A dominant geometrical parameter affecting the turbulent mixing rate in rod bundles

The trends of turbulent mixing data in rod arrays and various definitions of the mixing factors are examined to derive a new definition of a mixing factor. The proposed mixing factor is based on the eddy diffusivity of energy and it takes into account the degree of the turbulent mixing of various fluids having different Prandtl numbers. With this definition of a mixing factor it is found that the geometrical parameter δij/Dh is a dominant factor affecting a turbulent mixing, which correlates well with the reliable experimental data selected by considering the reliability of the measurement technique used in the experiments. A useful correlation of the turbulent mixing in rod arrays was developed as a function of δij/Dh. The correlation is applicable for both square and triangular arrays with a reasonable accuracy. It predicts a reasonable mixing at a higher δij/Dh or at a lower S/d ratio, which is the most improved feature of the correlation when compared with the existing ones. The proposed correlation may be applicable for the thermal-hydraulic design of a nuclear reactor with an improved accuracy.