# Finite Element Analysis of Combined Energy Piles with Long and Short Heat Exchanger Tubes
## 1 Introduction
This new energy pile technology can not only make full use of the advantages of buried pipes in the pile, it can also further extract the deep geothermal energy at the bottom of the pile foundation to increase the total heat exchange of the heat exchanger. Compared with traditional energy piles, deep-buried pipe energy piles can increase the heat transfer path and time and improve the overall heat exchange. Since the temperature of the deep formation is higher than that of the upper part in winter and lower than it in summer, this is conducive to heat transfer, so the heat exchange per unit tube length will also increase. The reasonable setting of the deep well can not only reduce the thermal interference caused by the heat accumulation of the pile foundation but also compensate for the adverse effect of the small spacing of the heat exchanger tubes on the heat exchange. However, geothermal energy within the depth range of the pile body is not fully utilized. The research methods for the above problems include field testing, theoretical research, numerical simulation, artificial intelligence, and other methods [3; 18; 19]. Among these references, numerical simulation can be used as an important research means to verify the new technology. [PERSON] et al. [20] conducted a multi-physics finite element analysis of energy piles and found that the mechanical properties of energy piles did not change much due to conventional temperature loading. [PERSON] et al. [21] established a thermo-mechanical coupling numerical model of energy pile considering groundwater seepage and concluded that the existence of groundwater seepage can effectively increase the heat transfer of energy pile but has no significant effect on the mechanical properties of energy pile. [PERSON] et al. [22] used the three-dimensional (3D) finite element to simulate the thermo-mechanical coupling numerical model of energy piles with different groundwater seepage flow rates and concluded that increases in the groundwater seepage flow rate can effectively increase the heat transfer of energy piles. [PERSON] et al. [23] employed a three-dimensional heat transfer numerical model to investigate the impact of heat transfer tube spacing, thermal conductivity, heat transfer liquid velocity, and the thermal conductivity of the heat transfer liquid on the heat transfer performance of energy piles. Their analysis revealed that, as the spacing between heat transfer tubes increases, the occurrence of thermal short circuits gradually diminishes, resulting in a gradual enhancement of the heat transfer performance of the energy piles. Based on this, the study introduces a new energy pile that combines long and short heat exchanger tubes. This technology arranges heat exchanger tubes with different lengths inside and outside the pile. Finite element simulation by ABAQUS is carried out to study the feasibility of this new technology. The temperature and mechanical behavior under different groundwater seepage flow velocities are analyzed and compared to provide a reference for future practical application of this new energy pile.