Slug-drop flow

In this regime, the dispersed phase flows in the forms of irregular slugs and drops. This flow occurs duringthe transition to or from slug flow either at low volumetric flow rate of the dispersed phase compared to the continuous phase, or at the flow rate higher than slug flow. 

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Figure 7.9 Flow regimes observed for liquid-liquid systems in microstructured reactors, (a) Drop flow in a Y-junction capillary microchannel, (b) Slug flow in a concentric microchannel, (c) Slug-drop flow in a concentric microchannel,...

Figure 35. Pressure drop flow diagram for slugging bed.

Fig. 4.45 Flow patterns in a horizontal, unheated tube a bubble flow b plug flow c stratified flow d wavy flow e slug flow f annular flow g spray or drop flow...

The treatment of bilge water and emulsions resembles that of the treatment of oil field brines and produced water. Chen et al. [25], using ferric chloride and other chemicals to enhance the performance of Membralox 0.2, 0.5 and 0.8 pm membranes, describe permeate fluxes between 1400 and 34001/m h. Without pretreatment however severe fouling occurred as well as break-through of oil. Zaidi et al. [26] report about the continuation of this work. They quote fluxes between 800 and 12001/m h, but also mention substantial lower fluxes in long term pilot tests using 0.8 pm membranes. In addition they indicate a drop in permeate flux caused by conditions of low pH, the presence of sea water, corrosion inhibitor, oil slugs or flow variations. 

F. 2.1 Flow configurations obtained in miciochannels a Drop flow, b Plug (slug) flow, c Plug-drop flow, d Deformed interface flow, e Aimular flow, f Parallel flow, g Plug-dispersed flow, h Dispersed flow (Kashid et al. 2011)... 

The flow regimes observed in liquid-liquid flow in microchannels such as drop, slug, slug-drop, deformed interface, annular, parallel, and dispersed flow are depicted in Figure 7.9. 

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]. 

Table 7.8 Mass transfer literature on slug-drop and deformed interface flow. ...

Figure 0.4 Observed flow regimes in the capillary microreactor (Y-junction ID = 1 mm, capillary ID = 1 mm), (a) Slug flow, (b) drop flow, and (c) deformed interface flow. (Adapted from Kashid, M.N. and Agar, D.W., Chem. Eng. J. 131, 1, 2007.)...

The flow patterns are combined into three regions surface tension-dominated region (slug flow), transition (slug-drop and deformed interface flow), and inertia dominated-region (annular or parallel flow). The following criteria were obtained ... 

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. 

In biphase systems velocity of the steam is often 10 times the velocity of the liquid. If condensate waves rise and fill a pipe, a seal is formed with the pressure of the steam behind it (Fig. 2). Since the steam cannot flow through the condensate seal, pressure drops on the downstream side. The condensate seal now becomes a piston accelerated downstream by this pressure differential. As it is driven downstream it picks up more liquid, which adds to the existing mass of the slug, and the velocity increases. 

Whenever two-phase flow is encountered in facility piping it is usually in flowlines and interfield transfer lines. Some designers size liquid lines downstream of control valves as two-phase lines. The amount of gas involved in these lines is low and thus the lines are often sized as singlephase liquid lines. Oversizing two-phase lines can lead to increased slugging and thus as small a diameter as possible should be used consistent with pressure drop available and velocity constraints discussed in Volume 1. 

Equations 8.57 and 8.58 are satisfactory except at low liquid rates when the frictional pressure drop is a very small proportion of the total pressure drop. Frictional effects can then even be negative, because the liquid may then flow downwards at the walls, with the gas passing upwards in slugs. 

The flow patterns (expansion of the bubbly, slug and annular regions of flow) affect the local pressure drop, as well as the pressure oscillations in micro-channels (Kandlikar et al. 2001 Wu and Cheng 2003a,b, 2004 Qu and Mudawar 2003 Hetsroni et al. 2005 Lee and Mudawar 2005a). 

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