Coupled Transport of Heat and Mass. Theory and Applications

Abstract

One natural and two technical processes with coupled transport of heat and mass are reviewed. The processes are frost heave, freeze concentration of juice, and salt transport in carbon cathode bottoms in the aluminium electrolysis cell. Recent non-equilibrium molecular dynamics simulation results have been used to establish molecular mechanisms of coupled heat and mass transport in liquids. Linear flux-force relationships have been found for extremely large temperature gradients. Use of linear irreversible thermodynamics is therefore Justified for all three practical cases considered here. Two criteria for local equilibrium are reviewed. In particular, local equilibrium was obtained in a liquid or dense gas with a temperature gradient if $l|\vec \nabla T| \leqslant \delta {\rm T} < 0.05{\rm T}$, where l is the dimension of the local control volume, $\vec \nabla T$ is the temperature gradient, and δT is the temperature fluctuation in the control volume. This criterion is fulfilled for the subcooled solution in the process of freeze concentration. The energy transported by water moving from a subsurface water table to an ice lens in clay capillaries during frost heave is mainly the enthalpy of freezing of water, lending support to the description of frost heave as a transport process. Similarly, the separation of salts in the cathode bottom of the aluminium electrolysis cell and the formation of salt lenses (bottom heave) can be understood as a way the system reacts to a temperature gradient in order to transport energy (heat) as effectively as possible. Computer simulations have confirmed the validity of the Onsager reciprocal relations (ORR) in liquids. The application of the ORR for average phenomenological coefficients across interfaces in the systems is discussed.