Coupled heat and moisture migration in unsaturated soils subjected to thermal gradients

Various mathematical formulations have been proposed to model moisture migration coupled with heat transfer in unsaturated soils under non-isothermal conditions. These formulations adopt different assumptions and approaches to incorporating phase change phenomena. This has led to confusion when assessing the performance of ground heat exchangers in unsaturated soils. This research provides insights into the development of fully coupled hydro-thermal numerical models for partially saturated soils under thermal loading. The multiphysics phenomenon involved is clearly discussed, and the governing equations are presented for both the equilibrium phase change (EPC) and non-equilibrium phase change (NEPC) approaches. A parallel comparison is then made between the two approaches through the estimation of variation in the degree of saturation in unsaturated soils when subjected to thermal gradients. The suitability of the two approaches for numerical modeling of unsaturated soils in the context of ground heat exchangers is then discussed. Considering the uncertainties in parameter identification, the results indicate that the EPC approach is sufficiently accurate and is often preferred over the NEPC approach in hydro-thermal modeling of ground heat exchangers in unsaturated soils.