Abstract: During soil freezing and thawing, water migration, phase transition and heat transfer are a multi-field coupled process in which these factors affect each other. In this study, a mathematical model for the coupling process of water migration and heat transfer was constructed by solving a simultaneous equation group, which contains the basic equations of water migration and heat transfer, and the equilibrium equation of water phase change and temperature. In the mathematical model, the effects of water phase change on hydraulic and thermal parameters were considered, so was the effect of latent heat in water phase change. Also, a code for the coupled heat transfer and water migration simulation was developed based on the finite volume method for the spatial discretization and fully implicit backward difference scheme for the time discretization. The code supporting unstructured mesh can ensure the conservation of mass and energy. It also has a parallel function, which can be used for the calculation of complex problems. Furthermore, an indoor soil freezing tests with two different temperature boundary conditions were conducted, and the corresponding numerical calculations were subsequently conducted. The experimental results generally matched the calculated ones, so the developed numerical simulation program can well simulate the evolution characteristics of the temperature field and water field during the soil freezing process. Hence, the established OpenFOAM program is an effective method to simulate the coupling process of water migration and heat transfer for the frozen soil.

Key words: freezing soil, coupled process of water migration and heat transfer, OpenFOAM, finite volume method, temperature, unfrozen moisture content, ice content