EXPERIMENTAL AND NUMERICAL INVESTIGATION OF TIME DEPENDENT NATURAL CONVECTION BOUNDARY LAYER FLOW OVER SPHERES IN FLUORIDE SALTS

摘要

An experimental and computational study was carried out to measure natural convection heat transfer coefficients from single spheres in fluoride salts. Fluoride salts are coolants in a class of nuclear reactors called fluoride-salt-cooled high temperature reactors (FHR). Single brass spheres were immersed in flinak (LiF-NaF-KF) salt and their transient temperatures were recorded until equilibrium was reached. The experimental range studied was for Prandtl numbers from 10 to 30 and Grashof numbers from 1500 to 350,000. The initial goal was to confirm that existing correlations for Nusselt number would satisfactorily predict experimental outcomes. However there was no match between the experimental Nusselt numbers and the correlation, because the experiment was not in quasi-steady state. A non-dimensional expression based on time scales of the sphere was derived: ρc_p/3πh. It was proposed that when this ratio becomes large, quasi-steady conditions are achieved. This was partially shown using COMSOL Multiphysics to simulate the fluid mechanics. Measuring the Nusselt number of single spheres is useful to validate methods to predict heat transfer behavior in the packed pebble bed core of FHRs; however it is difficult to heat spheres in experiments. Thus there is merit in designing a quasi-steady state experiment, where the thermal inertia of the sphere generates the heat flux.