Melt/freeze heat transfer measurements in cryolite-based electrolytes

Abstract

The need to maintain a solid electrolyte ledge on the sidewalls of aluminum smelting cells has recently provoked interest in the heat transfer rate from the molten to the frozen phase at this interface. In this work an electronic voltage to frequency conversion technique was developed to record thermocouple signals inside a vertical, air cooled graphite cylinder rotating in molten cryolite. Precise measurement of the solid electrolyte which freezes around the internally-cooled cylinder allowed determination of the interfacial heat transfer coefficient under both free and forced convection conditions. Correlation of the local Nusselt numbers for heat transfer to the stationary cylinder yields an expression similar to that obtained for natural convection in a rectangular enclosure. However, rotation of the cylinder at peripheral velocities of 0.09 to 0.12 m s⁻¹ caused forced convection to dominate in the cavity and increased the heat transfer coefficients by 34 to 45 pct, respectively. The data provide a basis on which to calculate the convective heat flow to the sidewall ledge for a range of hydrodynamic conditions in the cell.