Investigation on heat transfer characteristics of PHC energy piles in multi-layer strata

The PHC energy pile and traditional vertical ground heat exchanger have significant differences in heat exchanger structure, environmental sensitivity and heat transfer behavior. The long-term in-situ thermal response test of PHC energy pile in multi-layer strata was carried out at the energy pile testing site of Jiulonghu campus of Southeast University. The pile length is 24 m, and the heat exchange pipe is a single U-shape pipe. The duration of test reaches 1 100 h. The 20 m deep ground temperature borehole is set at 0.5, 0.65 and 1.15 m from the center of the energy pile to monitor the ground temperature. Field test combined with numerical simulation to analyze and study the heat transfer behavior of PHC energy pile in multi-layer strata and the influence of multi-layer strata on the heat transfer performance of energy pile. The research results show that: (1) At the beginning of heating, the closer the soil to the energy pile, the faster the temperature rises. After a long heating period, the amplitude and velocity of ground temperature rise of each soil layer have decreased. For instance, after heating for 760 h, the temperature rise rate of each point in the soil decreased by more than 80% compared with the first continuous heating. (2) When the thermal response power changes, the closer the soil is to the energy pile, the greater the temperature of the energy pile and the more sensitive the response. As the distance increases, the temperature influence becomes smaller. The soil temperature response has a delay effect, and the further away from the energy pile, the greater the delay. (3) At the soil layer with high thermal conductivity and thermal diffusivity, the ground temperature amplitude and velocity of the soil near the energy pile are both small. Conversely, at the soil layer with lower thermal conductivity, the soil temperature near the energy pile rises faster and rises at a greater speed. (4) After the TRT unit stopped heating, the ground temperature at all points decreased and recovered quickly in the vicinity, while in the distance, the ground temperature showed a first increasing and then decreasing trend due to the delay effect. (5) Numerical simulation results show that thermal conductivity and specific heat capacity produce different effects on soil heat transfer. The heat exchange of the energy pile increases when the soil thermal conductivity is higher. © 2020, Science Press. All right reserved.