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Flow patterns in oil-water carrying pipes vary due to the flow characteristics, fluid properties and pipe inclination. For inclined pipes, the gravity component along the pipe influences the flow patterns. Plug flow (PF) is one special flow pattern that occurs in slightly upward inclined pipe-lines. Mineral oil-Exxsol D60 (viscosity = 1.6 mPa.s, density =788 kg/m3) and water (viscosity = 1mPa.s, density = 997 kg/m3) were used as test-fluids. A test matrix was carried out to determine the possible flow patterns that occur at upward pipe inclinations +1° , +3° , +5° and +6° for low mixture velocities (0.2–0.5 m/s), and at water-cut 0.9. The plug flow regime was found only for +5° and +6° inclinations, while no plug flow was noticed at +1° and +3° inclinations. Plug flow was found only for lower flow velocity and higher water-cuts. Plug flow patterns were identified both through visual observation and by means of high-speed video imaging. Two new flow patterns ‘oil droplet clusters in continuous oil and water (OC/O&W)’ and ‘distinct oil droplet clusters in water (D-OC/W)’ were introduced, and they occur around the plug flow regime. High-speed images were post-processed for determination of the oil–water interface and subsequently used to calculate the water hold-up. The time averaged water hold up decreased with increasing mixture velocity due to the decrease of oil–water slip as a result of increased degree of dispersion. The oil plugs entrained more droplets as mixture velocity was increased, leading to high-frequency fluctuations of volume fraction of the oil plugs. Hold-up increased with increasing inclination due to the onset of plug flow, which leads to increased slip. The pressure drop over the test section was measured, and the frictional pressure drops were calculated using average water hold-up values. The frictional pressure drop increased with increasing mixture velocity, due to increased mixing and subsequent increase of effective viscosity. The frictional pressure drop decreased with increased inclination due to the appearing of oil plugs and the drag reduction effect associated with the plug flow.
flow patterns, frictional pressure drop, phase slip, plug flow, water hold-up
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