Churning regime

The homogeneous flow model and the separated flow model may be used to estimate the pressure drop for the churn regime but the former is not recommended for use with annular flow. The separated flow model of Martinelli and Nelson (1948), and developments thereof, may be used for annular flow. 

Figure 9.7 Maps of the water-nitrogen two-phase flow regimes in the microchannels (I) 100 X 50 pm [40], (II) 50 X 50 pm [45], (III) 300 x 100 pm [40], (IV) Ref [46], (V) Ref [47], and (VI) Ref [44]. The lines represent the boundaries of the transitions between regimes (la), (lb) slug/slug-annular (2a), (2b), (2c) slug-aimular/annulai (3a), (3b) annular/churn and (4) slug/ churn regimes.

Leung, J. C., Overpressure During Emergency Relief Venting In Bubbly and Churn-Turbulent Regimes, AIChEJ, 33 (6), 952-958, June 1987. 

Figure 5.3e shows the situation when the air velocity was increased to Ugs = 20 m/s. It is seen from this figure that the liquid bridges in churn flow disappeared and a liquid film formed at the side walls of the channel with a continuous gas core, in which a certain amount of liquid droplets existed. The pressure flucmations in this case became relatively weaker in comparison with the case of the churn flow. The flow pattern displayed in Fig. 5.3f indicates that as the air velocity became high enough, such as Ugs = 85 m/s, the liquid droplets entrained in the gas core disappeared and the flow became a pure annular flow. It is also observed from Fig. 5.3f that the flow fluctuation in this flow regime became weaker than that for the case shown in Fig. 5.3e, where Ugs = 20 m/s. 

The flow regime maps shown in Fig. 5.16a,b indicate that typical flow patterns encountered in the conventional, large-sized vertical circular tubes, such as bubbly flow, slug flow, churn flow and annular flow, were also observed in the channels having larger hydraulic diameters ([Pg.216]

Figure 5.16c indicates that as the channel size was reduced to Jh = 0.866 mm, the dispersed bubbly flow pattern vanished from the flow regime map. Figure 5.16a-c indicates that the slug-churn flow transition line shifted to the right, as the channel size was reduced. Similar trends were also found in small circular tubes by the... 

This map has been checked by many researchers, indicating that it is applicable to a wide range of conditions. Also shown in Figure 3.4 are correlations derived by Mishima and Ishii (1984), which used similar basic principles except for the slug-to-churn transition. These authors pointed out that, in view of the practical applications of the separate-fluid model to transient analysis, flow regime criteria based on the superficial velocities of the liquid and gas may not be consistent with the separate-flow model formulation. A direct geometric parameter such as the... 

Figure 3.4 Vertical upflow regime map (d = 2.5 cm), air-water at 25°C and 0.1 MPa. A, B, C, D, E. (From Taitel et al., 1980. Copyright 1980 by American Institute of Chemical Engineers, New York. Reprinted with permission.) A, D. D and D are the boundary between slug and churn flow. (From Mishima and Ishii, 1984. Copyright 1984 by Elsevier Sci. Ltd., Kidlington, UK. Reprinted with permission.)...

Figure 3.6 Vertical downflow regime map (1) bubbles (2) slugs (3) falling film (4) bubbly falling film (5) churn (6) dispersed annular. (From Oshimowo and Charles, 1974. Copyright 1974 by Canadian Society of Chemical Engineers, Ottawa, Ont. Reprinted with permission.)...

The Liao, Parlos, and Griffith model (a flow regime-based drift flux formulation including bubbly, churn-turbulent, and annular flows)... 

The increasing void fraction and acceleration of the flow also produce changes in the flow regime with downstream location. As shown in Figure 4.2, for vertical upward flow, bubbly flow at the onset location subsequently changes to slug, churn, and then annular flow. When there is a large difference in the liquid and vapor... 

For partial disengagement venting models (in bubbly and churn-turbulent regimes), refer to publications by Leung (1987) and Dalessan-dro (2004). 

In these models the phases are treated as if they are separate and flow in well defined but unspecified parts of the cross section. Only the simplest case, in which the phases are allowed to have different but uniform velocities, will be considered here. An overall momentum equation will be given and it will be seen that merely allowing the gas and liquid velocities to differ leads to considerable complexity. Two empirical correlations from the pioneering work of Martinelli and co-workers will then be described. These methods can be used for the churn and annular flow regimes. 

For a non-foamy system, the extent of the level swell and the fraction of gas/ vapour entering the pressure relief system, at a given gas/ vapour evolution rate, depends on the two-phase flow regime within the vessel. For non-viscous systems the two main flow regimes are bubbly-flow and churn-turbulent flow (see Figure 4.2 (b) and... 

As the gas or vapour production rate increases, the flow regime may change from churn-turbulent to droplet flow, in which a fluidised bed of liquid droplets is present in the reactor (see Figure A3.1). This is of less practical interest for relief system sizing because if the gas or vapour rate is so high as to give droplet flow, the relief system size is likely to be impractically large. 

The effect of moderately high viscosity (> 100 cP) is to prevent the formation of the churn-turbulent flow regime so that the bubbly flow regime persists at higher superficial gas/vapour velocitiesj(see Annex 3). . , -4... 

Experimental work for highly viscous fluids suggests that some disengagement does occur, in that bottomrvented tests results are different from top-vented test results12.1. The flow regime in highly viscous flow (>1000 cP) is unlikely to be one of those (churn-turbulent or bubbly flow) described in Annex 3. 

The different flow regimes during level swell (churn-turbulent, bubbly and droplet) were described in 4.3.1. In order to perform a level swell calculation, it is necessary to decide the flow regime. 

For a particular mixture, as the gas/ vapour superficial velocity increases, the flow regime moves from bubbly, through churn-turbulent and into droplet (see Figure A3.1). It should be remembered that the picture given by Figure A3.1 is a simplification of quite complex behaviour. 

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