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In single- and two-phase flow heat exchangers (in particular "coils"), besides the straight tubes there are also many singularities, in particular the 180° return bends (also called return bends or U-bends). However, contrary to the literature concerning pressure drops and heat transfer in straight tubes, where many experimental data and prediction methods are available, only a limited number of studies concerning U-bends can be found. Few reliable experimental data and no generally proven prediction method is available. Indeed, two-phase in U-bends is an old unresolved design problem in the heat transfer industry influencing evaporating and condensers.

Thus, the present study aims at providing a detailed insight into two-phase pressure drops and flows patterns in U-bends. Based on a new type of U-bend test section, an extensive experimental study was conducted. The experimental campaign covered five test sections with three internal diameters (7.8, 10.8 and 13.4mm), five bend diameters (24.8, 31.7, 38.1, 54.8 and 66.1mm), three flow orientations (horizontal, vertical upflow and vertical downflow), two fluids (R134a and R410A), two saturation temperatures (5 and 10°C) and mass velocities ranging from 150 to 1000kg s-1m-2.