Numerical Investigation For Heat Transfer Of Supercritical Water In Rod Bundles

Heat transfer of supercritical water in rod bundles is vital aspect forresearch and development of Supercritical Water-Cooled Reactor(SCWR). The objective of research work described in this essay is toachieve further understanding of thermal-hydraulic behavior ofsupercritical water in rod bundles and thus to provide information for thedesign of supercritical water cooled reactors based on numericalsimulation.In order to investigate the performance of turbulence models inpredicting supercritical water heat transfer in a rod bundle, numericalsimulation using different turbulence models, including the shear stresstransport k-ω model (k-ω SST) and realizable k-ε model, is first carriedout with OpenFOAM, an open source CFD toolbox to validate againstheat transfer experiment of supercritical water in4-rod bundles conductedin Shanghai Jiao Tong University. Comparison between the calculationresults and the experimental data and calculation results from the SSGmodel with CFX, the temperature distribution of internal fuel rod wall predicted by these three models is higher than that of the experimentalresults. As is shown in many cases, the k-ω model usually yields moreacceptable prediction in the near wall region than k-ε model. In areaswhere the effect of spacer grids is not prominent the temperaturedistribution derived from k-ω SST model is the closest to experimentaldata. However, in the vicinity of spacer grids, k-ω SST model andrealizable k-ε model are unable to provide accurate prediction due tomodification of turbulent viscosity which restrains the generation ofturbulent kinetic energy. Judging from the overall performance, the k-ωSST model is advised to perform the simulation and more suitableturbulence models should be developed in the future to yield moreaccurate results.Based on the evaluation of turbulence model, a three-dimensionalnumerical investigation is conducted using the k-ω SST model to explorethe effect of wire on the fuel on the wall shear stress and temperaturedistribution of external fuel rod surface, as well as on the flow and heattransfer of supercritical water in Supercritical Water-cooled Reactor-FuelQualification Test (SCWR-FQT) active channels. The calculation resultsshow that the flow direction of supercritical water is guided by thewrapped wire and that the distributions of streamlines, velocity, pressure,turbulent kinetic energy and temperature of fluid on the cross-section offlow channels present periodical changes along with the rotation of wire. The wire spacers enhance mixing and make the distribution of fluidtemperature and velocity on the cross-section more uniform. However, inSCWR-FQT active channels, wall heat flux is high in the region wherefluid temperature approaches the pseudo-critical temperature, which mayinduce the occurrence of heat transfer deterioration. Additionally, due tothe narrow surface of contact between the fuel rod and the wire, it ishighly possible for fluid to stagnate in these regions, which vitiates heattransfer between fluid and rod. These phenomena deserve specialattention in the process of wire-wrapped fuel assembly design.Finally, to understand the effect of pitch-to-diameter ratio on heattransfer in4-rod bundles within the SCWR, a numerical calculation isperformed with k-ω SST model. The results show that the effect of spacergrids on the flow and heat transfer of downstream fluid is more prominentwith the P/D ratio of1.18. The case with P/D ratio of1.18also showshigher fluid temperature, velocity and turbulence kinetic energydistribution along the flow direction. Besides, due to smaller ratiobetween heating perimeters and flow areas in sub-channels, thenon-uniformity effect at the cross-section of sub-channels is vitiated witha larger P/D ratio.