A visual observation of the air-water two-phase flow in small diameter tubes subject to the influence of vertical return bends

This study provides a visual observation of the two-phase flow patterns of the air-water mixtures inside two 3.0 mm and two 6.9 mm smooth tubes subject to the influence of vertical return bend. The effects of tube size (D = 6.9 and 3 mm) and curvature ratio (2R/D = 3 and 7) are examined. The total mass flux is from 50 to 800 kg/m² s. Some unique phenomena such as flow reversal, freezing slug are observed for the vertical return bend having D = 6.9 mm that are absent in previously reported results for horizontal configurations. However, the flow reversal and freezing slug phenomena are hardly seen at a smaller diameter tube (D = 3 mm) due to the influence of the flow inertia and surface tension. For D = 6.9 mm, the elongated bubble across the return bend reveals a separate and re-merged phenomenon but this phenomenon is nearly gone for D = 3 mm. Unlike that of D = 6.9 mm, for the same mass flux and vapor 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.