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We investigate the effect of a return U-bend on flow behaviour in the vertical upward section of a largediameter pipe. A wire mesh sensor was employed to study the void fraction distributions at axial distances of 5, 28 and 47 pipe diameters after the upstream bottom bend. It was found that, the bottom bend has considerable impacts on up-flow behaviours. In all conditions, contour plots of the crosssectional phase distribution measurement using the wire mesh sensor (WMS) show that centrifugal effect of the U-bend causes appreciable misdistribution in the adjacent straight section. However, flow asymmetry significantly reduces at an axial distance of 47D from the U-bend. Flow regime maps generated from three axial locations showed that, in addition to bubbly, intermittent and annular flows, oscillatory flow occurred particularly when gas and liquid flow rates were relatively low. At this position, the mean void fractions were in agreement with those from other large-pipe studies.Comparisons were made with existing void fraction correlations. Among the correlations surveyed, drift flux-type correlations were found to give the best predictive results.

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We present an experimental study on the flow behaviour of gas and liquid in the upward section of a vertical pipe system with an internal diameter of 101.6 mm and a serpentine geometry. The experimental matrix consists of superficial gas and liquid velocities in ranges of 0.15 to 30 m/s and from 0.07 to 1.5 m/s, respectively, which cover bubbly to annular flow. The effects on the flow behaviours downstream of the 180° return bend are significantly reduced when the flow reaches an axial distance of 47 pipe diameters from the U-bend. Therefore, reasonably developed flow is attained at this development length downstream of the bend. Other published measurements for large-diameter film thickness show similar trends with respect to the superficial gas velocity. However, the trends differ from those of small-diameter pipes, with which the film thickness decreases much faster with increasing gas flow. As a result, only a few of the published correlations for small pipe data agreed with the experimental data for large pipe film thickness. We therefore modified one of the bestperforming correlations, which produced a better fit. Qualitative and statistical analyses show that the new correlation provides improved predictions for two-phase flow film thickness in large-diameter pipes.

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