Experimental study on vertical downward air-water two-phase flow in a large diameter pipe

Downward two-phase flows in large diameter pipes are important in various industrial applications, especially for the safety analysis in nuclear reactors. To address the issue that few data of downward flow in large diameter pipes is available for model evaluation, experiments of air-water downward flow in a pipe with inner diameter of 203.2 mm have been performed. Area-averaged void fraction and pressure measurement, as well as flow visualization, have been conducted at several axial locations. The flow conditions for superficial gas velocity range from 0.05 m/s to 3.00 m/s and for superficial liquid velocity range from 0.1 m/s to 1.5 m/s, which cover cap-bubbly flow, churn-turbulent flow and annular/falling film flow. The flow structure at several axial locations and the transition from churn-turbulent flow to annular/falling film flow have been discussed. Current available drift-flux models developed for downward flow in regular pipes as well as for upward flow in large pipes are evaluated using newly collected data. For churn-turbulent flow, the data indicates a larger drift velocity than the model prediction. Corresponding drift-flux constitutive equations are suggested which can reduce the prediction error from 34.37% to 11.79%.