Simulation analysis and verification of heat transfer characteristics of truncated cone helix energy pile

Abstract: In order to reduce the thermal interference and to improve the heat transfer efficiency, a novel "truncated cone helix energy pile (CoHEP)" was proposed in this study. And in order to simulate the thermal performance of the novel CoHEP more accurately, a three-dimensional numerical model was established with consideration of the inhomogeneous initial soil temperature and the dynamic boundary condition above the soil surface. The heat transfer characteristic of the novel CoHEP of different cone angles was studied. In addition, in order to verify the accuracy of the numerical model, a verification test was conducted. The results showed that the novel CoHEP can be divided into four heat transfer stages along the flow direction: entrance stage → thermal short circuit stage → small temperature difference stage → exit stage. Among them, the heat transfer capacity of the entrance stage was the strongest, and the heat transfer capacity continued to decrease during the thermal short circuit stage and the small temperature difference stage, eventually the heat transfer capacity rebounded during the exit stage. The thermal interference at the bottom of the CoHEP was more serious, and the larger the cone angle, the more serious the thermal interference at the bottom. In contrast, the thermal interference effect was weaker at the top of the CoHEP due to the larger helix radius at the top. The heat flux per unit pipe length of the CoHEP increased linearly with the increase of cone angle. That was because with the same pipe length (Lpipe), pitch in the depth direction (b) and energy pile height (hpipe), increasing the cone angle would increase the top radius (rt) of the CoHEP, leading to small thermal interference in the upper part of the CoHEP. At the same time, more high-temperature fluid was located in the upper part of the CoHEP which directly contacted with the covered soil area. Thus the heat transfer capacity increased. When the system operating time was 12 h, the cone angle increased from 0? to 10? to 20?, the increasing rate of the heat flux was 2.54% and 3.53%, respectively. The thermal interference in the upper part of the novel CoHEP was much smaller than that of the traditional CyHEP, and more high-temperature fluid was located in the upper part of the energy pile which was good for heat transfer. In addition, the distance between the adjacent pipes in the axial direction (d) of the novel CoHEP was significantly larger than that of the traditional CyHEP under the same pitch in the depth direction (b), which can effectively reduce the axial thermal interference. Thus the heat flux per unit pipe length of the novel CoHEP was greater than the traditional CyHEP. And when the cone angle was 20?, the heat flux per unit pipe length of the novel CoHEP was 6.16% higher than that of the traditional CyHEP.