Characteristics of Cocurrent Two-Phase Downflow in Tubes

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Abstract
An experimental study was made in a cocurrent downflow for air-water system in tube on flow pattern, void fraction and pressure drop. In addition to wetted wall flow which is a distinguished feature in downflow, the same kinds of flow patterns as in up-flow were observed. They were represented on the flow map of downflow with the same variables as those of upflow. The flow maps showed that gas phase is relatively hard to exist in the form of bubble in the cocurrent downflow. General correlations of the void fraction and the pressure drop in a cocurrent down-flow were obtained by applying the following equations which have been established in upflow, i.e. α/(1-α)(1-Kα)=β/(1-β) for void fraction and φl=(1.α)- z for pressure drop. The determined values of K and Z by using the experimental results in the present study and other experimental works in cocurrent downflow wereK=2.0–0.4/β for β≤0.2, K=-0.25+1.250 for β≥0.2 and Z=0.90. Comparisons between the predicted values by the presented correlations and experimental data showed satisfactory agreement.

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