CFD investigation on reverse flow characteristics in U-tubes under two-phase natural circulation

Abstract

During the small break loss of coolant accident (SBLOCA) in a pressurized water reactor (PWR), single and two-phase natural circulations (NCs) as well as reflux condensation can be the three major flow modes in the primary side to remove the residual core heat. The performance of U-tube steam generators (UTSGs) during the accident is of great significance for the reactor safety and integrity. However, flow behaviors in UTSG parallel U-tubes are considerably complicated and the reverse flow phenomenon may occur during both the single and two-phase NCs. Despite the rich researches on the phenomenon occurring under the single-phase NC, studies concerning that under the two-phase NC are still quite scarce. In this paper, the vapor-liquid two-phase flow condensed in the U-tube is numerically simulated using the computational fluid dynamics (CFD) method. The distribution of vapor volume fraction αg in the U-tube is investigated and analyzed. It is shown that a negative slope region obviously exists in the curve of pressure drop versus mass flow rate, and two-phase flow excursion can occur if the mass flow rate reduces to a critical value, namely the critical mass flow rate Mc. Afterwards, some influence factors on αg and Mc are numerically studied, which include the inlet vapor volume fraction, the bubble diameter, the inlet slip ratio, the U-tube wall temperature and the primary side pressure. The results can provide references for the long-term core cooling during a SBLOCA and the optimization design of SGs.