Modeling of coupled transfer of water, heat and solute in saline loess considering sodium sulfate crystallization

The phase change of sodium sulfate significantly affects the salt weathering of loess slopes in seasonal frozen regions, which should be involved in the modeling of heat, water and solute transfer. The governing equation for heat transfer was established based on the Harlan model by incorporating ice-water phase change and crystallization of sodium sulfate and an empirical relationship for thermal conductivity of saline loess. Regarding the moisture field, a modified factor that characterizes the impedance of salt crystals and ice on water migration in frozen area was introduced into the Gardner model. The total suction as a function of water and salt contents was included in the unfrozen water migration equation. Considering the convection, diffusion and phase change of solutes, the equation for salinity field was established by Darcy's and Fick's laws, with a solubility function based on the solubility curve of sodium sulfate. The above models were numerically implemented on COMSOL Multiphysics through the user-defined partial differential equation module. In order to verify the rationality of the method, the profiles of temperature, water and salt contents of saline loess columns obtained by uniaxial freezing tests in closed system were compared with the numerical results. Comparisons show that the simulated profiles of temperature, water and salt contents at three cold-end temperatures (-5, -10 and - 20 degrees C) agree well with the measured data. According to the profile of the amount of crystalline salt, the concept of uncrystallized zone was proposed, which is found to be positively related to the cold-end temperature.

Automated summary

The phase change of sodium sulfate plays a significant role in salt weathering of loess slopes in seasonal frozen regions. Models for heat, water, and solute transfer were established based on various empirical relationships and laws. Numerical simulations were conducted using these models on COMSOL Multiphysics, showing good agreement with experimental results.