# 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  
Figure 6 shows the temperature variation curve of the pile body and cement soil in the pile with depth under different heat exchanger tube length combinations (\(L_{1}\) is the length of the outer heat exchanger tubes of the pile and \(L_{2}\) is the length of the inner heat exchanger tubes of the pile). It can be seen from Figure 6a that the temperature of the pile body decreased with the increase in depth under the heating condition. Under the cooling condition, the temperature of the pile body increased with the increase in depth. And in the case of rising and lowering temperature, the temperature of the pile end changed abruptly. After 10 days of warming process, when \(L_{2}\) = 0, 40, 60, and 80 m, the pile body changed by 18.35 \({}^{\circ}\)C, 20.97 \({}^{\circ}\)C, 19.61 \({}^{\circ}\)C, and 18.84 \({}^{\circ}\)C, respectively, compared with the initial average temperature. Compared with \(L_{2}\) = 0 m, when \(L_{2}\) = 40, 60, and 80 m, the pile temperature increased by 14.28%, 6.87%, and 2.67%, respectively. When \(L_{2}\) = 40 m, the increased heat exchange between the heat exchanger tubes and the pile body was 5.34 times that of \(L_{2}\) = 80 m and 2.08 times that of \(L_{2}\) = 60 m. After 10 days of the cooling process, when \(L_{2}\) = 0, 40, 60, and 80 m, the pile body changed by \(-\)7.77 \({}^{\circ}\)C, \(-\)9.09 \({}^{\circ}\)C, \(-\)8.77 \({}^{\circ}\)C, and \(-\)8.58 \({}^{\circ}\)C, respectively, compared with the initial average temperature. Compared with \(L_{2}\) = 0 m, when \(L_{2}\) = 40, 60, and 80 m, the pile temperature decreased by 17.00%, 12.87%, and 10.42%, respectively. When \(L_{2}\) = 40 m, the additional heat exchange between the heat exchanger tubes and the pile body was 1.63 times that of \(L_{2}\) = 80 m and 1.32 times that of \(L_{2}\) = 60 m.  
Figure 5: Pile top load-settlement numerical simulation curve. Figure 6: Temperature change curve of pile body and pile cement soil with different heat exchanger tube lengths under the condition of \(2\times 10^{-4}\) m/s flow rate. (**a**) File body temperature change curve with depth. (**b**) Change curve of cement-soil temperature with depth. The temperature variation pattern of the cement soil within the pile depth range Figure 6b was consistent with the temperature variation pattern of the pile body exhibited in Figure 6a. After 10 days of warming process, when \(L_{2}\) = 0, 40, 60, and 80 m, the cement soil in the pile changed by 18.30 \({}^{\circ}\)C, 24.49 \({}^{\circ}\)C, 22.02 \({}^{\circ}\)C, and 20.66 \({}^{\circ}\)C, respectively, compared with the initial average temperature. When this was compared with the pile temperature in the same situation, it was found that the temperature changed by \(-\)0.05 \({}^{\circ}\)C, 5.53 \({}^{\circ}\)C, 2.41 \({}^{\circ}\)C, and 1.82 \({}^{\circ}\)C, respectively. 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 nothing, and after the inner heat exchanger tubes of the pile were laid, the temperature difference between the two was relatively obvious. 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 5.53 \({}^{\circ}\)C to 1.82 \({}^{\circ}\)C. After 10 days of cooling process, when \(L_{2}\) = 0, 40, 60, and 80 m, the cement soil in the pile changed by \(-\)7.73 \({}^{\circ}\)C, \(-\)10.00 \({}^{\circ}\)C, \(-\)9.43 \({}^{\circ}\)C, and \(-\)9.09 \({}^{\circ}\)C, respectively, compared with the initial average temperature. When this was compared with the pile temperature in the same situation, it was found that the temperature changed by 0.04 \({}^{\circ}\)C, \(-\)0.91 \({}^{\circ}\)C, \(-\)0.66 \({}^{\circ}\)C, and \(-\)0.51 \({}^{\circ}\)C, respectively.