Ring coils heat source numerical modeling and thermal interference characteristics analysis of truncated cone helix energy pile

The truncated cone helix energy pile (CoHEP) is a new type of environmentally-friendly and cost-effective ground heat exchanger with a strong heat transfer capacity. However, its heat transfer characteristics, especially thermal interference, have not yet been fully interpreted due to its complex geometry and semi-infinite medium. In order to understand the thermal interference in the radial and generatrix directions, an efficient modeling method that regards a spiral pipe as a series of ring coils was proposed in this paper based on their structural characteristics and makes these ring coils equivalent to heat sources that are based on heat transfer in the pipe. The thermal interference index based on the temperatures at typical locations was defined to describe the thermal interference of CoHEP. And the accuracy and high efficiency of equivalent heat source modeling method was verified by field experiments. Finally, the CoHEPs of cylindrical energy pile at a cone angle of 0 degrees, 5 degrees, 10 degrees, 15 degrees and 20 degrees were numerically simulated. The results show that the thermal interference intensity decreased with the increase of the cone angle, the maximum thermal interference intensity in the radial direction was 4.54 m.degrees C/W, 4.41 m.degrees C/W, 4.37 m.degrees C/W, 4.31 m.degrees C/W, and 4.23 m.degrees C/W respectively, and that in the generatrix direction was 3.96 m.degrees C/W, 3.91 m.degrees C/W, 3.88 m.degrees C/W, 3.84 m.degrees C/W and 3.77 m.degrees C/W, respectively, proving that the CoHEP with a large cone angle can significantly reduce thermal interference. Meanwhile, a scientific zone criterion of the heat transfer characteristics of CoHEP was given quantitatively to evaluate the thermal interference of CoHEP from the interference intensity in the radial direction and directions. Particularly, three zones of weak thermal interference zone, transition thermal interference zone and serious thermal interference zone were presented. There was no difference in the proportions of weak zone, transition zone and serious zone for 0 degrees 5 degrees, 10 degrees and 15 degrees, which were 7.69%, 15.38% and 76.92%, respectively. The proportions of thermal interference weak zone, transition zone and serious zone in the radial direction of 20 degrees CoHEP were 7.69%, 23.08% and 69.23% respectively, and that of thermal interference in the generatrix direction were 7.69%, 17.05% and 75.26%, respectively, implying that CoHEP with a large cone angle exhibits better heat transfer characteristics and is not easy to develop into a region with severe thermal interference. (C) 2020 Elsevier B.V. All rights reserved.