Particle image velocimetry for characterizing the flow structure of oil–water flow in horizontal and slightly inclined pipes

Abstract

The simultaneous flow of oil and water in pipelines is a common occurrence in the chemical and process industry. An experimental investigation of oil–water flow in horizontal and slightly inclined pipes is presented in this paper. The experiments are performed in a 15 m long stainless steel pipe section with internal diameter 56 mm at room temperature and atmospheric outlet pressure. Exxsol D60 oil (density 790 kg/m 3 and viscosity 1.64 mPa s) and water (density 996 kg/m 3 and viscosity 1.00 mPa s) are used as test fluids. The pipe inclination is changed in the range from 5° upward to 5° downward. The measurements are made for two different mixture velocities, 0.50 and 1.00 m/s at water cut 0.50. The cross-sectional distribution of phase fractions in oil–water flow is measured using a traversable single-beam gamma densitometer. The different flow regimes are determined based on visual observations. The particle image velocimetry (PIV) is utilized in order to obtain non-invasive instantaneous velocity measurements of the flow field. Based on the instantaneous local velocities, mean velocities, root mean squared velocities and Reynolds stresses are calculated. Stratified flow with mixing at the interface is observed at mixture velocity 0.50 m/s. Interfacial waves are observed in upwardly and downwardly inclined flows. At mixture velocity 1.00 m/s, interfacial mixing is increased and dual continuous flows are observed. The degree of mixing largely depends on the pipe inclination. In general, higher water hold-up values are observed for upwardly inclined flows compared to the horizontal and downwardly inclined flows. The slip between the phases increases as the pipe inclination increases. The maximum mean axial velocity is detected in the more viscous oil phase at equal volumetric flow rates of oil and water. In addition, measured mean velocity and turbulence profiles show a strong dependency with pipe inclination. The largest root mean squared velocities and absolute values of the Reynolds stresses are observed close to the pipe wall due to higher mean axial velocity gradients. A damping effect of Reynolds stress is observed around the oil–water interface due to stable density stratification. The presence of interfacial waves enhances turbulence fluctuations in inclined oil–water flows.