# Long-Term Thermo-Hydraulic Numerical Assessment of Thermo-Active Piles--A Case of Tropical Soils
## 3 Results
### Results and Discussions
#### 3.2.3 Influence of Groundwater Flow  
The presence and movements of the groundwater can greatly influence the performance of thermo-active geostructures. [PERSON] et al. (1997) show that when the velocity of groundwater flow is in the range from 1e-6 to 1e-4 m/s, the groundwater flow has a non-negligible influence on the heat transfer of thermo-active piles ([PERSON] et al., 2016). [PERSON] et al. (2016) have also investigated the influences of groundwater flow velocity and of several other parameters on the heat exchange by conducting a numerical analysis ([PERSON] et al., 2016). When the groundwater flow is present, with an increase from 0 to 2 m/day, the exchanged heat improves by a factor of 3 to 8 times compared to the case without groundwater. In the summer heat injection period, the surrounding soil around the pile may trap heat. By dissipating the high temperature elsewhere where the soil temperature is lower, the groundwater flow could be favorable ([PERSON] et al., 2016). This can eventually result in a steady condition and ensure the system performance. The phenomenon, named natural thermal recharging or natural thermal recovery, corresponds to extracting the heat injected in summer to meet the heating demand during winter and vice versa. It is worth mentioning that soils with high groundwater flow (up to 1e-6 m/s) can produce natural heat regeneration ([PERSON] et al., 2016). The impact of groundwater flow is then discussed in this section, three groundwater flow velocities are considered: \(v_{D}\) = 0 m/day, 0.1 m/day, and 1 m/day. The thermal solicitation of T1- Brasilia is adopted for all the cases with a pile diameter of 1 m.  
It can be seen in Figure 13a that the pile circular geometry slightly affects the groundwater flow velocity around the pile in the surrounding soil, creating an asymmetric form around the pile-soil interface. Compared to the average flow velocity, the flow velocities on the pile upstream and downstream sides are small, while there is an increase in flow  
Figure 11: Temperature profiles around the pile with different diameters after 10 years of T1 thermal solicitation. Figure 12: The variation of average pile body temperatures over 10 years for three pile diameters. velocity near the tangency point of the pile-soil interface. Due to the advective heat transfer, asymmetry is also found in the temperature field (Figure 13b,c). A thermal plume is developed downstream of the pile and is maintained during the thermal solicitation cycles. The dissipation of energy leads to a moving thermal front whose velocity is proportional to the flow velocity. The average temperature of the pile-soil interface reached thermal equilibrium at the end of the 1 st year, except for the case of \(v_{D}\) = 0 m/day (Figure 14a), which implies even a small velocity of water flow can bring the pile temperature to thermal equilibrium quickly. The fluctuation range of average temperature is also reduced with the increase of water flow velocity. To better define the relationship between the average pile temperature fluctuation range after thermal equilibrium and the groundwater velocity, cases with \(v_{D}\) = 0.25 m/s, \(v_{D}\) = 0.5 m/s, \(v_{D}\) = 0.75 m/s are presented. As shown in Figure 14b, a a nonlinear relationship is observed and the fluctuation range for the pile-soil interface temperature follows the same trend. Compared to the case of \(v_{D}\) = 0 m/day, the fluctuation range of average temperature decreases by about 45% for the pile and 55% for the pile-soil interface in the case of \(v_{D}\) = 1 m/day. It implies that the groundwater flow significantly reduces the pile thermal disturbance induced by the thermal operation, improving the pile thermal sustainability. Table 4 visually quantifies the degradation in heat transfer capacity for different control volumes after 10 years. The energy exchanged induced by conduction and advection per unit of the control volume is denoted by \(Q_{cond}\) and \(Q_{adv}\), respectively.