Direct numerical simulation of turbulent free-surface high Prandtl number fluid flows in fusion reactors

Abstract

A high Pr fluid flow, such as FLiBe, has less heat transport capability because of low thermal conductivity and very thin thermal boundary layer. Until today, turbulent flow and heat transfer feature of high Pr fluid, especially as for a free-surface flow configuration, is not so clear because of the difficulties in velocity and temperature measurements in such a very thin boundary layer. In the present study, Direct Numerical Simulation (DNS) of turbulent free-surface flow of various Pr fluids has been carried out with a constant heat flux from the free surface and an adiabatic condition imposed on the wall. From the DNS, turbulent statistics have been obtained and the flow structure has been investigated via computational flow visualization technique. One of the highlights of this study is that the turbulent diffusivity near the free-surface increases when Pr increase and it leads to a very high-temperature region which could appear there, and then eventually a “laminarization” of fluid flow could occurr because of less turbulent heat flux there. This means that it is necessary to consider some turbulent heat transfer and thermal mixing augmentation for high Pr fluid flows.