An Anomaly in Natural Convection for Binary Mixtures subject to Flux Boundary Conditions

The problem of natural convection in a binary mixture subject to realistic boundary conditions of imposed zero mass flux on the solid walls shows solutions that might lead to unrealistic negative values of the mass fraction / solute ocncentration. This effect was investigated and presented in this paper and a possible way of addressing it is suggested via a mass-fraction-dependent thermodiffusion coefficient that can have negative values in regions of low mass fractions. The latter suggests the posibilty of particles moving locally up the mass fraction / solute concentration gradient rather than the usual Fick's law direction.