Investigation of heat transfer characteristics and energy balance analysis of FLiNaK in turbulent boundary layers of pipe flow

Abstract

The transport of energy in turbulent boundary layers is characterised by two complimentary transport mechanisms, vis-a-vis conduction and turbulent advection. These processes in the inertial sub-layer of fully developed turbulent flow in a round pipe at two Reynolds numbers are investigated to obtain conductive and advective heat fluxes. CFD simulations are carried out at these Reynolds numbers using Large Eddy Simulations (LES) to generate high quality transient velocity and temperature data. A fluoride salt with a eutectic composition of 46.5%LiF–11.5%NaF–42%KF (mol %) commonly known as FLiNaK (Prandtl number = 3), a potential candidate for high temperature heat transfer fluid is used as a fluid for simulations. Simulation results have been validated for the mean velocity profile using Direct Numerical Simulation data (DNS). Transient velocity information is used to identify and characterise turbulent structures using a wavelet transform based methodology called Wavelet Transform Modulus Maxima (WTMM) for obtaining the advective heat flux. Conductive flux is calculated using temperature gradients. Energy balance analysis with respect to input heat flux, was performed using conductive and advective heat fluxes. WTMM analysis is also used for visualising near wall flow structures, and to further find out the heat transfer coefficients. The predicted Nusselt numbers have shown good agreement with the available experimental data.