# Effect of Different Control Strategies on the Heat Transfer Mechanism of Helical Energy Piles
## 4 Conclusions  
This study used the G-function approach to analyze the heat transfer model of a parallel double helix energy pile at different pitches, and it studied and analyzed the temperature and velocity fields of a spiral pipe at various operating settings. The following conclusions were drawn from the results:  
(1) Lowering the helical pitch can significantly raise the SGHEs-P's heat transmission capacity while also improving the pile foundation's horizontal and vertical temperature distribution uniformity.  
(2) Over an extended period of time, intermittent operation, as opposed to continuous operation, can achieve a higher and more consistent daily heat transfer and successfully lessen heat damage to the soil that the energy pile system causes.  
(3) Under the intermittent operation approach, fluid thermal resistance makes up a clearly larger percentage of the energy pile's overall thermal resistance than it does under the continuous operation method. As a result, under the intermittent operation strategy, the fluid properties will have a greater influence on the energy pile's heat transfer effect.  
(4) The fluid velocity in the tube's core spirals faster as the spiral pitch rises, thickening the boundary layer. When the dimensionless pitch (SP) exceeds 32, a stable secondary circulation in the spiral cross-section would not been observed obviously, which hinders heat exchange and vice versa. It was also discovered that variations in spiral tube diameter have no appreciable impact on heat transfer.  
(5) In order to more precisely calculate the convective heat transfer coefficient in the spiral tube, correction factors for spiral pitch, spiral diameter, and spiral tube diameter were determined based on the simulation findings. A convective heat transfer coefficient calculation method was presented in relation to three correction variables. There is a significant link between the correction coefficient and SR and SP. For \(\varepsilon_{\text{rs}}\), a local minimum was seen, with SR = 16 resulting in the least value. Consequently, one can improve \(\varepsilon_{\text{rs}}\) by modifying the surface roughness (SR) in an upward or downward direction.  
Writing--original draft preparation, P.W. and K.L.; writing--review and editing, P.W. and K.L.; software--P.W., C.Y. and K.L.; resources, Q.H. All authors have read and agreed to the published version of the manuscript.  
This research was funded by the \"National Natural Science Foundation of China (NSFC)\", grant number 42202321.  
**Institutional Review Board Statement:** Not applicable.  
**Informed Consent Statement:** Not applicable.  
**Data Availability Statement:** The data presented in this study are available on request from the corresponding author.  
**Conflicts of Interest:** The authors declare no conflict of interest.  
**Nomenclature**  
\(T\) Temperature, K  
\(P\) Helical pitch, m  
\(r_{\text{s}}\) Helix radius, m  
\(d\) Diameter of spiral tube, m  
\(de\) Characteristic length, m  
\(h\) Height, m  
\(L\) Length, m  
\(\rho\) Densities, kg/m\({}^{3}\)  
\(c_{p}\) Specific heat capacity, J/ kg\(\cdot\)K  
\(k\) Thermal conductivity, W/m\(\cdot\)K  
\(q\) Heat flux, J/m\({}^{2}\)  
\(a_{\text{eff}}\) Convective heat transfer coefficient, W/m\({}^{2}\cdot\)K  
\(u\) Flow velocity, m/s  
\(\Phi\) Heat transmission, J  
\(R\) Dimensionless equivalent thermal resistance  
\(\theta\) Dimensionless temperature  
\(Re\) Reynolds number  
\(Pr\) Prandtl number  
\(Fo\) Fourier number  
\(Nu\) Nussle number  
C Inaccuracy  
\(Q\) Heat exchange, W  
**Greek alphabet**  
\(\tau\) Time, s  
\(\mu\) Dynamic viscosity, Pa\(\cdot\)s  
\(\
u\) Kinematic viscosity, m\({}^{2}\)/s  
\(\epsilon\) Correction factor  
\(\gamma\) Volume ratio  
**Suffix**  
\(st\) Tubes  
\(sp\) Spiral tube  
\(f\) Fluid  
\(g\) Soil  
\(p\) Pile  
\(b\) Pile wall  
\(int\) Internal  
\(ext\) External  
\(avg\) Average  
\(tube\) Tube wall