| As the vital component in the reactor,the thermal-hydraulic performance of fuel assembly has a significant influence on the safety,reliability and economy of nuclear power plants.Spacer grids are an important part of the skeleton of fuel assembly whose structural characteristics play a decisive role in the flow pattern and temperature pattern of the rod bundle channel.Mixing vanes are usually installed on the spacer grid to guide the cross flow of coolant,promote the coolant mixing and improve the heat transfer capacity of fuel assemblies.Therefore,the thermo-hydraulic characteristics of rod bundle channels with mixing vanes have been a hot issue of the scholars at home and abroad.However,it is difficult to restore the high-temperature and high-pressure environment in PWR by means of experiments.As a design tool with advantages of safety,low cost and short research period,numerical simulation can make up for the shortcomings of experiments.It is of great engineering value to explore the thermal-hydraulic characteristics of rod bundle channel by CFD simulation.Firstly,the three-dimensional modeling software of Solidworks was employed to model and assemble the rod bundle,spacer grid,mixing vanes and other components.ICEM CFD software was used to generate the mesh of the fluid domain via hybrid mesh generation technology,and the grid independence test was carried out.Then,by comparing the calculated lateral and axial velocity distribution,swirl factor and azimuthal Nu number fluctuations downstream of spacer grid with the experimental data,the reliability of the CFD method was verified,and the most suitable turbulence model was selected.Then CFD method was used to compare and analyze the effects of bending angle,length,gap distance and arrangement of the mixing vanes on the thermo-hydraulic characteristics of the rod bundle channel.Finally,by comparing and analyzing the flow pattern and temperature pattern of the four-channel model with periodic boundary conditions and the 5×5 model,the rationality of using periodic boundary conditions to simplify the size of the rod bundle in numerical simulation was investigated.The thermal-hydraulic characteristics of subchannels in different positions were also compared.By comparing and analyzing the results of numerical simulations,the following conclusions were drawn: The present CFD model has been validated using the measured data of previous works and can reproduce the data more closely than the previous work.Compared with the standard k-? model and Reynolds stress model,SST k-? model is more accurate in predicting the thermo-hydraulic characteristics in the rod bundle channel.It can be clearly revealed in the calculated results that the deflection angle of mixing vane shows the significant effect on the flow patterns,heat transfer and pressure drop;however,effects of the vane length is insignificant.The increase in the bending angle would enhance the heat transfer at the expanse of the increase in the pressure drop,while the increase in the length of the mixing vane could only increase the pressure drop and the heat transfer characteristics slightly.The pressure drop would change little as the gap distance increased,while the flow pattern would change obviously.The heat transfer capacity would also improve with the increase of rod gap distance.As the arrangement of mixing vane change,the flow pattern would change significantly,and the difference of pressure drop between different arrangement is small,among which the alternating arrangement has the best heat transfer performance.It is reasonable to use periodic boundary conditions to simplify the calculation of rod bundle size,but the calculation results exist differences from those of the 5×5 rod bundle channel model. |
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