Flow structure and flow regime transitions of downward two-phase flow in large diameter pipes

Downward two-phase flow in large diameter pipes appears in numerous industrial applications and nuclear reactor accidents. In this study, adiabatic air-water two-phase flow experiments in a 203.2 mm diameter pipe have been conducted to investigate flow regimes and their transitions in downward and horizontal flow. Three flow regimes (cap-bubbly, churn-turbulent and annular flow) were recognized in downward flow, as well three flow regimes (stratified, plug and pseudo-slug flow) were observed in horizontal section. Evolution of void fraction and flow structure along the loop under different flow conditions has been discussed. The Probability Density Function (PDF) and Cumulative Probability Density Function (CPDF) of area-averaged void fraction signals were utilized as the indicators for self-organized neural network (SONN) method to identify horizontal and vertical downward flow regimes, respectively. The downward flow regime maps for 203.2 mm diameter pipes have been proposed and compared with that for different diameter pipes. The results show that the flow regime maps agree well with that of 101.6 mm, but don't agree well with that of smaller diameter pipes (25.4 mm and 50.8 mm). It is found that the transition between churn-turbulent and annular flow occurs at a certain superficial liquid velocity regardless of superficial gas velocity. A set of new transition criteria have been developed for downward flow regime transitions in large diameter pipes, and validated by the experimental data of 203.2 mm and 101.6 mm diameter pipes. Compared with existing models, these criteria provide more accurate predictions for downward flow regime transitions in large diameter pipes.