterns are shown in Fig. 3. They are self-explanatory. In the field of absorptive bubble separation processes, only multiphase bubble flow and froth flow are of interest to the process engineer. 

Fig. 4.45 Flow patterns in a horizontal, unheated tube a bubble flow b plug flow c stratified flow d wavy flow e slug flow f annular flow g spray or drop flow...

FIGURE 17.51 Flow patterns in tube bundles (a) spray flow, (b) bubbly flow (vertical and horizontal), (c) chugging flow (vertical), (d) stratified spray flow, (e) horizontal stratified flow as defined by Grant and reported in Ref. 69. 

Convective condensation in horizontal and vertical tubes is most important with two flow patterns annular film flow and stratified flow. 

When two immiscible phases come into contact at a junction, an approximately parabolic flow profile can be obtained for a two-phase stratified flow in a common pressure-driven microfluidic system. On the other hand, in a droplet-based flow, stimulated by the shear between the stationary fluid at the channel wall and the slug axis, there exists internal recirculation in both continuous and dispersed phases [8]. Complex flow patterns may appear under different conditions, depending on the forces acting on the fluids. 

In small channels, a number of flow patterns can be observed, and the same terminology and classifications as in large channels are commonly used. Because of the dominance of the surface tension forces, stratified flow is rarely observed in small channels. In general, bubble flow appears at low gas flow rates. As the gas flow rate increases, Taylor bubbles form. With further increase in the gas flow rate, annular flow appears with the liquid forming an annulus which wets the wall. At high gas and liquid flow rates, chum flow occurs where there is a liquid film at the wall and the gas flow in the center is interrupted by the firequent appearance of frothy bubbles and slugs. 

The flow patterns and velocity distribution around elevated complex terrain (hills, mountains, or ridges) need to be imderstood before quantitative predictions of pollutant dispersion can be made. The broad aspects of the stracture of these stratified flows include ... 

It was shown by Brauner and Moalem Maron that linear stability analysis is insufficient to predict the stratified flow boundaries [40]. Parallel analyses on the stability as well as on the well-posedness of the (hyperbolic) equations which govern the stratified flow has been invoked. It has been shown that the departure from stratified configuration is associated with a buffer zone confined between the conditions derived from stability analysis (a lowerbound) and those obtained by requiring well-posedness of the transient governing equations (an upper-bound). These two bounds form a basis for the construction of the complete stratified/non-stratified transitional boundary to the various bounding flow patterns. 

In searching for the necessary conditions under which the smooth-stratified flow configuration is stable, linear stability analysis is carried out on the transient two-fluid continuity and momentum Equations 1, 2, 7. The equations are perturbed around the smooth fully developed stratified flow pattern. Following the route of temporal stability analysis h = heKkx-m). gi(icx-[Pg.327]

Thus, while the neutral stability boundary may represent preliminary transition from smooth-stratified flow to a wavy interfacial structure, the well-posedness boundary, which is within the wavy unstable region, represents an upper bound for the existence of a stratified wavy configuration. Beyond the well-posedness boundary transition to a different flow pattern takes place. In the ill-posed region, the model is no longer capable of describing the physical phenomena involved therefore, amplification rates predicted for ill-posed modes or numerical simulation of their growth is actually meaningless. 

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