Flow stratified

Jeong E K, Aitobeiii S A and Fukushima E 1994 NMR imaging studies of stratified flows in a horizontai rotating oyiinder Rhys. Fluids 6 2901-6... 

Pipe Lines The principal interest here will be for flow in which one hquid is dispersed in another as they flow cocurrently through a pipe (stratified flow produces too little interfacial area for use in hquid extraction or chemical reaction between liquids). Drop size of dispersed phase, if initially very fine at high concentrations, increases as the distance downstream increases, owing to coalescence [see Holland, loc. cit. Ward and Knudsen, Am. In.st. Chem. Eng. J., 13, 356 (1967)] or if initially large, decreases by breakup in regions of high shear [Sleicher, ibid., 8, 471 (1962) Chem. Eng. ScL, 20, 57 (1965)]. The maximum drop size is given by (Sleicher, loc. cit.)... 

Puttock, J. S. (ed.), "Stably Stratified Flow and Dense Gas Dispersion." Oxford University-Press, New York, 1988. 

Mcrrit, G., ind G. Redinger. 1973. Measurements of heat and impulse transfer coefficient m turbulent stratified flow. Journal of Rocket Technology and Astronautics, no. I I. 

Figure 12.33 shows the velocity distribution down through the full length of the exhibition hall. It is quite obvious that the flow is a stratified flow with the highest velocity in the occupied zone. Smoke measurements show that the cold air from the cooling device accelerates down into the occupied zone, due to gravity, and moves horizontally as a stratified flow along the floor in the restaurant and exhibition section. 

Measurements show that the velocity has a fairly constant level in the occupied zone even far downstream from the wall with the cooling device. The flow is plane, and general experience indicates that the velocity in a plane stratified flow is constant and independent of the distance from the inlet device. Prediction of the flow by coinpurational fluid dynamics shows a similar velocity level in the hall. - The full-scale measurements shown in Fig. 12.33 indicate a very low velocity in most of the hall due to the practical difficulties in obtaining a correct load during the full-scale experiment. 

The arrangement of baffle plates and nozzles. Figure 10-96C, are important to prevent (a) tube vibration, (b) maldistribution of the process boiling fluid, and (c) poor heat transfer coefficients due to uneven and stratified flow resulting in uneven and dry spot heat transfer from nonuniform tube wetting, and others. ... 

Lineham,J. H., M. Petrick, and M. M. El-Wakil, The Condensation of A Saturated Vapor on a Subcooled Film Drying Stratified Flow, AlChE Sym. on Heat Transfer, V. 66, No. 102. 

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. 

VOF or level-set models are used for stratified flows where the phases are separated and one objective is to calculate the location of the interface. In these models, the momentum equations are solved for the separated phases and only at the interface are additional models used. Additional variables, such as the volume fraction of each phase, are used to identify the phases. The simplest model uses a weight average of the viscosity and density in the computational cells that are shared between the phases. Very fine resolution is, however, required for systems when surface tension is important, since an accurate estimation of the curvature of the interface is required to calculate the normal force arising from the surface tension. Usually, VOF models simulate the surface position accurately, but the space resolution is not sufficient to simulate mass transfer in liquids. 

E. K. Jeong, S.A. Altobelli, E. Fukushima 1994, (NMR imaging studies of stratified flows in a horizontal rotating cylinder), Phys. Fluids 6, 2901. 

Two flow models are used to estimate the mean condensation coefficient in horizontal tubes stratified flow, Figure 12.45a, and annular flow, Figure 12.45. The stratified flow model represents the limiting condition at low condensate and vapour rates, and the annular model the condition at high vapour and low condensate rates. For the stratified flow model, the condensate film coefficient can be estimated from the Nusselt equation, applying a suitable correction for the reduction in the coefficient caused by... 

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

For condenser design, the mean coefficient should be evaluated using the correlations for both annular and stratified flow and the higher value selected. 

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 transition of slug to annular or slug to stratified flow can be interpreted as a flooding condition. This approach describes the condition in which a wave, formed on the liquid film, may become unstable and increase indefinitely until it forms a slug (Bankoff and Lee, 1983). 

A separated flow model for stratified flow was presented by Taitel and Dukler (1976a). They indicated analytically that the liquid holdup, R, and the dimensionless pressure drop, 4>G, can be calculated as unique f unctions of the Lockhart-Martinelli parameter, X (Lockhart and Martinelli, 1949). Considering equilibrium stratified flow (Fig. 3.37), the momentum balance equations for each phase are... 

The assumed dependence of pressure drop on X was first explained by Johannessen (1972) in evolving a theoretical model for stratified flow including some unnecessary simplifications. 

Figure 3.37 Equilibrium stratified flow. (From Taitel and Dukler, 1976a. Copyright 1976 by Elsevier Science Ltd., Kidlington, UK. Reprinted with permission.)... 

Figure 3.40 The stratified flow system under consideration. (From Cheremisinoff and Davis, 1979. Copyright 1979 by American Institute of Chemical Engineers, New York. Reprinted with permission.)... 

An iterative procedure to calculate the pressure drop was suggested by Cheremisinoff and Davis (1979) for turbulent/turbulent stratified flow ... 

Crowley, C. J., G. B. Wallis, and J. J. Barry, 1993, Dimensionless Form of a One-Dimensional Wave Model for the Stratified Flow Regime Transition, lnt. J. Multiphase Flow 7P(2) 369 376. (3) Cumo, M., 1972, Personal communication, Comitato Nazionale Per L energia Nucleare, Milan, Italy. (5)... 

Taitel, Y., and A. E. Dukler, 1976a, A Theoretical Approach to the Lockhart-Martinelli Correlation for Stratified Flow, Int. J. Multiphase Flow 2.591 595. (3)... 

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