The effects of surface tension on flow characteristics of two-phase annular flows in vertical pipes of 5 to 16 mm i.d. have been studied experimentally and analytically. In the adiabatic experiments, three kinds of liquids mainly with different surface tension were used as the test liquids while air as the test gas. The data obtained were instantaneous and mean void fraction, pressure drop and flow regime, etc. at near atmospheric pressure. The effects of surface tension on each flow parameter were studied by comparing the data for the different liquids. In the analysis, the above data were compared with calculations by various correlations in literatures. Regarding the void fraction, the calculations were conducted mainly by the well-known two-phase two-fluid model. The constitutive equations of the interfacial friction force were tested against the data. The results of the above experiments and comparisons are reported in this paper.

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