Slug-flow

This flow is identical to the bubbly flow of gas-liquid systems. It is characterized by the drops with diameters less than or equal to the microchannel diameter. In a microchannel, this flow pattern typically occurs at relatively high continuous flow velocities and low dispersed phase velocities. The drop size is restricted by channel dimensions. By varying the microchannel dimensions, the flow regimes can be changed from drop flow to slug flow and vice versa [12]. 

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Limiting Nusselt numbers for slug-flow annub may be predicted (for constant heat flux) from Trefethen (General Discu.s.sion.s on Heat Tran.sfer, London, ASME, New York, 1951, p. 436) ... 

Lockhart and Martinelh (ibid.) correlated pressure drop data from pipes 25 mm (1 in) in diameter or less within about 50 percent. In general, the predictions are high for stratified, wavy, ana slug flows and low for annular flow The correlation can be applied to pipe diameters up to about 0.1 m (4 in) with about the same accuracy. 

Slug flow must be avoided in all two-phase applications. The designer must be alert for two-phase flow developing in a system. In one case, absorber liquid going to a lower pressure stripper produced a two-phase mixture. The absorber stream entered the stripper in a line that was elled down onto the stripper tray. The two-phase mixture beat out a section of trays. A /4-in. protection plate was provided and this had a hole cut in it in two years. 

When the bubble diameter approaches the diameter of the containing vessel, slug flow is said to exist. In such cases, the bubble rise velocity is given by... 

In vertical flow, axial symmetry exists and flow patterns tend to be somewhat more stable. However, with slug flow in particular, oscillations in the flow can occur as a result of sudden changes in pressure as liquid slugs are discharged from the end of the pipe. 

A knowledge of hold-up is particularly important for vertical flow since the hydrostatic pressure gradient, which is frequently the major component of the total pressure gradient, is directly proportional to liquid hold-up. However, in slug flow, the situation is complicated by the fact that any liquid which is in the form of an annular film surrounding the gas slug does not contribute to the hydrostatic pressure 14. ... 

The flow of two-phase systems often causes erosion, and many empirical relationships have been suggested to define exactly when the effects are likely to be serious. Since high velocities may be desirable to avoid the instability associated with slug flow, there is a... 

It is apparent that some compromise may be essential between avoiding a slug-flow condition and velocities which are likely to cause erosion. 

The particles settle out as in slug flow but the dunes remain stationary with particles being conveyed above the dunes and also being swept from one dune to the next. 

Following slug flow, the particles, instead of forming stationary dunes, gradually build up over the cross-section until they eventually cause a blockage. This type of flow is less common than dune flow. 

Heywood, N. I. and Richardson, J. F. Chem. Eng. Sci. 34 (1979) 17-30. Slug flow of air-water mixtures in a horizontal pipe determination of liquid holdup by y-ray absorption. 

Figure 8.43. Liquid holdup 6/ and gas holdup g ( = 1 — ft) for slug flow in a 25 mm diameter pipe as a function of superficial gas velocity (u< ) and superficial liquid velocity ( /)...

A highly concentrated suspension of flocculated kaolin in water behaves as a pseudo-homogeneous fluid with shear-thinning characteristics which can be represented approximately by the Ostwald-de Waele power law, with an index of 0.15. It is found that, if air is injected into the suspension when in laminar flow, the pressure gradient may be reduced even though the flowrate of suspension is kept constant, Explain how this is possible in slug flow and estimate the possible reduction in pressure gradient for equal volumetric flowrates of suspension and air. 

Bubbly/slug flow. Bubbles both longer and shorter than the channel diameter (Fig. 2.30b). The bubble frequencies increase rapidly with the heat flux in the evaporator, reached a peak 900 Hz and then decreased due to coalescence. 

Slug flow. Vapor bubbles longer than the channel diameter, which is slightly smaller than that of the tube. The bubbles were separated from the inner channel... 

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