Effect of diameter and geometry on two-phase flow regimes and carry-over in a model pwr hot leg

This paper describes a series of tests investigating two-phase flow characterization and carry-over in a transparent model of a Babcock and Wilcox (BandW) pressurized water reactor (PWR) hot leg geometry. This work was performed, in part, to support the interpretation of results from the Once-Through Integral System (OTIS) and Multiloop Integral Test (MIST) facilities. Test conditions were selected to cover a wide range of gas and liquid superficial velocities expected to occur in a prototypical reactor geometry during a small break loss of coolant accident (SBLOCA). Tests at high gas superficial velocities were also performed for comparison with semi-analytical predictions. Tests were conducted in a test rig with 30.5-cm (12-inch) diameter pipe. Results include average void fraction, amount of water carry-over through the U-bend and a description of the two-phase flow phenomena. Results of these tests indicate that slug flow is not observed in large diameter pipes. Instead, as the air flow rate is increased, the flow regime progresses from bubbly to churn-type flow with the presence of large bubbles (approximately 15-cm diameter). The results also indicate that flow regimes and collapsed liquid level are more strongly dependent on air superficial velocity than the water superficial velocity and that the amount of water carry-over for a given air flow rate is a strong function of collapsed water level. Furthermore, the results show that similar thresholds for breakdown in natural circulation flow exist between small and large diameter pipes for gas and liquid superficial velocities expected in a SBLOCA