Flow instability analysis of U-tubes in SG based on CFD method

• The flow and heat transfer in U-tubes of steam generator are investigated with CFD.
• The curve of the total pressure drop with mass flow rate is obtained.
• In the curve there exists the negative slope used to judge where the flow instability occurs.
• The inlet subcooling and U-tube length have large influence on the reverse flow.

It is known that the parallel flow in inverted U-tubes of steam generator (SG) may occur the non-uniform or unstable flow under the natural circulation condition. Though the unstable flow phenomena in the SG are thought as the Ledinegg-type flow excursion that is dominated by the flow resistance and gravitation pressure drop, it has not been studied well. In the present work, the U-tubes in the SG are divided into three groups by their length, and the flow behavior of single phase water in the U-tubes is investigated with CFD method. The curves of total pressure drop with mass flow rate are obtained. The influences of the inlet subcooling and U-tube length on the flow stability are discussed. It is found that when the total pressure drop of parallel U-tubes is given, the higher the inlet subcooling is, the more easily the reverse flow occurs. As the inlet subcooling is low, the reverse flow occurs more easily in U-tubes with the shorter length. However as the inlet subcooling is high, the reverse flow occurs more easily in U-tubes with the longer length.