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

A high Pr fluid flow, such as FLiBe, has less heat transport capability bec ause of low thermal conductivity and very thin thermal boundary layer. Unti l today, turbulent flow and heat transfer feature of high Pr fluid, especia lly as for a free-surface Row 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 Row of various Pr fluids has been carried out with a constant heat Aux 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 diffus ivity near the free-surface increases when Pr increase and it leads to a ve ry high-temperature region which could appear there, and then eventually a "laminarization" of fluid Row could occurr because of less turbulent heat f lux there. This means that it is necessary to consider some turbulent heat transfer and thermal mixing augmentation for high Pr fluid flows. (C) 2001 Elsevier Science B.V. All rights reserved.