An investigation of the effect of split-type mixing vane on extent of crossflow between subchannels through the fuel rod gaps

Abstract

Subchannels of a fuel bundle may have varying pressure drop characteristics due to the prevalence of different thermal hydraulic conditions. Diversion crossflow between two adjacent subchannels results from lateral pressure fluctuations between the two subchannels. Pressure fluctuations, may also be introduced by varying subchannel cross-sections. In addition to the contribution to crossflow by varying subchannel cross-section, flow deflectors such as mixing vanes attached to spacer grids contribute greatly to the extent of crossflow through gaps connecting adjacent subchannels.To establish the extent to which flow deflection by virtue of split-type mixing vane contribute to crossflow in rod bundles, computational fluid dynamic simulation using STAR-CCM+ was performed for flow of water at a Reynolds number of RE1=3.4×104 through a 5×5 rod bundle geometry for which the rod to rod pitch to diameter ratio was 1.33 and the wall to rod 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 flow through the geometry with imposed cyclic periodic interface boundary condition of fully developed flow type at flow inlet and outlet boundaries.The objectives were to primarily investigate the extent of predictability of the physical problem by the computational fluid dynamic (CFD) code as a measure of reliability of the models employed. Another objective is to assess through comparative analysis the effect of split type mixing vanes on crossflow through gaps connecting adjacent subchannels.Validation of simulation results with experimental data showed good correlation of mean flow parameters with experimental data whiles turbulent fluctuations deviated largely from experimental trends. 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. Relative to the non-linear standard k-epsilon and the k-Omega models, the two layer realizable k-epsilon turbulence model employed was unable to appreciably predict turbulent velocity fluctuations. Flow diversions at the position of the mixing vanes were strengthened greatly and observed to be due to the contribution of flow sweeping by the split-type mixing vanes. As observed from the magnitude of diversion and directed crossflow, the mixing vanes contributed largely to the extent of crossflow through the gaps which should result in improved distribution of flow properties between subchannels.