This study provides a qualitatively visual observation of the two-phase flow patterns for air-water mixtures inside a 3 mm smooth tube with the presence of vertical return bend. The curvature ratio (2R/D) is 3.2 whereas the total mass flux is from 70 to 800 kg/m2 s. The flow can be either entering from the upper of the tube or from the lower tube. However, it is found that there is no great difference between those flow entering at the upper tube and that of the lower tube if the inlet mass flux and vapor quality is the same. For a mass flux of 70 kg/m2 s at a vapor quality to 0.009, as the flow is approaching the return bend, one can observe a fluctuating phenomenon at the tail of the long slug that leads to a liquid ripple around the periphery. When the air slug is trying to penetrate the preceding liquid in the return bend, the shape at the front of the air slug was sharpened. A further increase of the vapor quality to 0.05, the flow after the return bend was temporarily turned from stratified flow into the annular flow. At a higher mass flux of 300 kg/m2 s, unlike those flow pattern at 70 kg/m2 s, the increase of the vapor shear interacts with the centrifugal force and the accumulated liquid within the return bend forces the Taylor bubble to be completely disordered. There is no separating and re-merging phenomenon of the air slug for the slug flow pattern across the return bend even for a very low vapor quality of 0.001. This is quite different from those with larger diameter tube (Chen et al. 2002, Wang et al. 2003b, 2003c).

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