Numerical Investigations of Coherent Structures in Axial Flow in Single Rod-Channel Geometry

In order to ramp up the conversion ratios and burn-up of nuclear reactors, it is inevitable to go to tightly packed fuel rods in the reactor. These nuclear reactors with tightly packed rod-bundles are characterized by interesting flow patterns, different from the ones encountered in regular channel and pipe flows. The correct prediction and control of the flow distribution is essential for the reactor design and safety assessment, and has been an active area of research in reactor thermal-hydraulics. Apart from the axial flow of coolant parallel to the rod bundles, there exists cross-flow between the sub-channels. The cross-flow promotes homogeneous enthalpy distribution and enhanced mixing between the coolant flowing in the sub-channels. Turbulent mixing is an important phenomenon, which influences the flow and temperature patterns in the rod bundles. Large-scale coherent structures along with transverse flow pulsations have been identified in the rod-rod and rod-wall gap regions. This large-scale structure has a quasi-periodic behavior and is considered an important factor for high mixing-rate. The aim of this work is to get a better understanding of the flow in a rod-bundle. This is done by performing numerical investigations on a simplified rod-channel geometry. The Unsteady Reynolds Averaged Navier Stokes equations are solved using the Computational Fluid Dynamics software OpenFOAM. Extensive benchmark and validation studies were done in order to determine the simulation technique that offers a good balance between computational cost and accuracy. The flow dynamics and the transport and mixing of a passive scalar due to the coherent structures are studied. Different turbulence models were used to study their effect on flow dynamics, and no major differences were observed. Following this, the computationally cheaper k-epsilon turbulence model with wall functions was chosen for the simulations. The time required for flow development in this geometry was significantly higher than that in regular turbulent channel or pipe flow. This led to different results than the ones observed in the experiments and in previous simulation results published in the literature. It was concluded that the flow in the experiments was not fully developed and that probably not enough time was used to allow flow development in the previous simulations. Our results indicate that the shear-layer becomes thinner and the number of structures decreased with flow development, which would explain the higher number of structures found in previous simulations. High values of velocity fluctuations and the kinetic energy due to these fluctuations indicated the presence of structures in the near-gap region. Large-scale three-dimensional counter-rotating sledge-shaped structures were observed via the flow visualization of resolved velocity. These structures were not only restricted to the gap region, but encompassed the entire flow domain. The high periodicity and stability of these structures indicate that they are not turbulence structures. The effect of gap-size on the coherent structures was studied, and this study suggested that the presence of more than one mechanism for the formation of these structures. A critical gap-size was obtained, at which the intensity of the structures has a maximum value, and a cut-off gap size was identified, at which a transition takes place between the two mechanisms. The coherent structures were found to play a significant role in both the transport and mixing of the passive scalar. The contribution were similar to that of the turbulent diffusion. The simulations indicate that the effect of the coherent structures on the transport and mixing of a passive scalar is of the same order of magnitude of the effect of the turbulent diffusion.