Two-phase flow patterns in U-bends and their contiguous straight tubes for different orientations, tube and bend diameters

In two-phase systems there are many singularities in piping systems, one of which in particular are U-bends (also called 180° return bends or U-tubes) found very often in air conditioning coils, refrigerant systems, etc. Contrary to the literature concerning two-phase flow patterns in straight tubes, where many studies are available, only a limited number of studies concerning bent tubes can be found. In this study, observations of two-phase flows in U-bends and their contiguous straight tubes were made with R134a at 5 °C for three different orientations (horizontal flow, vertical upflow and vertical downflow). Five different glass test sections with three different internal diameters (13, 11 and 8 mm) and five different bend diameters (67, 38, 55, 32, 25 mm) were used. The experiments were made for three mass fluxes (150, 300 and 500 kg s−1 m−2) and covered vapor qualities ranging from 0.05 to 0.95. A total of 342 flow pattern observations were made during this experimental campaign. Analysis of the videos showed that, for the present range of experimental conditions, the centrifugal force was dominant over that of the gravity. In the U-bend, the flow patterns are qualitatively similar to those observed in the straight tube sections before the bends. Upstream of the U-bend, in the straight tubes the flow pattern are not modified. Downstream of the U-bend the flow patterns are generally reestablished shortly, except for vertical upflow orientations.