| Theories of thermal diffusion, or Ludwig-Soret effect, were studied in detail based on comparisons with available experimental data, and challenges in thermal diffusion modeling were clarified. The Firoozabadi model was chosen and used in numerical simulations of thermo-solutal convection in porous media in order to investigate the compositional variation due to processes of thermal diffusion, pressure diffusion, molecular diffusion and convection.; A thermal diffusion model can be fully established based on the conservation laws of transport theory and laws of non-equilibrium thermodynamics including a recent concept, the heat of transport. All thermal diffusion models can precisely be identified by their different definitions of the heat of transport. Existing models of Rutherford, Haase, Dougherty and Drickamer, Kempers, and Firoozabadi were studied for applications to water-alcohol mixtures, and their characteristics were investigated. The Firoozabadi model performed better than all other models. The energy of viscous flow used in the Firoozabadi model improved the modelling of Ludwig-Soret effect.; A numerical scheme was developed to apply the Firoozabadi model to simulate thermo-solutal convection in porous media. Methane and n-butane binary mixture was used to investigate physical details of the thermo-solutal convection in porous media. Numerical results for a two-dimensional cavity with lateral heating revealed three patterns of thermo-solutal convection based on the magnitude of the permeability of porous media, that is, Soret dominant, combined Soret/convection dominant, and convection dominant patterns in terms of compositional variations. This conclusion is strengthened in a porous cavity with heterogeneous permeability. In three dimensional simulations, different contributions from thermal diffusion, molecular diffusion, and pressure diffusion with lateral heating and combined heating conditions were investigated. All three spatial components of a diffusion flux were considered in order to properly evaluate their contributions. With given properties, certain criteria have been derived to determine which diffusion is dominant in the compositional variation process during thermo-solutal convection. |
