The Role of Atmospheric Demand and Internal Heat Sources Embedded in the Vadose Zone on the Thermal and Moisture Fluctuations in the Shallow Subsurface soil

The careful assessment of transient heat and moisture transfer in the vicinity of a buried energy geo-structures and underground electrical cables in shallow subsurface soils requires an advanced coupled thermo-hydraulic theoretical framework. However, most studies have been performed based on rather simplifying assumptions, such as the Equilibrium Phase Change (EPC) and neglecting the adsorptive regime of Soil-Water Retention Characteristics (SWRC), which disregard some of the most significant factors in the strongly coupled thermo-hydraulic process. The existing experimental observations in the laboratory- and field-scale for sands and sandy loams suggest that the phase change is a time-dependent process, especially in the transition from the first to the second stage of evaporation. Moreover, in the hydraulically dry condition, where the adsorptive forces are dominant in the total matric suction, the liquid film flow can significantly contribute to the overall liquid advective flux. In the present study, we present a complete non-isothermal multiphase flow model that utilizes: The Non-equilibrium Phase Change (NEPC) approach to handling the evaporation/condensation process; a non-isothermal SWRC model which covers capillary and adsorptive regimes; the Hydraulic Conductivity Function (HCF) to consider capillary and film flow; and the temperature- and saturation-dependent Thermal Conductivity Function (TCF). The objective of our research is to: (a) calibrating the theoretical parameters of the current model, which have physical meanings, with experimental observations. (b) Comparing the NEPC with EPC models to distinguish the key differences of these two approaches. (c) Performing a numerical study of heat, liquid, and vapor flow from an in-field bare sandy loam for a year under the real meteorological condition (i.e., natural evaporation and rainfall) when it is also subjected to thermal loading from a buried horizontal heat source. The outcome of our study marks the fundamental differences of EPC and NEPC approaches and advantages of the NEPC model with respect to the cyclic thermal loading as well as wetting induced by rainfall in the soil. In addition, the numerical results highlight the importance of temperature- and saturation-dependency of HCF and TCF that we incorporate in our model.