Analysis on variations of ground temperature field and thermal radius caused by ground heat exchanger crossing an aquifer layer

Due to the groundwater migration in the underground aquifer, the heat transfer between ground heat exchangers and surrounding ground changes from heat conduction to the conjugated conduction–convection mode. To investigate the aquifer effects on the ground temperature distribution surrounding the ground heat exchanger, a realistic model was established and numerically solved, including a ground heat exchanger and alternatively stacked aquifer and aquifuge layers. The results show that a variation in groundwater velocity would result in a significant fluctuation in the aquifer temperature field close to the ground heat exchanger, but has less effect on the aquifer temperature field away from the ground heat exchanger. The difference between the initial temperature and local stable ground temperature, and the time for the aquifer to reach the stable temperature are both negatively correlated with the groundwater velocity, and positively correlated with the distance to ground heat exchanger on the downstream. The thermal influence radii are ranging from 7.4 m to 143.0 m in the aquifer layer under tested groundwater velocities ranged from 3.15 m/a to 315 m/a respectively, while the radii of aquifuge layer are about 8.3–8.4 m. There exists a critical velocity that makes the radius of thermal influence in the aquifer layer the same as that in the aquifuge layer. When the groundwater velocity is greater than the critical velocity, the thermal influence radius shows an increasing trend with the increase of aquifer layer thickness, while it shows a reversed trend for the velocity lower than the critical velocity.