Experimental investigation on air-water-high viscosity oil three-phase flow in 15° inclined downward pipe

This study presents an experimental investigation of air–water–high-viscosity oil three-phase flow in a 15◦ downward-inclined pipe with an internal diameter of 40 mm, addressing key gaps in three-phase flow, downward-inclined configurations, and high-viscosity oil systems. Flow visualization revealed three distinct regimes, namely: ST–DC (stratified dual-continuous), ST–WC (stratified water-continuous), and IN–WC (intermittent water-continuous), which are based on gas–liquid and liquid–liquid interfacial dynamics. Analysis of the void fraction revealed the presence of inertia-driven, homogeneous, and gravity-driven regimes, with the gas superficial velocity governing the onset of gravity effects. A new void fraction correlation based on the superficial velocities of the three phases is proposed. Frictional pressure drop results demonstrated the limitations of conventional separated flow model, motivating the development of a new correlation based on alternative dimensionless scaling. Furthermore, analysis of pressure signal fluctuations showed that classical dimensionless parameters developed for gas-liquid two-phase flow fail to predict characteristic frequency behavior. Overall, this work provides a comprehensive experimental database and discusses new approaches for modeling the main hydrodynamic parameters of three-phase flow in inclined pipelines, with important implications for flow assurance and well/piping design.