Investigation of flow and heat characteristics and structure identification of FLiNaK in pipe using CFD simulations

Abstract The demand for hydrogen as a transportation fuel is rising day by day. Hydrogen can be produced thermo-chemically or electrolytically at high temperatures. A fluoride salt with eutectic composition of 46.5%LiF–11.5%NaF–42%KF (mol %) commonly known as FLiNaK is a leading candidate for heat transfer fluid. It can be used for transferring heat from high temperature heat source to hydrogen production plant. Computational fluid dynamics (CFD) simulations at three Reynolds numbers were carried out using large eddy simulation (LES) to investigate the flow and heat characteristics of molten FLiNaK in a cylindrical pipe. Simulation results have been validated with the help of mean velocity profile using direct numerical simulation (DNS) data. Transient velocity information was used to identify and characterise turbulent structures which are important for transfer of heat across solid–fluid interface. A wavelet transform based methodology called wavelet transform modulus maxima (WTMM) was used to identify and characterise the singularities. WTMM analysis was also used for flow visualisation, and to calculate heat transfer coefficient using small eddy model. The predicted Nusselt number showed good agreement with the available experimental data.