TWO-PHASE FLOW AND HEAT TRANSFER IN VERTICAL U-SHAPED TUBES(Ⅰ)FLOW PATTERN TRANSITIONS IN THE BEND

The experiments on transitions over wide velocity ranges between two-phase flow patterns during co-cur- rent air-water flow have been conducted in 21.5 mm I.D.vertical U-shaped tubes with a 180°bend of radii 694,500 and 320 mm,respectively.The flow pattern were determined by visual observation and electrical con- ductance probe.The transition data were compared with the Mandhane et al.’s and Weisman et al.’s transi- tion correlations for horizontal tubes.An overall flow pattern diagram based on the present tests was established in terms of superficial gas and liquid velocities V_SG and V_SL,and correlations for determining flow pattern transitions were suggested.