Experimental analysis of downward liquid-gas slug flow in slightly inclined pipes

Downward gas-liquid slug flow occurs in hilly-terrain pipelines and is frequently observed in oil and gas transportation lines. The onset of this kind of flow owes to instabilities generated by irregular pipe topography. Therefore, a solid understanding on the hydrodynamics of the slug flow is paramount in the design of crude oil production lines as well as in the project of equipment involved in oil and gas operations. The goal of this work is to analyze the mean values of the translational velocity, slug frequency, bubble and slug lengths in ducts with inclination angles from - 4 degrees down to - 13 degrees on experimental grounds. The analyses were performed at different gas-liquid volumetric flow rates for which the slug flow regime was observed. An existing experimental rig in the NUEM/UTFPR labs was used to collect data. A pair of wire-mesh sensors was used to evaluate the flow structure, and thence void fraction temporal series were obtained. From those temporal series, the experimental data were obtained and comparative analyses could then be made. The mean bubble (C-0) and drifty velocity (C-infinity) coefficients were determined for the translational velocity. The mean bubble and slug lengths were compared for minimum and maximum values. A schematic of the unit cell behavior in downward slug flow is proposed.