• CFD
• Fuel Bundles
• heat transfer
• Heated Channels
• Mixed Convection
• SCWRs
• Stability
• Supercrtical Pressure Fluids
• Turbulent Models

Supercritical water reactors are considered in the Generation IV as one of the promising nuclear reactor concepts to be commercialized in the next decades. Studies are ongoing worldwide in order to establish the most important design choices of a proposed plant whose main purpose is to achieve a high efficiency in power conversion. In fact, the use of water at supercritical pressures will benefit from experience of the well established PWRs as well as fossil-fired supercritical steam generator technologies.
Nevertheless, the presence of large variations in fluid properties in the vicinity of the pseudocritical temperature poses new problems to be tackled by detailed analyses. Thermal phenomena like heat transfer enhancement and deterioration observed at sufficiently low and high heat flux to mass velocity ratios, respectively, challenge the capabilities of both engineering correlations and CFD models. In addition, the change in the fluid density through the reactor core is large enough to raise problems related to dynamic instabilities similar to those in boiling channels.
The aim of this study is to address the problems of prediction of the unusual heat transfer characteristics under supercritical conditions and the instability phenomena using CFD models. An in-house CFD code has been developed as a research tool to study the problem of heat transfer in simple two and three-dimensional geometries using different turbulence models. Different experimental data have been addressed for this purpose. In addition, the Fluent CFD code has been employed to simulate flows in complex geometries like non-circular channels and fuel bundles.
The capabilities of different turbulence closure laws in reproducing the experimental data for different channel geometries and at different heat transfer conditions are evaluated. It is found that the turbulent models and approaches adopted in this study are able to predict the heat transfer enhancement phenomenon fairly well, especially at low heat flux to mass velocity ratios. On the other hand, only the low-Reynolds number k-e models are found able to respond to the effect of buoyancy and to show the heat transfer deterioration observed in experimental data. Sensitivity analysis on the effect of the turbulent Prandtl number on the prediction of experimental data is made using different models which estimate the turbulent Prandtl number for fluids with the molecular Prandtl number far from one.
Based on the evaluations of different turbulence models for different geometries for which experimental data are available, heat transfer in subchannels of rod bundle are studied. Low-Reynolds number turbulence models are used to study subchannels with both triangular and square pitch assemblies showing interesting three-dimensional phenomena. The onset of heat transfer deterioration is also estimated based on the computational findings. Further calculations are made for 1/8 slice of a rod bundle in comparison with data available from subchannel codes. It is shown that CFD calculations offer a much greater detail than subchannel codes in the analysis of local temperature distributions on the cladding surface. The predictions by the standard k-e turbulence model and the Reynolds stress model, both based on the wall functions approach, are also compared, providing information on their capabilities in predicting secondary flows and mixing among subchannels.
Dimensionless parameters for studying the instability phenomena in heated channels at supercritical pressures have been proposed as an extension to those adopted for boiling channels (Ambrosini and Sharabi, 2006). CFD showed the occurrence of density wave oscillations at relatively large power-to-flow ratio in heated channels with supercritical fluids. Prediction of the onset of unstable behaviour is in close agreement with one-dimensional linearised codes and the transient non-linear RELAP5/MOD3.3 code for the both circular tube and three-dimensional subchannels geometries. This suggests that the density wave mechanism is depicted in a similar way by both one-dimensional models and the CFD models, and to some extent is independent of the geometry details.
On the other hand, the heat transfer behaviour observed after the occurrence of instabilities, as predicted by the low-Reynolds number Yang and Shih (1993) turbulence model for the circular tube case, has produced interesting additional information, showing the possibility of obtaining cycles of deterioration and restoration of heat transfer effectiveness, in close similarity with the phenomenon of cyclical boiling transition and quenching expected in boiling channels at some unstable operating conditions. Moreover, the heat transfer deterioration is predicted to occur before the onset of unstable behaviour which may result in different oscillation characteristics to those obtained by the standard k-e turbulence model with the wall functions treatment.