In order to clarify the cause of tube failures in high pressure waste heat boilers with vertical U-shaped tubes

a series of tests was made to study flow pattern transitions over wide velocity ranges of ambient air-water mixtures in a 21.5mm 1. D. vertical U-shaped tubes with 180°-bends of radii 694

500 and 320 mm respectively. The flow patterns observed in these bends were similar to those in horizontal two-phase flow. The data obtained showed that the differences from the horizontal flow were the expansion of stratified flow and restriction of wavy and intermittent flow. Both annular and dispersed transitions were quite close to those in horizontal tubes. The transition data obtained in the present tests were compared with the transition correlations of Mandhane et al. and Weisman et al. forhorizontal tubes. An overall flow pattern diagram based on the present tests was established in terms of superficial gas and liquid velocities

VsG and VSL. Correlations for determining flow pattern transitions were suggested. The effects of the bend radii on flow pattern transitions are small within the test range.