Long-term Thermo-hydraulic Response of the Shallow Subsurface soil in the Vicinity of a Buried Horizontal Heat Source

Abstract

Heat and mass transfer in the shallow subsurface and in the vicinity of buried heat sources has been often studied based on rather simplifying assumptions, such as the Equilibrium Phase Change (EPC), disregarding film flow, and adsorptive soil-water characteristics. In this study, a fully coupled non-isothermal multiphase flow model is developed that utilizes the Non-equilibrium Phase Change (NEPC) approach. First, the performance of the EPC and NEPC approaches is assessed and validated against the in-field experimental data. The second and main objective of this study is to analyze the thermo-hydraulic response of granular soil close to a buried horizontal heat source (e.g., electrical cables) for one year under real meteorological conditions. Numerical results indicate that the NEPC model has higher robustness in predicting the volumetric water content in the soil close to the soil-atmospheric boundary. However, NEPC and EPC approaches estimate the same temperature variations in the soil. In the case of precipitation (e.g., rainfall), the NEPC model predicts higher degrees of saturation in deeper soil in which the infiltrated water reaches 150 cm below the surface, while, in the EPC model, the infiltrated water barely reaches the 100 cm depth. The increase in moisture content close to the heat source facilitates the heat transfer in the medium and thus results in a 40% to 50% reduction in soil temperature close to the heat source. An accurate prediction of the soil moisture content in the medium is needed to evaluate the performance and thermal capacity of buried electrical cables. Furthermore, the heating/cooling cycles contribute to gradually drying the soil in the vicinity of the heater until the hydraulically equilibrated condition is attained.