Numerical study of influence of Karst fracture water on heat transfer performance of borehole heat exchanger

Groundwater flow within karst fractures can significantly enhance the heat exchange efficiency between a borehole heat exchanger (BHE) and the surrounding rock. The development of artificial fractures to intensify heat transfer between the BHE and rock has emerged as a promising direction in geothermal exploration. This study presents a three-dimensional finite element simulation model that integrates fracture flow with BHE heat transfer, accounting for various characteristics of horizontal fractures. Data analysis was conducted using range analysis and multi-criteria comprehensive evaluation, based on the principles of orthogonal experiments. The results indicate: (i) Fracture water flow substantially improves BHE heat transfer performance in summer, with even the lowest-performing configuration in the orthogonal test showing a 5.36 % increase in heat transfer per unit length of the BHE (HPLU) compared to the natural control group without fractures; (ii) The influence of different fracture characteristics on BHE heat transfer performance follows this order: fracture water velocity > fracture aperture > fracture depth > fracture flow direction > fracture water temperature; (iii) The optimal configuration enhances HPLU by 16.95 % over the natural control group, demonstrating that developing well-designed artificial fractures in karst regions can substantially improve BHE heat transfer efficiency.