Two-Phase Pressure Drops in Adiabatic Horizontal Circular Smooth U-Bends and Contiguous Straight Pipes (RP-1444)

Curved channels in the form of bends are encountered in many industrial two-phase flow applications, such as U-bends in air-conditioning and refrigeration evaporators and condensers. For engineering design purposes, the evaluation of the pressure loss in two-phase flows is necessary. Basically, for the same flow conditions the pressure loss is higher in a U-bend than in a straight pipe with an equal cross-flow section and mean length. Moreover, the curved tube causes a perturbation that propagates up- and downstream of the U-bend itself Many studies have been made on two-phase flows in bent tubes, but none have yet focused on the axial evolution of the pressure gradients before and after the U-bend or on the peripheral pressure differences that may be created. Therefore, this study reports new data obtained for R-134a at saturation temperatures ranging from 3 degrees C to 5 degrees C (37 degrees F to 41 degrees F), flowing at mass velocities ranging from 155 to 533 kg.s(-1).m(-2) (114.10(3) to 393-10(3) ft(-2)) through a horizontal U-bend tube with internal tube and bend centerline diameters of 13.4 mm (0.52 in.) and 66.1 mm (2.6 in.), respectively, over a wide range of vapor qualities. The experimental results show that the differences between the peripheral inner and outer pressures up- and downstream of the bend are negligible, even very close to the bend (at 4 internal diameters). The frictional pressure gradients in the U-bend were found experimentally to be about two times that in a straight pipe. Close to the bend, up- and downstream, the pressure gradients are highly affected. For lower and medium mass velocities the perturbation caused by the U-bend does not propagate further than 6 internal diameters upstream from the bend. For the higher mass velocity, that perturbation seems to propagate further. Downstream of the U-bend the perturbed length extends over 100 internal diameters.