Numerical simulation of the influence of karst structure on heat transfer efficiency of buried pipeline group

The buried pipeline is the part where the rock mass heat pump system performs direct heat transfer with the ground. The water-bearing structure in the karst area is complex and diverse, which has a significant impact on the heat transfer and storage of the buried pipeline group. Herein, the heat transfer process of a group of 27 vertical buried pipelines in four geological structure types was simulated, including the non-karst structure, fractured karst structure, karst conduit structure and mixed karst structure. Meanwhile, comparison was made for the change of parameters, such as the temperature field in the rock mass, the outlet water temperature of buried pipeline, the coefficient of performance (COP) of heat pump unit, and the heat change in unit well depth. The results show that the flow of groundwater can significantly relieve the heat build-up in the rock mass where fractured karst structure and karst conduit structure exist in different model temperature fields under the refrigeration conditions, and the distance between the karst water-conducting structure and the buried pipeline is also an important influencing factor. When the model operates to the end of the first refrigeration cycle, the temperature difference between the inlet and outlet of rock mass in fractured karst structure and karst conduit structure is 0.87 K and 4.00 K higher than that in mixed karst structure respectively, while the temperature difference between the inlet and outlet of rock mass in non-karst structure is 1.16 K lower than that in mixed karst structure. The COPs of rock mass in non-karst structure, fractured karst structure, karst conduit structure, and mixed karst structure are 7.2, 7.4, 7.8 and 7.3 respectively. Besides, the heat transfer per well depth is 64.1, 90.3, 130.7 and 79.1 W/m respectively. The research results show that the karst water-conducting structure significantly enhances the heat transfer efficiency of the buried pipeline group, and different geological structure types have different effects on the heat transfer efficiency of the buried pipeline.