Slug flow pressure drop

For the two-phase flow, the total pressure drop will be approximately equal to the sum of the pressure drops across the individual liquid slugs, the pressure drop across the gas slugs being negligible in comparison with that for the liquid, slugs. For a power-law fluid in laminar flow at a velocity of in a pipe of length L, the pressine drop (—Api) is given by (equation 3.6, Chapter 3) ... 

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. 

Figure 35. Pressure drop flow diagram for slugging bed.

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). 

The frictional pressure gradient is obtained by different correlations described in following sections. In a horizontal flow, (dp/dz)elev = 0, it is an ideal case to perform experiments excluding the term of elevation pressure drop. Because of nonhomogeneity of the slug flow, the acceleration pressure gradient term is different from that shown above it is given in Section 3.5.2.2. 

Slug flow. As a very complex, unsteady, turbulent two-phase flow, slug flow typically has high acceleration and decelerations of liquid, which causes steep pressure drops. Dukler and Hubbard (1975) considered the basic hydrodynamic structure of a slug unit as consisting of several regions (Fig. 3.39) ... 

Barnea, D., 1990, Effect of Bubble Shape on Pressure-Drop Calculations in Vertical Slug Flow, Int. J. Multiphase Flow, 16 79-89. (3)... 

Jens, W. H., and P. A. Lottes, 1951, Analysis of Heat Transfer, Burnout, Pressure Drop, and Density Data for High Pressure Water, USAEC Rep. ANL-4627, Argonne Natl. Lab., Argonne, IL. (4) Jepson, W. P., and R. E. Taylor, 1993, Slug Flow and Its Transitions in Large-Diameter Horizontal Pipes, Int. J. Multiphase Flow 19(3) 410420. (3)... 

The calculation of pressure drop in vertical slug flow requires an understanding of the distribution of shear stress and pressure around a... 

In the region of slug flow, Chenoweth and Martin s correlation of superficial liquid volume fraction against a pressure-drop ratio can be represented by an expression... 

Dense-phase (low-velocity slug-flow) solids mass flow = 6th-1, air mass flow =0.065kgs-1, solids loading = 26, pipehne pressure drop = 120 kPa, air velocity = 2.9 to 6.3 ms-1. 

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