Core-annular flow is an efficient flow regime for the transportation of viscous oils. The viscous oil in the core is surrounded and lubricated with an annulus of water. Water has a low viscosity and therefore reduces the pressure drop. Numerical simulations are performed for horizontal core-annular flow by using the Volume of fluid (VOF) method to solve the Reynolds-averaged Navier-Stokes equations (RANS) in OpenFOAM. Periodic boundary conditions are used for a small pipe section. With periodic boundary conditions the maximum wavelength at the oil-water interface is imposed together with the oil and water holdup fractions.

Numerical results are compared with recent experimental data. Oil viscosities between 3338 cSt at 20°C and 383 cSt at 50°C are solved with a fixed total flow rate. The 20°C simulation converged to the desired water cut of 20%, similar to the experiment. At this 20% water cut a comparable pressure
gradient is found with the experiment. At higher temperatures (i.e. lower viscosities) deviations from the desired water cut of 20% are obtained. The different water cut values lead to differences between the numerical and experimental pressure gradients.

Additional simulations are carried out for the oil viscosity of 718 cSt at 40°C. Instead of a fixed total flow rate these simulations are solved with a fixed pressure gradient. This solving method is found to be considerable faster. Different holdup fractions and pressure gradients are imposed which resulted in different water cut and flow rate values. Numerical results are interpolated for the experimental water cut values of 9%, 12% and 15% at similar flow rates. Differences of 98%, 24% and 37% are found for the water cut of 9%, 12% and 15% respectively. An interpolation is not possible for the experimental water cut of 20% as this experiment is outside the covered solution region of the numerical results. This difference is caused due to an incorrectly used domain length.

The study is finished with holdup estimations of the experiments. Flow visualizations from a high speed camera are used which are made during the experiments. Oil holdup fractions of 0.749 are found for a water cut of 20%. This holdup fraction corresponds exactly with the imposed holdup fraction for the different viscosity simulations which are solves with the fixed total flow rate. Only the 3338 cSt at 20°C converged, however, to the water cut of 20%.