An Experimental Study of Oil-Water Flow in Pipes

Abstract

The study reported in this thesis aims at improving the understanding of oil-water flows in horizontal and slightly inclined pipes.

The experimental activities are carried out in the multiphase flow facility at Telemark University College in Porsgrunn, Norway. The experiments are performed using Exxsol D60 oil (density 790 kg/m3 and viscosity 1.64 mPa s) and water (996 kg/m3 and viscosity 1.00 mPa s) as test fluids at room temperature and atmospheric outlet pressure. The test section is a 15 m long steel pipe with inner diameter equal to 56 mm. The pipe inclination is changed in the range from 5° upward to 5° downward. The cross-sectional distribution of phase fractions in the flow is measured using a traversable single-beam gamma densitometer. The total pressure drop over the test section is measured using differential pressure meters and the frictional pressure drop is estimated based on water hold-up measurements. The instantaneous local velocities are measured using Particle Image Velocimetry (PIV) and based on the instantaneous local velocities mean velocities and turbulence fluctuations are estimated.

The observed flow patterns are categorized into eight different flow regimes and presented in terms of flow pattern maps for different pipe inclinations. In general, higher water hold-up values are observed for upwardly inclined pipes. In downwardly inclined pipes, lower water hold-up values are observed compared to the horizontal and upwardly inclined pipes. The measurements show that the water hold-up of oil-water flow is very sensitive for pipe inclination at lower mixture velocities for near horizontal flows (-1º and +1º). At low mixture velocities, the slip ratio decreases as inlet water cut increases. This decrease is prominent in the upward inclinations, while it is more moderate in the downward inclinations. As the mixture velocity increases the slip ratio approaches one, due to increased level of mixing.

The frictional pressure drop is estimated based on measured total pressure drop and averaged water hold-up data. At low mixture velocity, there are smaller variations of frictional pressure drop with inlet water cut. A peak in pressure drop is observed at higher inlet water cuts at higher mixture velocities. The observed peak in frictional pressure drop can be attributed to the dispersion effect in the oil phase. The measured frictional pressure drop profiles for horizontal and inclined flows show some similarities in their overall shape. However, the frictional pressure drop in upwardly inclined pipes is slightly higher than the corresponding flow in horizontal pipes. This can be directly attributed to the increased turbulence level due to the interfacial waves observed in upwardly inclined flows. However, marginal differences in the frictional pressure drop are observed between horizontal and downwardly inclined flows. PIV measurements are validated using single-phase flow data. Two-phase PIV measurements are presented for horizontal and slightly inclined oil-water flows.

PIV measurements of horizontal oil-water flow are compared with LDA measurements that were performed on a similar experimental set-up by Elseth (2001). A good agreement is observed between PIV and LDA measurements. A damping effect of Reynolds stress was observed close to the interface due to stable density stratification and the damping effect is pronounced at inlet water cut 0.50. PIV measurements of inclined oil-water flows show that the mean velocity and turbulence profiles strongly depend on the pipe inclination.

Two-dimensional axisymmetric computational scheme has been developed for predicting single-phase flow. The predictions show a good agreement with the experimental data. Stratified oil-water flows have been simulated based on Volume of Fluid (VOF) model and two equation turbulence models. Observed deviations between the predicted and measured data are discussed.