Patterns annular flow

In annular flow, liquid flows as a thin film along the pipe wall and gas flows in the core. Some liquid is entrained as droplets in the gas core. At veiy high gas velocities, nearly all the liquid is entrained as small droplets. Inis pattern is called spray, dispersed, or mist flow. 

In high heat flux (heat transfer rate per unit area) boilers, such as power water tube (WT) boilers, the continued and more rapid convection of a steam bubble-water mixture away from the source of heat (bubbly flow), results in a gradual thinning of the water film at the heat-transfer surface. A point is eventually reached at which most of the flow is principally steam (but still contains entrained water droplets) and surface evaporation occurs. Flow patterns include intermediate flow (churn flow), annular flow, and mist flow (droplet flow). These various steam flow patterns are forms of convective boiling. 

The annular flow regime is very extensive, and the above mechanism of burn-out is stated (S7) to be consistent with the film-flow measurement data over a range of exit qualities from 10 to 100% for uniformly heated round tubes. A summary of experimental observations on flow patterns produced... 

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. 

Notably, the higher the mass flux, the earlier annular flow is reached. Bubbly flow is more or less non-existent for mass fluxes exceeding 1,000 kg/m s. The most important observation about the flow patterns is that their transitions are controlled primarily by the rate of coalescence, which is not recognized as a contributing factor by any of the micro-scale or macro-scale flow pattern maps. 

How patterns in conventional size channels deviate significantly from those in micro-channels. Slug and annular flow constitute dominant flow patterns in con-... 

At relatively low liquid superficial velocities, increasing the mixture volumetric flux led to longer bubbles and shorter liquid slugs, eventually leading to the merging of elongated bubbles, and the development of the slug-annular flow pattern, repre-... 

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. 

Annular flow pattern is characterized by a thin water film, which flows along the channel wall with the nitrogen comprising the central core. Unlike annular macro-... 

Suo and Griffith (1964) performed experiments in horizontal channels with 0.5 and 0.7 mm diameters, and could identify slug, slug-bubbly, and annular flow patterns. For transition from slug to slug-bubbly, they suggest ReWe = 2.8 x 10, where Re = We = dpildljla, = 1. 2(Gls + 1/gs)- Re and We rep-... 

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]

Flow patterns and heat transfer were also investigated by Ghajar et al. (2004) in slug and annular flow. The different flow regimes depicted in Fig. 5.40 illustrate parameters in their experiments in the tube of t/ = 25.4 mm. 

Figure 5.6 Flow pattern map for a gas/liquid flow regime in micro channels. Annular flow wavy annular flow (WA) wavy annular-dry flow (WAD) slug flow bubbly flow annular-dry flow (AD). Transition lines for nitrogen/acetonitrile flows in a triangular channel (224 pm) (solid line). Transition lines for air/water flows in triangular channels (1.097 mm) (dashed lines). Region 2 presents flow conditions in the dual-channel reactor ( ), with the acetonitrile/nitrogen system between the limits of channeling (I) and partially dried walls (III). Flow conditions in rectangular channels for a 32-channel reactor (150 pm) (T) and singlechannel reactor (500 pm) (A) [13].

The need for analytical (flow-pattern) characterization in advance of the experiment is less than for the dispersive mixers forming slug and annular flow patterns, because the dispersion typically is formed in an attached tube. This tube is commonly made of glass and mostly of larger inner diameter. Hence visual inspection by the operator is routinely possible. 

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

Annular flow. In annular flow there is a continuous liquid in an annulus along the wall and a continuous gas/vapor phase in the core. The gas core may contain entrained droplets—dispersed mist—while the discontinuous gas phase appears as bubbles in the annulus. This flow pattern occurs at high void fractions and high flow velocities. A special case of annular flow is that where there is a gas/vapor film along the wall and a liquid core in the center. This type is called inverse annular flow and appears only in subcooled stable film boiling (see Sec. 3.4.6.3)... 

Comparison of the boundaries of the observed flow patterns with the analytical criteria derived by Quandt showed that the bubble, dispersed, and annular flow patterns are subclasses of a pressure gradient-controlled flow. Similarly, flow patterns identified as slug, wave, stratified, and f ailing film are subclasses of a gravity-controlled situation. 

M Annular to slug churn Matching of voids for slug and annular flow pattern... 

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