Assessment of two-equation and second-moment closures for flow and energy predictions in wire-wrapped sub-channel of liquid metal reactor using unstructured finite volume codes

Abstract

This study provides the comprehensive thermal hydraulic analysis in triangular interior sub-channel of liquid metal fast breeder reactor. A stepwise approach is adopted to computational fluid dynamics simulations to investigate the flow, temperature fields and thermal mixing behavior with proper modeling procedures. Conventional advanced eddy-viscosity models and second-moment closure model are benchmarked referring to direct numerical simulation results. In the simplified rectangular flow channel with round obstacle which has a role in wire-wrapped structure in sub-channel, three cases of simulations are calculated with six different turbulent models. For resolving the viscous sub-layer region, the low $y^+$ approach is selected as a wall treatment where is available in candidate turbulent models. Both qualitative and quantitative comparisons are performed at special locations defined by the flow structures imposed by a combination of the axial and crossflow. In terms of flow statistics, the second-moment closure model, elliptic blending Reynolds stress transport model produces the most detailed results. Based on the swept flow simulations, two additional sets of heat transfer conditions are applied, Pr=1 and $Pr=10^{-2}$. In case of Pr=1, overall under-predictions are observed in case of elliptic blending RSM with all $y^+$ wall treatment which has a limitation in buffer-layer region. However, the lower Pr range, this tendency is completely disappeared due to the domination of the thermal diffusivity. For modeling the unitized triangular sub-channel assembly, two pairs of interior channels are combined to achieve the periodicity with helically wire-wrapped structure. Mainly observed variable in unit channel structure is temperature distribution with radial velocity streamlines. Also the Nu variations are investigated and compared to conventional correlations with provided geometry in terms of pitch-to-diameter ratio, periodic length of the wire-to-diameter ratio and Peclet number. From the calculation results, three individual formulations are suggested including the wire effect.