Effect of Tube Wall Wettability on the Onset of Churning in Upward Gas-Liquid Annular Flow

The influence of hydrophobic and hydrophilic tube walls on the pressure drop and liquid loading behavior was investigated in previous projects in the past. In this paper, results of visualization experiments and of experiments at different liquid-to-gas ratios and inclination angles are presented for hydrophobic tube walls. Experiments were performed in coated and uncoated steel and Perspex pipes of different diameters: 20 mm and 50 mm. Experiments with different deviation angles ranging from 0° (vertical) to close to horizontal (≈80°) were performed for the 20mm diameter tube. The impact of the wall wettability on the flow patterns was examined by performing flow visualizations with a high speed camera in coated and uncoated Perspex tubes. From the experiments it becomes clear that the hydrophobic coating prevents the formation of a liquid film on the tube wall. As a result, the transport of the liquid phase is solely in the form of droplets/ligaments. In the hydrophobic coated tube, the onset of churning is at a lower gas velocity than in the uncoated tube. The presence of the coating reduces the critical gas rate corresponding to the minimum pressure drop by as much as 40%. This decrease reduces for high liquid rates and for larger inclination angles. However, for a liquid content of LGR = 1000 Sm³/10⁶ Sm³, the benefit is still 33% at a vertical flow and a reduction in the critical rate remains up to a deviation angle of 80°. The hydrophobic coating increases the friction at higher gas rate but reduces the hydrostatic head at lower gas rate with a 70% reduction at low liquid content.