Annular mist flow regime

For circular tubes, the experimental data set consisted of a total of 603 points. Of these, 77 points lie in the intermittent regime, 448 in the disperse/annular/mist flow regime, and the remaining 78 data points are in the overlap zone between these two regimes. Pressure drop models for these regimes are described below. 

The onset of erosion-corrosion in iminhibited systems is associated with the onset of an annular mist flow regime in multi-phase flow in the siuface piping. 

At higher gas flow rates an annular regime is found as in vertical flow. At very high flow rates the liquid film may be very thin, the majority of the liquid being dispersed as droplets in the gas core. This type of flow may be called the spray or mist flow regime. 

If the gas-flow rate is increased, one eventuaHy observes a phase transition for the abovementioned regimes. Coalescence of the gas bubbles becomes important and a regime with both continuous gas and Hquid phases is reestabHshed, this time as a gas-flUed core surrounded by a predominantly Hquid annular film. Under these conditions there is usuaHy some gas dispersed as bubbles in the Hquid and some Hquid dispersed as droplets in the gas. The flow is then annular. Various qualifying adjectives maybe added to further characterize this regime. Thus there are semiannular, pulsing annular, and annular mist regimes. Over a wide variety of flow rates, the annular Hquid film covers the entire pipe waH. For very low Hquid-flow rates, however, there may be insufficient Hquid to wet the entire surface, giving rise to rivulet flow. 

For condensation inside long tubes, when the condensation rate or the length are large, the flow regime becomes annular with a vapour core. At even higher vapour velocities, the vapour core may contain a mist of liquid droplets. 

Different authors have identified various flow regimes in large channels. In both vertical and horizontal configurations these include bubbly, dispersed bubbly, slug, pseudo-slug, churn, annular, annular mist and dispersed droplet flows. An important difference in minichannels is that the liquid flow is preferentially laminar. Surface tension effects have more and more influence as the hydraulic diameter is reduced. Gravity becomes negligible compared to surface tension so that the orientation is less influential. 

Mist flow. At very high gas flow rates, the annular film is thiimed by the shear of the gas core on the interface until it becomes unstable and is destroyed, such that all the liquid is entrained as droplets in the continuous gas phase analogous to the inverse of the bubbly flow regime. Impinging liquid droplets intermittently wet the tube wall locally. The droplets in the mist are often too small to be seen without special lighting and/or magnification. 

Several authors have observed that erosion-corrosion happens in annular mist regimes. It is also indicated that the increase in corrosion rate with velocity in the Khuff Gas sour production system was associated with the onset of an annular mist regime in multi-phase flow. Here, without inhibitor injection, the corrosion rate dramatically increased at about 5 m/s flow rate with inhibitor injection, on straight sections, this change happened at about 7-8 m/s. Of course these figures are strictly vahd for the Khuff Gas experience only, and cannot be extrapolated to other conditions. From this experience, the following general rules can be derived ... 

Figure 5.22 Various flow regimes (a) bubble flow (b) slug (c) froth (d) annular mist. (Adapted from Ref. [34].)...

Multiphase flow in gas channels is characterized by different regimes. In PEFC flow channels, slug, annular, and mist flow can occur depending on the flow velocity. In porous media, the description of multiphase transport is more complex. The momentum equation reduces to Darcy s law ... 

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