Performance of drift-flux correlations for predicting void fraction of two-phase flow in tight-lattice rod bundles

In view of the energy system design involving two-phase flow in various industrial apparatus, the void fraction is one of the most critical parameters, which characterizes gas fraction in a two-phase mixture. Accurate prediction of a void fraction is indispensable to estimate two-phase mixture levels and represent the flow behaviors in a given flow channel. A drift-flux type correlation has been utilized in existing one-dimensional thermal-hydraulic system analysis codes to calculate various two-phase flow parameters in a rod bundle. Due to the demand for higher performance of equipment, a tight-lattice configuration of the rod array has been given increased attention. The tight-lattice rod-bundle geometry has a much smaller rod diameter-to-rod pitch length than a typical rod-bundle utilized in equipment, such as compact heat exchangers, advanced steam generators, and next-generation nuclear reactors. As a result, it is expected that the flow characteristics may differ from conventional rod-bundle geometries. Therefore, developing the reliable void fraction prediction model for accurately simulating the two-phase flow behavior in a tight-lattice rod-bundle will be essential from the safe, economical, and robust design aspect. The present study extensively reviews existing drift-flux correlations applicable to rod-bundle geometries. Their performances were assessed against available experimental data collected from test facilities equipped with tight-lattice configuration, and applicability towards tight-lattice bundle geometry was considered.