Using ANSYS/FLOTRAN® and CFX-4® for the solution of a mixed convection flow benchmark

For the design, optimisation and safety analysis of nuclear plants, the use of 1D system codes is state of the art. However, for the modelling of passive equipment the range of validity of the 1D-codes is exceeded, because in these structures 3D-flow phenomena are often dominating. The fast progress of the computer technique and of Computational-Fluid-Dynamics (CFD-) codes gives the opportunity, to apply CFD-codes to the assessment of the efficiency of those components. Most of these codes are based either on the finite-volume or the finite-element method.
Integrated simulation of thermo-hydraulic phenomena and the resulting thermo-mechanical loadings for the surrounding structures (e. g. the reactor pressure vessel) can be achieved using a finite-element-code like ANSYS/Multiphysics®. The finite volume code CFX® has been proven to be an appropriate tool for modelling of 3D single phase fluid phenomena.
The governing mechanism in passive components for decay heat removal is natural convection and heat transfer with internal heating. To assess the capability of both codes describing a mixed convection flow, post test calculations of an IAHR (International Association for Hydraulic Research) benchmark exercise were performed (Kamide et. al, 1991). The working fluid was water in the temperature range of 15 to 50°C at atmospheric pressures.
This paper presents a discussion of the problems and capabilities of each code to calculate complex flow regimes and temperature fields.