Summary

Return bends are found in various applications involving two-phase ow, including heat exchangers, transport pipes and separators. Gasliquid ows in bends are aected by centrifugal forces that tend to separate both phases. If the bend is oriented vertically, the ow is also subjected to gravitational eects. The resulting eect of such forces is a change of the ow conguration as it passes through the bend, which depends on several aspects, such as ow direction (i.e., upward and downward), curvature, ow rates, and physical properties. Thus, the ow shows a distinct behavior in the return bend if compared to a straight tube. The main objective of this study is the characterization of air-water two-phase ows in 180 return bends that connect two 5-m long 26-mm ID horizontal tubes. The bend lies in the vertical position and the two-phase ow can be set as upward or downward. The behavior of the static pressure downstream and upstream of the return bend was measured for a wide range of ow regimes, as was the pressure drop and change in gas holdup associated with the return bend itself. The behavior of the phases in the bend was investigated with a high-speed camera, illustrating several particular features of the two-phase ow in the bend in both directions. The experiments were carried out with bend curvatures 2R=d of 6.1, 8.7, and 12.2 in both upward and downward directions. Supercial velocities varied from 0.2 to 40 m/s for the gas, and were set as 0.05, 0.2 and 1 m/s for the liquid. The irreversible pressure changes in the bend were determined based on the dierential pressure and gas holdup measurements, resulting in an empirical correlation. The gas holdup was measured at 12 dierent positions, downstream and upstream of the bend, covering the plug, slug and annular ow regimes in both upward and downward ow directions. This allowed a better understanding of the in uence of the return bend on the ow and the evaluation of ow parameters along the axis of the tube.

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