Magneto Hydro-Dynamics and Heat Transfer in Liquid Metal Flows

Abstract

Liquid metals are considered to be the most promising coolants for high temperature applications like nuclear fusion reactors because of the inherent high thermal diffusivity, thermal conductivity and hence excellent heat transfer characteristics. The coolant used in nuclear reactor should have high heat extraction rate. The high melting point and boiling point which eliminates the possibility of local boiling makes liquid metals more attractive to high temperature applications. The thermal entrance length of liquid metals are relatively high leading to flow never reaching fully developed condition which is always advantageous for heat transfer applications as the Nusselt number value is higher in a developing flow than a fully developed flow. The molecular properties of liquid metals are such that the thermal diffusion is faster than momentum diffusion having Prandtl number << 1. The thermal boundary layer for liquid metal flow is not only confined to the near wall region but also extends to the turbulent core region which makes the turbulent structures important in transfer of heat. As turbulence plays an important role in transfer of heat from the viscous sub-layer to the core flow, it is necessary to maintain high turbulence to achieve high convective heat transfer rates. It may also be noted that surrounding magnetic fields reduce the turbulence and flow becomes more streamlined. Lithium is the lightest of all metals and has the highest specific heat per unit mass. Lithium is characterized by large thermal conductivity and thermal diffusivity, low viscosity, low vapor pressure as shown in table 1. Lithium is the most promising coolant for thermonuclear power installations. Tritium is a component of fusionable fuel. Tritium does not occur in nature in large amounts and is unstable with its half life 12 years. Tritium can be obtained from lithium with nuclear reactions in fission nuclear reactors or blankets of fusion reactors. Thus, lithium provides raw nuclear fuel to implement fusion reaction.