Study of cross flows and large flow structures in nuclear reactors

The aim of the present PhD project is to develop a better understanding of the complex thermal hydraulic phenomenon of nuclear reactors using advanced computational fluids dynamics (CFD). In particular, this PhD project investigates the horizontal flows in the AGRs, and the large flow structure and flow instability in tightly packed fuel assemblies. The horizontal flows in Hartlepool/Heysham 2 advanced gas-cooled reactors are investigated by solving RANS/URANS equations with RNG k-ε model. Even though it has been known for some time that there are some horizontal flows in these reactors, but their effects on the cooling of graphite bricks have not been previously assessed. The study reveals that the horizontal cross flow helps to improve the cooling of the moderator brick, while the HIBL flow can result in local hot spots in some narrow gap regions. The axial variation of the brick geometry has also been found to significantly influence the distribution of the temperature within the brick. It is also found that there may be flow instabilities in the flows in the lower level of the AGR fuel channels. Large-eddy simulation (LES) with WALE subgrid scale model is applied to study the buoyancy effect on the behaviour of the ascending flow in the tightly-packed-rod-bundle geometry. The buoyancy effect is modelled using the Boussinesq approximation. The existence and behaviours of large flow structures in the gaps of the flow passage are demonstrated using instantaneous flow fields, spectrum analysis and correlation analysis. The relationship between the Strouhal number of flow structures and the buoyancy parameter Bo* is established. The study also demonstrates that the local buoyancy effect on the turbulence in the non-uniform flow passage is non-uniform as well. It has been shown that buoyancy-induced heat transfer deterioration in the non-uniform sub-channel considered herein is much weaker than that in a circular pipe under similar conditions. LES and WALE SGS model is again utilized to investigate the effect of strong variations of fluid properties on the flow in the tightly-packed-rod-bundles geometry addressed earlier under supercritical pressure. Even through the flow has been significantly influenced by the variation of fluid properties, especially, thermal expansion, the large unsteady coherent flow structures remain similar. The swinging flow in the narrow gap is still an important feature. It has been found that the r.m.s of the horizontal fluctuating velocity is always increased by the heating due to the variation of fluids properties. In contrast, the r.m.s of the vertical fluctuating velocity is first suppressed when the heating is weak to medium, but is enhanced when the heating is very strong.