Correlation for cross-flow resistance coefficient using STAR-CCM+ simulation data for flow of water through rod bundle supported by spacer grid with split-type mixing vane

Abstract

Mass transfer by diversion cross-flow through gaps is an important inter-subchannel interaction in fuel bundle of power reactors. It is normally due to the lateral pressure difference between adjacent sub-channels. This phenomenon is augmented in the presence of flow deflectors and is referred to as, directed cross-flow. Diversion cross-flow carries the momentum and energy of flow and hence affects the velocity and temperature profile in the rod bundle. The resistance to cross-flow in the transverse momentum equations is specified by the cross-flow resistant coefficient which is the subject of concern in the present study.In order to obtain data to correlate cross-flow resistance coefficient, computational fluid dynamic simulation using STAR-CCM+ was performed for flow of water at the bundle Reynolds number of Re1=3.4×104 through a 5×5 rod bundle geometry supported by spacer grid with 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+ were adopted for an isothermal single phase (water) flow through the geometry. 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 apply the simulation data to develop correlations for determining resistance coefficient to cross-flow.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.Cross-flow resistance coefficients were observed to be dependent not only on the ratio of lateral to axial flow velocity but also on the hydraulic diameter of the interconnected sub-channel with larger cross-section, the centroid to centroid distance and the gap width. The coefficient is also dependent on the dynamics of the flow in the neighborhood of specific gaps. These dynamics were attributed to the behavior of secondary flows in the sub-channels and especially across the gaps. Profiles of cross-flow resistant coefficient showed a decreasing trend as the ratio of lateral to axial flow velocity increased.