Numerical Simulations of Counter-current Flow Limitation at Lower End of a Vertical Pipe Simulating Lower Part of Steam Generator U-tubes

Numerical simulations were done for counter-current flow limitation (CCFL) at the lower end of a vertical pipe simulating lower part of steam generator U-tubes by using the volume of fluid method (VOF) implemented in the CFD software FLUENT6.3.26. The simulated CCFL characteristics agreed well with air-water experimental data but flooding in simulations appeared at the upper end of the vertical pipe. To avoid flooding at the upper end, water was supplied through the pipe wall simulating condensation on the inner surface and flooding at the lower end was successfully simulated. However, computations by the standard k-ε turbulence model became unstable for pressures lower than 1.0 MPa and significantly underestimated falling water flow rates. On the other hand, computations by the laminar flow model were stable even for low pressures and significantly overestimated falling water flow rates. Computations by the k-ω SST turbulence model were unstable for pressures lower than 1.0 MPa but gave good agreement of a CCFL value with a steam-water experimental value at 1.0 MPa.