Numerical Analysis of the Factors Influencing a Vertical U-Tube Ground Heat Exchanger

The development of a three-dimensional, unsteady state model, which couples heat transfer with groundwater seepage for a vertical U-tube ground heat exchanger (GHE) is presented. The influence of underground soil thermal properties, grout materials, inlet water temperature and velocity, and groundwater seepage on heat transfer in the GHE is examined. The results indicate that before the heat in the borehole is saturated, the heat flux in the GHE is directly proportional to the thermal conductivity coefficient of the grout materials. The radius of the thermal effect of the GHE and the recovery rate of the temperature in the soil are also proportional to the thermal diffusion coefficient of the soil. In cooling mode, the increase of the inlet water temperature of the GHE results in enhanced heat transfer. However, this may cause issues with heat buildup. The increase of the inlet water velocity in the GHE enhances heat convection in the tube. The effect of thermal-seepage coupling in groundwater can reduce the accumulated heat, thus resulting in the effective enhancement of the heat transfer in the GHE.