On the boundary conditions of the k-ε model in 3D coarse-mesh models for containment analysis

摘要

In CFD-type codes like the containment analysis code GOTHlC, one of the options that can be used for modelling of turbulence is the k--εE model. Though, in contrast to commercial general purpose CFD codes which are tailored for making detailed CFD calculations with a large number of spatial meshes, in codes like GOTHIC which are primarily aiming at calculating transients in reactor containments, for practical reasons, one generally uses coarse meshes. The solution of the two parabolic k--ε model equations requires the definition of boundary conditions at physical boundaries and this, in turn, requires very small spatial meshes near these boundaries. Hence, while in CFD codes these boundary conditions are properly defined, in codes like GOTHIC, this is done in an indirect and non-rigorous fashion, exactly due to the fact that except near sharp velocity gradients (e.g. near the walls), one uses large spatial meshes, this can have catastrophic consequences during the calculation of a transient and in this work, we shall give some examples of this and outline one aspect of a general approach which would aim at bypassing this problem. The problems reported here were addressed by the code developers in the later code versions (after the GOTHIC6.0(QA)) in a way different to that addressed here.