Heat Transfer Model And Simulation Analysis Of Regularly Arranged Multi-well Mid-deep Buried Tube Heat Exchangers

In order to achieve the goal of "peak carbon dioxide emission and carbon neutrality",the construction industry has an increasingly urgent demand of clean energy(including geothermal energy)applications.Many places in China have begun to promote the use of mid-deep geothermal energy for heating.The deep coaxial ground source heat pump(GSHP),which takes heat but not takes water,has less influence on underground environment than the open system and has been paid more and more attention and application.Present research on heat transfer of mid-deep buried pipe heat exchanger mainly focuses on single buried pipe heat exchanger,which can usually adopt analytical method and numerical method based on cylindrical coordinates.For the heat transfer analysis of multiple buried pipe heat exchangers,the heat transfer model in three-dimensional rectangular coordinate system is usually used.Due to the large calculation domain and geological stratification,the calculation workload is very huge.At present,there is no effective method to quickly simulate the operation of multiple buried pipe heat exchangers in their whole life cycle.Therefore,the current engineering application lacks relevant theoretical guidance.In view of the above situation,this thesis proposes a numerical heat transfer model for the multiple mid-deep buried pipe heat exchangers with regular polygon arrangement.Based on the characteristics of regular arrangement of boreholes,the model simplifies the calculation of multiple buried pipe heat exchangers to that of single buried pipe heat exchanger by reasonably processing the calculation domain,which greatly reduces the calculation workload without reducing the calculation accuracy.The computing speed is several orders of magnitude higher than using commercial simulation software.It takes only a few minutes to simulate and analyze the life-cycle heat transfer of the regularly arranged multiple buried pipe heat exchangers by using an ordinary computer.The established model,the long-term operation characteristics of multiple mid-deep buried pipe heat exchangers,the evolution of circulating liquid temperature and heat transfer capacity of the heat exchanger with time under continuous operation and intermittent operation modes,and the influence of borehole spacing on the heat transfer performance of multiple buried pipe heat exchangers are studied by using the established model.The influence of the operation of buried pipe heat exchanger on the temperature field of rock and soil layer,and the influence of the geotechnical thermophysical parameters on the heat extraction capacity of buried pipe heat exchanger are also discussed.The results show that the thermal interference among boreholes has a negative effect on the operation of buried pipe heat exchanger,and this negative effect will increase with the extension of heat extraction time and the increase of heat extraction flux.When determining the borehole spacing of ground source heat pump heating projects of mid-deep multiple buried pipe heat exchangers,it is necessary to comprehensively consider the factors such as operation time,heat extraction power,geotechnical physical parameters and so on.This paper also discusses the influence of borehole related parameters and circulating fluid flow rate on the heat extraction capacity of buried pipe heat exchangers.The results show that the heat extraction capacity of heat exchanger can be effectively improved by appropriately increasing the thermal conductivity of backfill material,the size of outer pipe and circulating fluid flow rate.The variable fluid flow rate operation mode can improve the economy of operation and meet the requirements of heat pump unit for circulating fluid temperature.