Research On The Alleviation Effect Of Karst Structure On Thermal Imbalance In Shallow Geothermal Development

The thermal imbalance of rock and soil in the operation of ground source heat pumps is the main problem that restricts ground source heat pumps to further improve their economic benefits and promote their application.It is also a frontier and hot spot in the field of ground source heat pump technology research.In southern our country,karst is widely distributed,with complex geological conditions and abundant groundwater.The unique geological structure and active groundwater movement in karst areas will have a positive impact on the thermal imbalance of buried pipe heat transfer area.Therefore,in order to explore the influence of fissure water in karst area on the thermal imbalance of underground rock-soil heat exchange area,Established a physics experiment design platform,By comparing the resultant experimental data of a rock mass with no fractures and a rock mass with horizontal single fractures,Understand the law of influence of horizontal single fissure on the temperature field of rock mass when water flows through.Then,according to the COMSOL numerical simulation software,a three-dimensional heat-fluid coupled heat transfer model of the fluid heat transfer in the buried pipe and the heat transfer of the fractured rock mass is established.The feasibility of the model is verified by comparing the results of numerical simulation with the field test data of ground source heat pumps in typical karst areas.In this paper,six numerical models are established according to different fracture parameter structures.By changing the fracture water velocity,the number of fractures,and the distribution of fractures in the rock mass,the changes of the water temperature of the buried pipe heat exchanger,the heat transfer per well depth and the heating performance coefficient COP_hare studied.The research found that by drawing the physical experiment data of the two rock mass models,it can be clearly observed that the horizontal fissure flow can positively alleviate the thermal imbalance of the rock and soil.Then,numerical simulation is used to study the change of the heat transfer efficiency of the buried pipe heat exchange area under long-term operation.Numerical simulation studies show that when the fracture water velocity is higher than the critical value of 5e-6m/s,there is no significant difference in the effect of the fracture water velocity on the water outlet temperature of the buried pipe,the heat transfer per well depth and the coefficient of performance COP_h.Under the same fracture distribution and the operating state of the fracture water velocity of 5mm/s,the greater the number of fractures,the stronger the mitigation effect on the thermal imbalance of the rock mass.Under the same fissure water flow rate and the same number of fissures operating conditions,horizontal fissures are the most obvious to alleviate the imbalance of cold and heat load of rock and soil.Moreover,the heat transfer per unit well depth is 33.89%and 12.86%higher than that of vertical fractures and cross-fractured rock masses,respectively,and the heating performance coefficient COP_his 2.53%and 1.91%higher than that of vertical fractures and cross-fissured rock masses,respectively.The reason why the heat transfer efficiency of the rock mass with the same number of horizontal fractures is higher than that of the rock mass with vertical fractures is that the contact between the horizontal fractures and the vertical borehole heat exchanger can effectively alleviate the accumulation of cold near the borehole.Among the six numerical models proposed in this paper,the two horizontal fractured rock masses have the highest heat transfer per well depth and heating performance coefficients,which are 75.56%and 4.89%higher than those of the non-fractured rock mass,respectively.Therefore,the fractured structure in the karst area studied in this paper can effectively reduce the risk of thermal imbalance in the buried pipe heat transfer area,Improve the heat transfer performance of the buried heat exchanger,Extend the operating life of the unit.The characteristics of the research in this paper:clarify the nature and key of the problem of thermal imbalance,and find a new way of thinking.It is no longer limited to adjusting the design and operating parameters of underground pipes,units and other engineering facilities.It takes the special hydrogeology of the karst area as the basic condition,Focus on the study of the mechanism of heat exchange and storage in fractured rock and the mitigation effect of karst structure on thermal imbalance.The innovation of this paper:simulating the heat transfer process of fractured rock mass in karst area through physical experiment,The method of numerical simulation is used to study the influence of the fractured water flow under different karst structures on the heat transfer performance of the buried pipe heat transfer area under the long-term operating conditions of the ground source heat pump.Fundamentally solve the problem of thermal imbalance in energy storage rock masses in karst areas,It has practical engineering significance to promote the development of ground source heat pump technology in karst areas.