Determination of the heat transfer coefficient between molten cryolite and a solid surface

Abstract

During the aluminum production process in a Hall-Héroult cell, it is necessary to maintain a ledge of frozen electrolyte on the sidewalls of the cell for the dual purposes of erosion protection and heat insulation. Since the thickness of this ledge is governed by the convective heat transfer at the bath/ledge interface, it is important to have good knowledge of the heat transfer coefficient at this interface. An experimental transient technique was used to determine values of the heat transfer coefficient for both natural and forced convection. The experiments were performed in two different fluids, namely water and cryolite. The heat transfer coefficient was found to increase with increasing fluid temperature in natural convection, and increasing fluid flow velocity in forced convection. The size of the copper cylinder used was also found to affect the value of the heat transfer coefficient in both modes of convection, and the following dimensionless correlations were derived based on the results: Nu d = 0.24Gr L 0.36 for natural convectio; Nu d = 1.38 + 1.27Re d 1/2 for forced convection.