Thermal-hydraulic characteristics of upward two-phase flows in vertical large size square channels

Economic Simplified Boiling Water Reactor (ESBWR) concept has been proposed to improve the reactor safety feature. The core flow in ESBWR is driven by natural circulation induced by the difference between two-phase flow density in a partitioned chimney and single-phase flow density in a downcomer. The accurate prediction of the void fraction in the chimney is the key to calculate the two-phase flow density. The geometry of the partitioned chimney is a square channel with 0.61 m in the current ESBWR design, which is considered a large-sized square channel. Extensive research has been conducted for two-phase flows in large-sized circular channels, but the research for two-phase flows in large-sized square channels is very limited. In view of this, the current paper summarized the current understanding of the two-phase flow behavior in large-sized square channels and reviewed existing data collected in large-sized square channels. The two-phase flow parameters considered in the assessment were critical channel size at the boundary between medium and large-sized channels, developing length, void fraction, interfacial area concentration, flow regime transition, and frictional pressure drop. The assessment clearly indicated that experimental study in large-sized channels was very limited. Most datasets were collected in atmospheric air-water bubbly flows. Thus, the constitutive equations of two-phase flows were not fully evaluated. Some analytical studies for critical channel size and distribution parameter were introduced to give insight into the geometrical effect on the two-phase flow parameters. This paper strongly recommended collecting fundamental data.