Estimation of the turbulent mixing rate for low Prandtl number flows through rod bundles

The prediction of the turbulent mixing rate is very important in the thermal-hydraulic design of nuclear reactors. In this study, the turbulent mixing rate for low Prandtl number fluid flow through rod bundles is estimated with the scale analysis on the flow pulsation phenomenon which is pointed out as a main cause of the mixing in rod bundles. Based upon the assumption that the turbulent mixing is composed of molecular motion, isotropic turbulent motion (turbulent motion without the flow pulsation), and flow pulsation, the scale relation is derived as a function of P/D, Re, and Pr. The derived scale relation is compared with published experimental results and shows good agreements.