CFD calculation and evaluation of RELAP5 coarse-mesh cross-flow model

A variety of cross-flow models are used in one-dimensional subchannel analysis and system thermal hydraulic simulation codes to account for 2-D and 3-D momentum and energy transfer between adjacent computational cells. Of particular interest to this study is the evaluation of RELAP5 cross-flow model for simulation of reactor core problems involving 2-D and 3-D flow and heat transfer. For comparison purposes, an industry standard CFD code, FLUENT, is used to perform high-resolution crossflow calculations in parallel channels. Initial and boundary conditions for the RELAP5 model are used to develop FLUENT model for a pair of parallel reactor core channels. Two models were developed for FLUENT calculation of cross-flow. The first model is a simple tube with axisymmetric non-uniform inlet flow velocities. The second model included different power generation rates in the inner tube and the outer annulus. Results of flow rate calculations in the transverse direction were compared with RELAP5 cross-flow calculations under similar geometry and boundary conditions. All FLUENT calculations were performed using simple orthogonal mesh using finer mesh in the region of interest. The k-epsilon turbulence model is used to calculate turbulent viscosity. The power density is adjusted to account for the uniform heat generation inside the reactor core. Results of this study show the limitations and shortcomings of the RELAP5 to account for multi-dimensional flow phenomena using the cross-flow model