The present study has attempted to investigate phase inversion and frictional pressure gradients during
simultaneous vertical flow of oil and water two-phase through upward and downward pipes. The liquids
selected were white oil (44 mPa s viscosity and 860 kg/m3 density) and water. The measurements were
made for phase velocities varying from 0 to 1.24 m/s for water and from 0 to 1.87 m/s for oil, respectively.
Experiments were carried either by keeping the mixture velocity constant and increasing the dispersed
phase fraction or by keeping the continuous phase superficial velocity constant and increasing the dispersed
phase superficial velocity. From the experimental results, it is shown that the frictional pressure
gradient reaches to its lower value at the phase inversion point in this work. The points of phase inversion
are always close to an input oil fraction of 0.8 for upward flow and of 0.75 for downward flow, respectively.
A few models published in the literature are used to predict the phase inversion point and to compare
the results with available experimental data. Suitable methods are suggested to predict the critical
oil holdup at phase inversion based on the different viscosity ratio ranges. Furthermore, the frictional
pressure gradient is analyzed with several suitable theoretical models according to the existing flow patterns.
The analysis reveals that both the theoretical curves and the experimental data exhibit the same
trend and the overall agreement of predicted values with experimental data is good, especially for a high