Investigating the Effects of Interface Topology on Flow Development in Rod Bundle Supported by Spacer Grid with Split Mixing Vane Using STAR-CCM+

Abstract

Flow development downstream of a spacer grid is dependent on the upstream conditions and the imposed interface topology, especially at inlet and outlet boundaries. In STAR-CCM+, all interfaces fall into two groups, direct and indirect. A direct interface directly joins together two boundaries composing the interface either permanently or temporarily, for the case of rigid body motion. An explicit connection is created between cells on each side of the interface, so that mass and energy or either of them will occur across the interface. Three options of interface topology namely, in-place, periodic and repeating are available to be imposed at the inlet-outlet boundaries for a flow problem. In the present work, computational fluid dynamic simulation using STAR-CCM+ was performed for the flow of water at a bundle’s Reynolds number of Re1 = 3.4 × 104 through a 5 × 5 rod bundle geometry supported by spacer grid with and without split mixing vanes for which the rod-to-rod pitch to diameter ratio was 1.33 and the rod to wall pitch to diameter ratio was 0.74. The two-layer k-epsilon turbulence model with an all y+ automatic wall treatment function in STAR-CCM+ was adopted for an isothermal single phase (water) flow through the geometry with and without imposed cyclic periodic interface boundary condition of fully developed flow type at inlet and outlet boundaries. The objectives were to primarily investigate the extent of predictability of the experimental data by the Computational Fluid Dynamic (CFD) simulation as a measure of reliability on the CFD code employed, and also study the effects of the imposed interface topology on flow redistribution in the presence and absence of split mixing vane. Validation of simulation results with experimental data showed a good correlation of mean flow parameters with experimental data. Generally, the agreement of simulation results with data obtained from the experimental investigation confirmed the suitability of the CFD code, STAR-CCM+ to analyze the physical problem considered. Trends of flow redistribution downstream of the spacer grid indicate that, the split mixing vanes acted to quickly bring the flow to an equitable redistribution downstream of the spacer grid irrespective of the imposed inlet-outlet interface topology. For the case of the spacer grid without mixing vanes, some extents of deviation were realized between the model with no imposed interface topology and that with imposed periodic interface topology. The variation in trends shows that, a much longer inlet segment of the domain is required to completely nullify the effect of the inlet-outlet interface topology on flow distribution in the absence of mixing vanes which may lead to a relatively higher demand for computational resources than required in the presence of mixing vanes.