Analysis of reverse flow in U-tubes of UTSG in rocking motion

Highlights: • The flow and heat transfer of fluid in U-tubes in the ocean are different from that in the land. • The relation between pressure drop(PD) and mass flow rate(MFR) in U-tubes in rocking motion is derived. • The reverse flow in U-tubes in rolling and pitching motion is analyzed theoretically. • The rocking motion will change the location of inflection on the curves of PD with MFR. • The effect of rolling motion on the reverse flow in U-tubes is greater than that of pitching motion. - Abstract: For natural circulation, it is shown that reverse flow occurs in the tubes of inverted U-tubes steam generator (UTSG), which has a negative effect on the operation of reactor. Under the ocean conditions, the UTSG will rock with the movement of ship, and the situation of flow and heat transfer of fluid in U-tubes will also be different from that under the land conditions. The typical rocking motion can be divided into the rolling and pitching motion. Based on the models of frictional resistance and heat transfer for the incompressible turbulent flow in tubes in rocking motion, the relationships between pressure drop and mass flow rate of single-phase water in U-tubes under the rolling and pitching conditions are derived. And the phenomenon of reverse flow in U-tubes is analyzed theoretically. The research shows that the rolling and pitching motion will change the location of inflection on the curves of pressure drop versus mass flow rate, which has an effect on the phenomenon of reverse flow. The oscillating amplitudes of critical pressure drop and critical mass flow rate will increase with the decrease of rocking period and the increase of rocking amplitude, and the effect of rolling motion on the reverse flow in U-tubes is greater than that of pitching motion.