On the problem of natural convection in liquid phase thermotransport coefficients measurements

We focus in this paper on the effect of natural convection in thermodiffusion coefficients measurements in liquid metal alloys both for normal and microgravity conditions. Our previous experimental results are briefly recalled, with a special emphasis on the data recently obtained from the EURECA space mission. With respect to the ground based values, it is seen that the solutal separation is always significantly higher in microgravity, even in systems where solutal stabilization of the flow has an effect. Simple scaling analysis arguments show that the error induced by additional convective transport scales with the square of the fluid velocity. Such a result compares favorably with existing three dimensional (3D) numerical data. The theory also accounts qualitatively for the reduced separation observed experimentally in ground based set-ups. We conclude that it is in principle possible to perform accurate measurements in space, but that the size of the capillaries used in the experiments should always be limited to roughly two millimeters. On Earth on the other hand, the risk of convective interference cannot be avoided.