Visual observations of air-water two-phase flow across small diameter tubes with vertical return bends

This study provides a qualitatively visual observation of the two-phase flow patterns for the air-water mixtures inside a 3.0 mm and two 6.9 mm smooth tubes with the presence of vertical return bends. The curvature ratios (2R/D) for the 6.9 mm tubes are 3 and 7, whereas 2R/D = 3 for the 3 mm tube. The total mass flux is from 50 to 800 kg/m2·s. Some special phenomena such as flow reversal and freezing slug are observed for the current vertical return bend with D = 6.9 mm relative to previously reported results of horizontal configurations. However, the flow reversal and freezing slug phenomena arc hardly seen in a smaller diameter tube (D = 3 mm) due to the influence of the how inertia and surface tension. Quantitative descriptions of the flow regime of flow reversal and the predicted equation of the retreat length are proposed. For D = 6.9 mm. the air slug across the return bend displays a separated and remerged phenomenon, but it is nearly gone for D = 3 mm. Unlike that of D = 6.9 mm, for the same mass flux and vapour quality with D = 3.0 mm, there is no great difference of the two-phase flow patterns between the flow entering in the upper tube or the flow entering from the lower tube. (author)