# Finite Element Analysis of Combined Energy Piles with Long and Short Heat Exchanger Tubes
## 4 Analysis of the Results
### Temperature of the Pile Body and the Cement Soil Inside the Pile
In the scenario where heat exchange pipes were arranged inside the pile, significant variations in temperature were observed in the cement soil located below the pile's depth range. After a 10-day heating process, the average temperature of the cement soil at \(L_{2}\) = 80 m was 30.3 \({}^{\circ}\)C. At \(L_{2}\) = 60 m, the average temperature of the cement soil was 32.8 \({}^{\circ}\)C. After a 10-day cooling process, the average temperature of the cement soil at \(L_{2}\) = 80 m was 14.9 \({}^{\circ}\)C. At \(L_{2}\) = 60 m, the average temperature of the cement soil was 13.9 \({}^{\circ}\)C. The results revealed that a greater amount of heat exchange occurred between the heat exchange tube and the soil at \(L_{2}\) = 80 m.  
Since the length of the heat exchanger tubes on the outside of the pile is 40 m, the same length as the pile, the heat exchange between the heat exchanger tubes and the pile body at the pile end position will be significantly reduced, so the pile end temperature will have a sudden change in the same direction along the depth change trend. From the above data, it can be seen that, after adding the heat exchanger tubes on the inside of the pile, the temperature of the pile body changed significantly, but with the increase of the embedding depth of the heat exchanger tubes, the change of the temperature of the pile body gradually decreased. This is because, after the addition of the heat exchanger tubes on the inside of the pile, the heat exchange between the pile body and the heat exchanger tubes increased significantly, but with the increase of the length of the heat exchanger tubes on the inside of the pile, the heat exchanger tubes and the soil below the pile carried out more heat exchange, so the temperature difference between the heat exchange liquid and the pile body in the heat exchanger tubes decreased, leading to the decrease of the change of the temperature of the pile body and cement soil. When the inner heat exchanger tubes of the pile were not laid, the average temperature difference between the cement soil in the pipe pile and the pile body was almost negligible, and after the inner heat exchanger tubes of the pile were laid, the temperature difference between the two was relatively obvious, and with the increase of the length of the heat exchanger tubes on the inside of the pile, the temperature difference between the two gradually decreased from \(-\)0.91 \({}^{\circ}\)C to \(-\)0.51 \({}^{\circ}\)C. This is because, as the length of the heat exchanger tubes inside the pile increases, the heat exchange between the heat exchanger tubes and the surrounding soil body increases significantly. However, this also leads to a decrease in the heat exchange between the heat exchanger tubes and the cement soil. The temperature of the cement soil below the pile depth range was significantly lower than that of the cement soil within the pile depth range. This phenomenon occurred because the temperature of the heat exchange tube gradually decreased as it penetrated deeper into the soil, resulting in a gradual reduction in the temperature deviation between the heat exchange tube and the soil. In particular, the temperature of the cement soil experienced a significant variation at 40 m, which was consistent with the abrupt variation in the pile temperature at this depth. Figure 7 shows the temperature change curve of the pile body and cement soil in the pile with depth under different groundwater flow velocities (\(v\) is the groundwater seepage flow rate). From Figure 7a, it can be seen that the temperature of the pile body decreased with the increase in depth, and the temperature changed suddenly when reaching the pile end.