Coarse-Grid-CFD for pressure loss evaluation in rod bundles

CFD simulation of the complete reactor core of a nuclear power plant requires exceedingly huge computational resources so that this crude power approach has not been pursued yet. The traditional approach is the use of one dimensional sub channel analysis, employing calibrated transport models. Coarse-Grid-CFD is an attractive alternative technique based on strongly under-resolved CFD and the inviscid Euler equations. Obviously, using inviscid equations and coarse grids does not resolve all the relevant physics. The neglected physics is restored by additional volumetric source terms modeling viscosity and other sub-grid effects. The source terms are implemented via correlations derived from fully resolved representative simulations. The new simulation technique is demonstrated for a rod bundle experiment performed at the KALLA experiment of the Karlsruhe Institute of Technology (KIT) which includes three spacer grids. In this paper a 3D CFD study of the rod bundle is presented. A representative computational domain of the bundle is selected for the Coarse-Grid-CFD study. Comparison of the 3D CFD results to experimental results shows that a good agreement is obtained by an adequate selection of the computational grid. Coarse-Grid- CFD study results in good agreement with 3D CFD results. Careful selection of the representative coarse grid in geometrically complex regions like spacers is recommended. (authors)