Significant two-phase flow-induced forces were previously measured on a 20 mm diameter U-tubes over a range of homogeneous void fractions. To investigate the effect of tube diameter, new experiments were conducted using vertical 52 mm diameter U-tubes. In the experiments, two parameters were varied, the homogeneous void fraction and the homogeneous velocity. All the results were compared with those for the smaller diameter tubes. The geometrical characteristics of the smaller diameter test sections are similar. L/D = 56 and the ratio of the bend curvature radius to inner diameter: R/D = 4. For all conditions tested, we compared the vertical upward flow-patterns for four different tube diameters. The observed flow regimes agreed relatively well with those predicted from two-phase flow pattern maps, except for the slug/churn transition. A specific two-phase flow pattern map is proposed for the 52 mm diameter tube, using a more accurate transition model from slug to churn flow. The forces comparison confirms the dependency with tube diameter. The RMS forces were found to increase with tube diameter for all void fractions studied. Conversely, the dominant frequency decreases with tube diameter, linearly for void fractions of 50 and 75%. Dimensionless forces and spectra for the smaller diameter tubes show good data collapse using the proposed model. Two-phase flows in the 52 mm diameter tube generate lower dimensionless spectra. Finally, a dimensionless model is proposed for practical application.

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