Mist 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. 

Fair s presentation provides the development of the design technique. The method recognizes two-phase flow in a vertical reboiler and points out that slug-type flow is most predominate and that mist-flow should be avoided. 

Economic optimum design usually implies high circulation rates, although not high enough to give mist flow. 

Figure LI Steam generation from a heated surface, showing nucleate boiling, leading to bubbly, intermediate, annular and mist flow forms of convective boiling. Steam bubbles in water (a) leading to water droplets in steam (b).

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. 

Mist flow, one component In a one-component system with finely dispersed drops in the mist flow, the mass transfer between phases over a large interfacial area has to be considered. For the compression wave the frozen state can be assumed to be subcooled liquid, superheated vapor conditions generated by the wave are fairly stable, and the expressions for the two-component system are valid (Henry, 1971) ... 

Henry, R. E., 1971, Pressure Wave Propagation through Annular and Mist Flows, AIChE Chem. Eng. Prog. Symp. Ser. 67( 113) 38-47. (3)... 

ANNULAR MIST FLOW TRANSITION REGION MIST FLOW... 

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. 

Two-phase flow model in channels mist flow X mist flow annular film mist flow X X X un- specified... 

At a constant low liquid-flow rate with steadily increasing gas flow, the patterns observed will tend to be stratified, wavy, annular and mist flow. At a somewhat higher liquid rate, stratified, plug, slug, annular, and mist flow occur while at high liquid flows the patterns follow the order bubble, plug, slug, annular, and mist, as gas flow increases. 

The basic assumptions implied in the homogeneous model, which is most frequently applied to single-component two-phase flow at high velocities (with annular and mist flow-patterns) are that (a) the velocities of the two phases are equal (b) if vaporization or condensation occurs, physical equilibrium is approached at all points and (c) a single-phase friction factor can be applied to the mixture if the Reynolds number is properly defined. The first assumption is true only if the bulk of the liquid is present as a dispersed spray. The second assumption (which is also implied in the Lockhart-Martinelli and Chenoweth-Martin models) seems to be reasonably justified from the very limited evidence available. 

Note that the tube contains a liquid before the bubbly flow regime and a vapor after the mist-flow regime. Heat transfer in those two cases can be determined using the appropriate relations for single-phase convection heat transfer. Many correlations are proposed for the determination of heat transfer... 

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. 

This annular/disperse-wave/mist flow pressure drop model development above uses a physical representation where die liquid forms an annular film aroimd a gas core however, as noted above, the resulting correlation is also... 

A comparison of the measured pressure drops and those calculated using the intermittent and annular/disperse-wave/mist flow models is shown in Figure 11 for each tube considered. In the overlap zone (Figure 7), the flow exhibits both the adjoining mechanisms (intermittent and annular/disperse-wave/mist flow). Therefore, for calculating the pressure drops in the overlap zones in Figure 11, the four-point interpolation scheme described above in connection with the transition between laminar and turbulent data was applied to the pressure drops calculated using the intermittent and annular/disperse-wave/mist flow models. This combined model for the... 

Dispersed, Spray, or Mist Flow. Here, all of the liquid is entrained as fine droplets by the gas phase. Dispersed flow occurs for gas velocities greater than 200 ft/sec (60 m/sec). 

---