A numerical study on two-phase core-annular flows of waxy crude oil/water in inclined pipes

Abstract

In the present work, a 3D numerical study has been performed to investigate the core-annular flow of waxy crude oil/water in inclined pipes with a gradual expansion. Waxy crudes are highly viscous crude oils that exhibit non-Newtonian flow behavior, and their efficient transportation is still a technical challenge. In the current work, the use of the core-annular method for the transportation of waxy crudes, which are modeled as viscoplastic fluids, is examined comprehensively where the core oil flows in the laminar flow regime, and the water flow field is turbulent. The volume of fluid (VOF) multiphase flow model is used to capture the oil/water interface, and the SST k-ω turbulence model has been employed to predict the turbulent features of the water flow field. The governing equations are discretized and solved using the finite volume method on a structured numerical grid. The effects of several parameters, such as the wax content of the crude oil, inlet velocities, the expansion angle, and the inclination angle of the pipe have been investigated on the non-Newtonian core-annular flow comprehensively. The results revealed that as the wax content of the crude oil increased, the use of the core-annular flow became more economically lucrative for the transportation of waxy crude oils in comparison to the conventional single-phase oil transportation. Moreover, The simulation results indicated that increasing the expansion angle in the core-annular regime from 3.7° to 45° elevated the overall pressure drop more than fourfold. Finally, it is shown that for downward flows, by increasing the inclination angle, the overall pressure drop monotonically decreased. However, in upward flows, the overall pressure drop profile as a function of the inclination angle had a local maximum around 45°.