Radiation Heat Transfer in the Molten Salt FLiNaK

The molten fluoride salt eutectic LiF-NaF-KF (FLiNaK) has been proposed as a coolant for use in Generation IV reactors designed to operate at temperatures at which radiation heat transfer (RHT) may be significant. Little research has been performed into the absorption coefficient of FLiNaK as it pertains to thermal RHT. An estimate of the spectral absorption coefficient for FLiNaK has been generated using informed assumptions and existing data for the constituent salts. The effect of heat transfer, as it pertains to flowing salt in circular cross-section pipes with heated walls, has been investigated for laminar flow using a mathematical model. The combined energy equation, in various geometries, was solved for laminar flow, with the radiative heat flux calculated using the differential approximation. The percentage of energy transferred by radiation to the salt was found to be primarily a function of pipe diameter, wall temperature, and the salt absorption coefficient. A map of temperatures and pipe diameters has been generated, which indicates where RHT is significant. A correlation has been proposed, based on the mathematical model, to account for increase in Nusselt number due to radiation. Additional discussion is included on the effects of wall emissivity and high Reynolds flows.