An experimental investigation has been carried out to analyze the effect of the diameter on two-phase flows in vertical large-diameter pipes. This study generated and gathered experimental data for diameters ranging from 19 to 297 mm. The working fluids for the experimental database are water and air, for superficial liquid velocities ranging from 0.2 to 0.7 m/s, and superficial gas velocities ranging from 0.1 to 30 m/s.
From the experimental observations of flow regimes in diameters larger than 97 mm, churn flow is observed under conditions that slug flow would be expected in smaller diameter pipes. The experimental data indicate that the holdup and pressure gradient is systematically higher for the 297 mm pipe, compared to the smaller diameters, when the total pressure gradient is gravitationally dominated. However, the total pressure gradient only changes approximately 25% for pipe diameters ranging from 97 to 297 mm and superficial gas velocities lower than the minimum point in the pressure gradient curve. In contrast, the liquid holdup varies up to 100% for the same range of pipe diameters and gas/liquid velocities. The measurements of liquid holdup and pressure gradient are also compared to a simplified model based on the Drift-flux concept. The results from this simplified model shows that the liquid holdup can only be roughly estimated, however, the pressure gradient predictions stay mostly within 25% of the experimental data.