Effect of pipe inclination on the hydrodynamics of slug flow

Abstract

There is a general lack of studies addressing the optimization of hydrocarbon flow conditions in pipes passing through hilly terrains. Most of the studies carried out on two-phase flow are for fluids flowing through vertical and horizontal pipes. For studies carried out on inclined pipes, a large proportion are on air-water mixtures. To address the aforementioned challenge, there is a need to study the hydrodynamics of slug flow in various pipe inclinations with a focus on fluids with properties that replicate the behavior of effluents seen during crude oil production. This study provides a foundational perspective into the physical phenomenon that governs the behaviour of slug flow and the variation of slug flow parameters as the pipe deviated from horizontal to vertical. To clarify the effect of pipe inclination on the hydrodynamics of slug flow, the characteristics of fully developed multiphase air-silicone oil mixture flowing in a 67 mm diameter pipe and inclined at 0, 30, 45 and 90 to the horizontal was investigated. Fluid flow properties over the range of superficial gas and liquid velocity were measured using Electrical Capacitance Tomography (ECT) and analyzed using Probability Density Function (PDF) and Power Spectral Density (PSD) plots to determine; mean void fraction, void fraction in the liquid slug and Taylor bubble, structure velocity, slug frequency and slug unit. The mean void fraction decreased as the pipe inclination advanced from 0° to ~45°, it then increased between ~45° and 90° to the horizontal; the angle at which it begins to increase seems to be around 45°. The void fraction in the liquid slug increases as pipe inclination tends towards the vertical from the horizontal axis. In the case of the void fraction of the Taylor bubble, no unique trend was observed as the pipe inclination changed. The slug frequency increased with pipe inclination and liquid superficial velocity at a constant gas superficial velocity. In addition, the structure velocity increased as the pipe inclination advanced from 0° to 45°, it slightly decreased again between 45 and 90 to the horizontal. The slug unit in horizontal pipes is far greater than that obtainable in pipes inclined between 30° and 90°, at superficial velocities of 0.05 m/s and 0.157 m/s. However, at higher superficial velocities between 0.262 m/s and 0.514 m/s, the slug unit ranges from 0.6 to 1.8 m for all pipe inclinations. It was also confirmed that for upward inclined flow, the dominant flow pattern is intermittent flow.