Insights into thermal-mechanical behaviors of energy tunnels with groundwater through full-scale experiments and simulations

As a sustainable geothermal structure, energy tunnels have garnered attention for their potential as a low-carbon energy supply source. Their thermal and mechanical behaviors are significantly affected by groundwater, particularly in soils with a high water table. This study investigates the thermal-hydro-mechanical performance of energy tunnels through full-scale experiments and numerical simulation. The experimental setup utilized prototype concrete segments embedded with heat exchange pipes in sandy soils. The effects of water temperature and flow velocity, as well as groundwater seepage, on the heat exchange efficiency of energy segments were investigated. Additionally, a three-dimensional numerical model was established, which reproduced the heat transfer process of the energy segments and extended parametric studies. The results demonstrate that increasing the flow velocity inside pipes can reduce the thermal boundary layer thickness and thereby enhance the heat exchange rate. Groundwater seepage exhibits a dual beneficial effect: it acts as a dominant driver for heat transfer, increasing the heat exchange rate by 36.7%, while reducing thermally induced stresses by 22.8% through convective cooling. Based on the experimental and numerical data, a generalized dimensionless fitting formula was derived. The findings provide support and design criteria for the application of energy tunnels in saturated strata.