Experimental study of air–water two-phase flow in an 8×8 rod bundle under pool condition for one-dimensional drift-flux analysis

► Adiabatic two-phase flow experiments in an 8×8 rod bundle were performed under pool conditions. ► An extensive database of adiabatic/boiling two-phase flows in pool rod bundle systems was collected. ► Effect of large casing on two-phase flow structure was observed in experimental data for medium 〈jg+〉 conditions. ► Existing rod bundle models were benchmarked with the present database. ► All of the void fraction prediction errors of each model are tabulated. In order to establish a reliable rod bundle database under pool conditions and to benchmark the existing models, a well-scaled 8×8 rod bundle test loop was designed based on the scaling criteria and a series of experiments was carried out with adiabatic air–water two-phase flow. Experiments for pool conditions covered the area-averaged void fraction 〈α〉 range of 0.12–0.93. Existing models and experimental data including boiling and air–water two-phase flow were compared and analyzed. Experimental results show that differences exist between large and small casing rod bundles as the flow structure changes with the casing scale. In addition, traditional drift-flux models for pipes may reflect the casing scale effect, but cannot be directly applied to the rod bundle geometry in pool conditions. Among the existing models for rod bundles, the Murase’s model (1986), Kamei’s model (2008) and Ishizuka’s model (1995) give relatively better predictions in most regions. All benchmark results of existing models are tabulated in terms of void fraction prediction error for the present database.