Numerical simulations for turbulent oil-water core-annular flow

Abstract

The transportation of high viscosity oil is an ongoing research topic for quite some years because of its importance in industrial applications and the increasing energy requirements of the world population. The water lubricated oil core with a core-annular flow configuration is a viable option for the transport of high viscosity oil. It was observed that there are significant differences between the predictions from the RANS simulations and the experimental data for turbulent oil-water core-annular flow obtained in the flow loop in the lab at TU Delft. Thus, this study is performed to understand the difference between the RANS simulations and the experimental data. 1D and 2D OpenFOAM simulations for turbulent vertical core-annular flow are performed using the Launder-Sharma low-Re k-ε turbulence model. The pressure drop and the oil holdup are imposed while the total flow rate and the water-cut is obtained as the output from the simulations. The results from the OpenFOAM simulations are compared with the DNS data available in literature to understand the impact of the turbulence characteristics and the interfacial waves. Both 1D and 2D simulations were carried out. The 1D model does not include the interface waves. The 2D axisymmetric model is able to capture the travelling interface waves. The 1D OpenFOAM simulations are carried out for five cases with different oil holdups and the results are compared with the asymptotic wall laws. It is concluded that the turbulence is sufficiently resolved by the 1D simulations. The 2D simulations are also performed for five different oil holdups. The 2D simulations are first validated by performing grid independence tests. The dependence of the simulation results on the streamwise domain length, presence of gravity and oil viscosity is also analysed. It was found that the streamwise domain length and the oil viscosity used does not have a significant influence on the prediction of the results. The comparison of the RANS simulations with the DNS data shows that the difference depends on the turbulence levels inside the simulation domain. The difference for the total flow rate between the 2D OpenFOAMsimulations and the DNS data is found to be 10% for fully turbulent core-annular flow while the difference increases to 29% for core-annular flow with no turbulence. The friction factor for the 2D Open- FOAM simulations is found to be about 17% lower for fully turbulent flow while it is 39% lower than the DNS predictions for the simulation with no turbulence. The difference for the holdup ratio (when comparing 2D OpenFOAM simulations with respect to DNS) decreases first from 15% higher for the low oil holdup cases to 8% higher for simulation with oil holdup fraction 0.71 and then increases to 14% higher for the highest oil holdup case. It is also observed from the comparison of the mean streamwise velocities and the mean shear stresses that the turbulence level in the simulation domain impact the extent of the difference. The agreement in the wave characteristics between the 2D OpenFOAM simulations and the DNS data depends on the mixture of wavelengths present in the simulation domain. The RANS simulations predict a specific dominant wavelength while the DNS data gives a mixture of wavelengths for lower oil holdups. The dominance of a specific wavelength increases with increasing oil holdup for the DNS predictions.