Experimental validation of the porous media approach for rod bundle geometry under crossflow conditions

At present, there are two possible approaches which are used in practice for the thermal-hydraulic analysis of the reactor core in simulations with relevant crossflow effects are: the representation of the core by a system of separated parallel flow subchannels with provision of a forced crossflow mixing by implementation of additional formulations and the porous body approach, where the core geometry is replaced by a structure of homogenized zones of porous media. If crossflow is comparable with flow along the bundles, the porous body approach is the only one suitable method to perform steady-state and transient flow calculations for safety analyses. The CFD-code CFX-4 offers the porous region model for the modelling of the core geometry as a homogenized medium. This model is characterised by a set of properties (volume porosity, resistance to flow and so on). The Core Crossflow Experimental Facility (CCEF) was built with the objective to obtain experimental data for the flow in a rod bundle under the conditions of forced crossflow with relatively low Reynolds numbers and variable flow angle. The test section of CCEF (a plexigalss model) contains the test rod bundle. The test rod bundle is based on the geometry of a typical PWR with a pitch/diameter ratio of 1.33 and was scaled with a factor 1.5. The crossflow was induced by an asymmetrical outlet condition for the bundle. The test bundle consists of the 100 plexiglass rods, arranged in 4 rows. Additional internals, which are typical for a reactor core geometry, were not installed in the test section. The working fluid in the experiments was water. For the velocity measurements the one-component laser-Doppler anemometer (LDA) was used. Calculations for the comparison with experimental data were performed in two different approaches. The detailed geometry was created for the calculation of flow fields with k-ε turbulence model and a relative simple geometry of the test section was built by means of the CFX-4 PreProcessing for the flow simulations in the porous region approach. The results of the experimental investigations on the CCEF and the comparison with the calculations, performed with CFX-4 code in the geometry of the test section are presented in this paper. (author)