3D core thermal hydraulics with the PORFLO code: Turbulence modelling and porous medium with porosity steps

VTT is developing a 3D two-phase flow solver code called PORFLO with the objective of enabling coupled reactor dynamics simulations through a parallel computation and porous medium description of complex geometries. The current features include unstructured mesh usage, 6-equation flow solution, k-ε turbulence modeling and MPI parallelization. The present applications are fuel bundles and cores of VVER and EPR, with a steam generator and core debris bed having been simulated previously. In this work, the main new developments are improved turbulence modeling, improved interpolation of face velocities in porous medium, and new improved modeling of EPR plant. The recent work examines porous medium based macroscopic models for turbulence quantities in the fuel assembly sub-channels. A few modifications in the standard k-ε turbulence model were tested against experimental data from the AGATE, MATiS-H and IVO VVER LDA measurements. In the coupled solution of pressure and velocities, problems were encountered at locations with a large change in the porosity. In the latest development, the pressure weighted momentum interpolation of face velocities was modified to explicitly account for the interphase drag term, in a manner similar to the Coupled Phasic Exchange Algorithm. In this work, the improved two-phase solver was tested in single-phase cases.