System–CFD coupled simulations of flow instability in steam generator U tubes

• System–CFD coupled simulations are performed for flow instability in SG U tubes.
• Flow instability with temperature fluctuation in long U tube is simulated well.
• Upward flow occurs simultaneously with downward flow in one U tube.

The transient behavior of flow instability in SG U tubes is simulated numerically by performing system–CFD coupled simulations. The system analysis code RELAP5 is used to simulate the overall system behavior in the test facility, which consists of the heating loop and the model SG, and the commercial CFD code FLUENT is used to simulate detailed flow phenomena in the model SG. The flow conditions obtained by RELAP5 are used as the boundary conditions for FLUENT, and the calculated results by FLUENT are used for RELAP5. The flow instability with temperature fluctuation observed experimentally in the long U tube is simulated well, and the downward flow is shown to occur simultaneously with the upward flow in one U tube.