Flow instability analysis in U-tubes of steam generator under rolling movement

• The reverse flow in U-tubes under rolling movement is investigated.
• A thermal hydraulic model for the analysis of reverse flow is established.
• The effect of rolling period and amplitude is analyzed by small perturbation theory.
• The transverse swing has more effect on the reverse flow than the longitudinal swing.

The reverse flow in U-tubes under natural circulation is a typical Ledinegg-type flow instability. Under rolling movement, the flow instability in U-tubes of marine steam generator is affected by an additional force, which makes the phenomenon more complicated. In the present work, a one-dimensional thermal hydraulic model for U-tubes under the natural circulation and rolling movement conditions is established. By the small perturbation theory, the critical pressure drop (CPD) and critical mass flow rate (CMFR) are proposed and analyzed, which determine the occurrence of reverse flow in U-tubes. The effects of rolling period and amplitude on the flow instability in U-tubes with different length are discussed. It is found that the fluctuation period of CPD is the same as the rolling period under the transverse swing movement, but it is equal to half of the rolling period under the longitudinal swing movement. The relationships between the fluctuation amplitude of the CPD and rolling amplitude are in the form of linear and square functions under the transverse and longitudinal swing movements, respectively. The transverse swing movement has some effect on the spatial distribution of reverse flow, but the longitudinal swing movement has little effect on that parameter.