Investigation on the role of mass flow rate in UTSG reverse flow under natural circulation condition

Abstract

In view of the phenomenon of reverse flow in partial inverted U-tubes of inverted U-tube steam generator (UTSG) under natural circulation condition, the numerical calculation method is used to study the flow and heat transfer in the primary side of UTSG. There are many factors influencing reverse flow in UTSG, such as the inverted U-tube length, coolant mass flow, operating pressure and temperature difference between primary and secondary side. However, in a specific UTSG, the lengths of different inverted U-tubes and the variations of internal mass flow rates are not uniform, so it is difficult to study the reverse flow mechanism. In this paper, we study the reverse flow of UTSG with identical tubes. Results show that under natural circulation condition, it is more favorable to reduce reverse flow as total inlet mass flow rate in UTSG increases. In addition, the critical value of reversion is calculated, which provides a reference for the engineering design and reactor safety analysis of UTSG. The spatial distribution of the reverse tubes is related to the inlet initial mass flow rate of U-tube and the U-tube with small inlet initial mass flow rate is more likely to happen reverse flow. U-tube with least inlet mass flow rate firstly triggers to the inflexion point of reversion, and the second least one happens reverse flow next while the total inlet mass flow rate in UTSG decreases gradually.