Effect of convection on liquid diffusion experiments

The knowledge of diffusion coefficients in the liquid state is necessary to get a better understanding of the structure and the transport phenomena in metallic and semi-conducting melts. In liquid diffusion measurements, one of the main problems is the additional transport by convection. On ground, the measured diffusion coefficients of liquids are often too high due to the additional mass transport of the natural convection, especially at high temperatures. In microgravity, self diffusion experiments were performed successfully but interdiffusion experiments with major density differences failed because of residual accelerations. With the finite element rnethod the convection suppression by stabilizing concentration gradients was proofed, For liquid diffusion experiments on ground, the influence of steady-state convection is analysed quantitatively with the Monte-Carlo method. This method is enhanced to calculate the influence of transient convection caused by oscillatory acceleration in space. Taylor's expression for effective diffusity D(eff) = D + alpha (fr(2)/D) v(0)(2)r(2)/D with a frequency dependent proportionality constant was derived for different velocity fields. For the experimental proof an experiment using the diffusion and heat conduction analogy was tested during parabolic flights.