Effect of Grout Thermal Conductivity and Leg-spacing on the Borehole Length of U-tube Ground Heat Exchanger
DOI:
https://doi.org/10.38032/scse.2025.3.169Keywords:
Ground Coupled Heat Exchanger, U-tube Heat Exchanger, Grout Materials, Python SimulationAbstract
This study explores the effects of grout thermal conductivity and leg spacing between a single U tube pipe on the borehole length of a ground-coupled heat exchanger, which is required to dissipate heat from a 1-ton air conditioner to the ground under the climatic conditions of Bangladesh. The result obtained from the zeroth-order multipole approach has been compared with first-order multipole, and Liao et al. methods to predict borehole length. Additionally, the impact of flow rate and inner pipe diameter on the pressure drop of U tube pipe has been evaluated here. Python-based simulations have been conducted to analyze them. The results demonstrate that increasing grout thermal conductivity significantly reduces borehole length—up to 31%—by minimizing grout thermal resistance. However, at higher conductivities, borehole length approaches an asymptotic behavior as other factors, such as ground thermal conductivity, dominate. Comparisons revealed that zeroth-order and first-order methods yield almost identical results, while minor deviations (<6%) occur with the Liao et al. method in the low-conductivity regime due to differing heat transfer assumptions. It has been investigated that larger leg spacings were shown to reduce thermal interference between U-tube legs, thereby shortening borehole lengths. The study also highlights that smaller pipe diameters lead to higher pressure drops, necessitating lower flow rates to minimize frictional resistance and higher pumping costs. However, ensuring sufficient flow rates to maintain turbulent flow is recommended for effective heat transfer. These findings underscore the importance of optimizing grout conductivity, leg spacing, and flow conditions to enhance borehole heat exchanger efficiency. The results provide a robust foundation for further research and practical applications in sustainable Ground Heat Exchanger (GHE) technologies over conventional air-cooling systems.
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