# Effect of Different Control Strategies on the Heat Transfer Mechanism of Helical Energy Piles
###### Abstract  
In this paper, numerical simulations of a special energy pile, which constitutes a spiral-injected pipe and one straight discharge pile for Geothermal Heat Pump Systems (SGHEs-P(parallel)), were conducted by Fluent software. The effects of the spiral pitches on the heat transfer rate based on the G-function method and peripheral soil temperature of the pile were investigated under continuous and intermittent operation strategies. The impact of spiral tube sizing on the surface heat transfer coefficients was studied. The results indicated that SGHEs-P may be preferred for office buildings under intermittent operation conditions. For a short period, the temperature profiles and heat transfer efficiency of SGHEs-P were mainly influenced by the fluid type, length of the spiral tube, and spiral pitch. The smaller the spiral pitch, the more uniform the temperature distribution, and the better the heat transfer effect, but the heat transfer per unit depth of pile decreased. The average temperature variation curve of the soil around the energy pile with different spiral pitches was simulated and obtained over time. Meanwhile, the impact of spiral radius, spiral pitch, and spiral tube radius on the convective heat transfer coefficient was also presented. Through data fitting, the formulas for the correction coefficients of spiral radius, spiral pitch, and spiral tube radius on convective heat transfer coefficient were obtained, respectively. helical energy piles; intermittent operation; numerical modeling; soil thermal damage; correction factor +
Footnote †: journal: _Applied_  
liquid or a gas. In the same year, [PERSON] et al. [10; 12] and [PERSON] et al. [13; 14] studied the heat transfer characteristics of SGHEs with varying spiral pitches, respectively, through on-site construction, COMSOL calculation software, and the G-function approach. They found that the changes in fluid temperature and internal thermal resistance both decreased as the spiral radius decreased and that heat transfer efficiency was not directly proportional to the length of the internal pipe, suggesting that there might be thermal interference between the adjacent buried pipes in SGHEs. [PERSON] [15] and [PERSON] et al. [16] used experimental methods and finite element numerical modeling to investigate the fluid thermal resistance and heat transfer efficiency of SGHEs at different flow rates under different helical shapes and found that the flow rate did not affect the heat transfer efficiency of SGHEs under turbulent conditions. [PERSON] et al. [17; 18] investigated the effect of the spatial arrangement of helical tubes in a pile body on heat transfer efficiency using CFD and found that increasing the number of helical tube arrangements increased heat transfer efficiency while decreasing the number of arrangements led to a more stable heat transfer efficiency. [PERSON] et al. [19] studied the heat transfer process of double-spiral-type energy piles in series by spiral injection and spiral discharge (SGHEs-S) and compared the heat transfer efficiency of six pitches and found that the heat transfer efficiency of SGHEs-S could be improved by shortening the pitch. In their study on heat transfer in energy piles, [PERSON] et al. [20] found that the greater the difference in thermophysical properties between the soil and the pile foundation, the more the accuracy of the heat transfer prediction model is affected. [PERSON] [21] and [PERSON] et al. [22] studied the effect of simultaneously considering permafrost in the soil, groundwater seepage, and heat transfer through the water pipe of an energy pile and found that increasing the distance between the energy piles reduced the heat transfer effect of the piles but improved year-round performance. [PERSON] [23] and [PERSON] et al. [24] investigated the effects of groundwater seepage on heat exchange in energy piles and found that flowing groundwater was conducive to soil temperature recovery and the heat exchange of an energy pile.