Numerical investigation on mixing performance in rod bundle with spacer grid based on anisotropic turbulent mixing model

Fuel assemblies are essential elements of pressurized water reactors. Each fuel assembly comprises of several fuel rods with open subchannels between them thus resulting in inter-channel mixing between the adjustment subchannels. However, the assemblies are fixed with spacer grids to reinforce the fluid turbulence between the fuel rods. To this end, a detailed investigation of the efficiency and safety of thermal-hydraulic performance in fuel rods with spacer grids is highly desirable. Recently, the subchannel approach-involving dividing the complex geometries of the fuel assembly into smaller sections have been identified as a promising solution.

It provides relatively accurate results on the flow and temperature distributions across the rod thus more effective for design calculation of pressurized water reactor in the core. However, the complexity involved in the modeling of flow interactions in lateral directions between the adjacent channels is the main challenge in the subchannel analysis approach.

The turbulent mixing model in subchannel thermal-hydraulic performance analysis of fuel bundles has attracted significant research attention. In the turbulent mixing model, it is generally assumed that the fluctuation of the turbulent velocity is proportional to the average velocity of both the subchannels and the mixing coefficient of the introduced turbulence. Additionally, the general turbulent mixing correlation has been represented by regarding the turbulent mixing coefficient as either a constant value or as a function of the Reynolds number. Even though turbulent mixing models and coefficients have been described in different ways, subchannel analysis of rod bundles with spacer grids produce averagely larger discrepancies.

Recently, Chongqing University researchers: Xiang Li (graduate student) and Professor Deqi Chen and Dr. Lian Hu (postdoctoral fellow) developed an anisotropic turbulent mixing model comprising of a turbulent mixing coefficient matrix for analyzing the thermal-hydraulic performance of fuel assembly and especially those with spacer grids. They used a 5 by 5 fuel rod bundle with spacer grid and mixing vanes. This helped in analyzing the mixing performance of different subchannels in the fuel rod. The feasibility of the model was validated by comparing the numerical results with the experimental results and more so for the outlet temperature distribution of the fuel assembly for both the anisotropic and isotropic turbulent mixing models. The research work is currently published in International Journal of Heat and Mass Transfer.

Unlike the isotropic turbulent mixing model, the anisotropic turbulent mixing model achieved more accurate prediction of the mixing characteristics of the inter-channels. For instance, this resulted in uniform outlet temperature distributions of the subchannels which further led to an increase in the critical heat flux and departure from the nuclear boiling ratio of the fuel assembly. In general, the anisotropic turbulent mixing model developed by Professor Deqi Chen and his team accurately exhibited the inter-channel mixing characteristics of the spacer grids and thus will advance the thermal-hydraulic performance of fuel assembly subchannels of pressured water reactors.