The helical-coiled once-through steam generator (H-OTSG) has the advantages of compact structure and strong heat transfer ability, which is appropriate for lead-cooled fast reactor (LFR). The two-phase flow instability may cause mechanical vibration and thermal fatigue of heat transfer tube bundles, posing a serious threat to the safe operation of steam generators.

This study aims to explore the oscillation modes and influencing laws of two-phase flow instability of H-OTSG, providing reference for industrial design.

Firstly, RELAP5/MOD3.4 code was applied to modelling the helical-coiled once-through steam generator with 14 parallel heat exchange tubes. The primary working fluid of H-OTSG was liquid lead bismuth eutectic (LBE) and the secondary fluid was water. Then, the oscillation behavior during start-up was studied based on time-domain method and the oscillation characteristics and the parameter sensitivity of the stable boundary were analyzed. The limit cycles of the oscillation in each channel were shown on the pressure-drop vs. flow-rate plane. Finally, the influence of structural parameters on system stability were explored, so did that of operating parameters such as the pressure, flow rate, and temperature of the secondary fluid.

The results indicate that the operating parameters exhibit density wave oscillations at the heating section in a (n-2,2) pattern, with superimposed flow pattern transition instability. The smaller the flow amplitude, the shorter and thinner the corresponding limit cycle, and the closer to the circle. In the same channel, as the driving force increases, the flow amplitude gradually decreases, and the limit cycle also gradually shrinks. In addition, as the inlet throttling has been increased from 1 100 to 1 700, the duration of oscillation shortened from approximately 6 000 s to less than 1 000 s, and the amplitude decreased by nearly 30%. With the increase of the outlet throttling from 0 to 200, the duration of oscillation has been lengthened from less than 5 400 s to approximately 18 000 s. In addition, the steam temperature and the power-flow ratio of the channel increase with the increase of outlet throttling, resulting in reduced system stability. As the system pressure is increased from 3.7 MPa to 4.4 MPa, the oscillation duration of the flow curve shortens from 10 000 s to less than 5 000 s, and the amplitude also decreases. The density difference between the liquid phase and vapor phase decrease with the increase of system pressure. As a result, two-phase frictional resistance is decreased, the self-sustained oscillation of mass flow is suppressed, and the system stability is increased.

Results of this study demonstrate that the system stability of the helical-coiled once-through steam generator can be improved by increasing inlet throttling and system pressure and reducing outlet throttling, and involved structural and operational parameters should be focused on during the design process.