CFD investigation of the cold wall effect on CHF in a 5 × 5 rod bundle for PWRs

In this study, a set of mathematic models suitable for flow boiling simulation and CHF prediction in rod bundle channels of Pressurized Water Reactor (PWR), including Eulerian two-phase model combined with wall boiling model, were established using CFD method. The models were validated against the international flow boiling and Critical Heat Flux (CHF) experiment benchmark. The results show that the error between the calculated CHF and the experimental data is less than 10%. Then a 3-D model of 5 × 5 rod bundles with cold wall was built and the detailed three-dimensional thermal–hydraulic characteristics were investigated, especially emphasising on the cold wall effect. The existance of cold wall would affect the coolant thermal mixing intensity, leading to a certain degree influence of the CHF. Results show that under the same inlet conditions, the calculated CHF value after introducing the cold wall is higher by 11.4%, while the overall power decreases due to the existence of cold wall. Besides, the pressure drop and temperature in the channel would decrease, while the distribution of void fraction basically remain unchanged, which is still mainly affected by the spacer grids and mixing vanes. Compared with the modified W-3 formula, the error of the CHF value calculated by CFD method is less than 10%, and the CFD method can show the three-dimensional distribution characteristics and evolution process of physical parameters.