Measurement of solute diffusivities. Part I. Analysis of coupled solute buoyancy-driven convection and mass transport

An effective diffusivity model is developed for mass transport by molecular diffusion and unsteady solute buoyancy-driven convection in a horizontal c apillary by a method similar to that used for Taylor dispersion in forced f low. A ID non-linear diffusion equation is obtained and an analytical asymp totic solution is found for dominant convective mass transport, the solutio n for dominant molecular diffusion is already well known. Both the effectiv e diffusivity model and the asymptotic solutions are validated numerically. A scaling analysis clarifies the boundary between dominant molecular diffu sion and dominant convective mass transport. The use of a steady vertical m agnetic field for liquid metals and semi-conductors is found to damp the co nvective mass transport by a factor of Ha(-4), where Ha is the Hartmann num ber, characterising the MHD problem. (C) 2001 Elsevier Science Ltd. All rig hts reserved.