Prandtl Number Effect on Heat Transfer Degradation in MHD Turbulent Shear Flows by Means of High-Resolution DNS

Abstract

Estimation of the heat transfer degradation effected by Magneto-Hydro-Dynamics (MHD) forces is one of the key issues of the fusion reactor designs utilized molten salt coolant. FLiBe which is the molten salt mixture of LiF and BeF, is one of the coolant candidates in the first wall and blanket of the fusion reactors, and has several advantages which are little MHD pressure loss, good chemical stability, less solubility of tritium and so on. In contrast, heat transfer degradation for the high Prandtl number, (Pr=ν/α, Prandtl number, ν is the kinetic viscosity, α is the thermal diffusivity) characteristics caused by the low thermal diffusivity and high viscosity (Sagara et al, 1995), was one of the issues of concern. MHD turbulent wall-bounded flows have been investigated extensively by both experimental and numerical studies (Blum, 1967, Reed & Lykoudis, 1978, Simomura, 1991, Lee & Choi, 2001, Satake et al., 2006, Boeck et al, 2007, etc.) and much important information such as the drag reduction, the turbulent modulation, similarity of velocity profile, and heat transfer have been obtained. On the other hands, MHD turbulent heat transfer in a high-Pr fluid has not been understood well. The previous experimental and direct numerical simulation (DNS) studies still have conducted for Prandtl number up to Pr=5.7. Therefore, the knowledge of the MHD heat transfer on higher-Pr fluids such as FLiBe (Pr=20–40), is highly demanded to verify and validate the MHD turbulent heat transfer models for the fusion reactor designs. The objective of this study is to perform a direct numerical simulation of MHD turbulent channel flow for Prandtl number up to Pr=25, where all essential scales of turbulence are resolved. In this study, we report that the MHD turbulent heat transfer characteristics in Pr=25 for the first time and discuss that the MHD pressure loss and heat transfer degradation under the wide-range Pr conditions. The obtained database is of considerable value for the quantitative and qualitative studies of the MHD turbulent heat transfer models for the blanket design of a fusion reactor.