Turbulent flow of water in a 3x3 rod bundle - Numerical results from RANS, URANS and LES

The coolant flow behavior inside fuel bundles influences the subchannel mass and momentum exchange and the cladding temperature, which should be considered when designing fuel assemblies. In this paper, three numerical methodologies of RANS (Reynolds-Averaged Navier-Stokes), URANS (Unsteady Reynolds-Averaged NavierStokes) and LES (Large Eddy Simulation) were adopted to investigate the turbulent flow characteristics of water inside a 3 x 3 bare bundle at atmospheric pressure. Emphasis was focused on clarifying the features related to the flow through rod bundles, such as the distribution of mean flow, Reynolds stresses and the characteristics of large-scale flow pulsation. It was found that the prediction of RANS simulation deviates relatively large from the experimental data, on the other hand, the predictions given by URANS and LES are comparable and agree well with the experimental measurements. The time-averaged velocity, Reynolds stresses and turbulent kinetic energy were plotted along various lines of interest. The quasi-periodic large-scale flow pulsation was identified by URANS and LES, but the pulsation period and oscillation magnitude deviate slightly. Finally, power spectrum density was conducted for URANS and LES to study the axial and lateral velocity variations in the frequency domain.