Research On Heat Transfer Performance Of Underground Tube Ground Source Heat Pump System In Abandoned Mine

With the transformation of the world’s energy structure and the increasingly prominent environmental problems,geothermal energy has attracted more and more attention as a highly efficient and clean energy source,especially the technology of ground-source heat pump has been greatly enriched and improved in recent years,and the application is also more and more extensive.Due to the limited reserves of shallow geothermal energy and the greater impact of climate,it is difficult to effectively use the ground source heat pump for long-term buried pipes.On the other hand,with the gradual depletion of mineral resources and the shallow mineral resources being exhausted,mining depth has been gradually developed into deeper strata,therefore a large number of outages or abandoned mines appeared,which contain a lot of mine geothermal resources but being underappreciated.This paper proposes the application of ground-source heat pump technology in the field of geothermal development in abandoned mines.Based on the structural characteristics of underground mines,a radial heat exchanger layout is proposed,which utilizes the heat exchange between the circulating fluid in the heat exchanger and the underground rock mass,achieving the development and utilization of mine geothermal resources and providing a new idea for the transformation of abandoned mines.Based on the heat transfer theory,the heat transfer mechanism of the underground heat exchangers in mines is analyzed.Combined with the energy balance theory,a heat transfer model for buried tubes in mines is proposed.On this basis,the factors influencing the heat exchange performance of underground heat exchangers in mines are summarized in terms of mines natural conditions,engineering design and so on,laying a foundation for the analysis of the operating characteristics of mine ground-source heat pump.The main research is as follows:(1)A set of miniature test benches has been designed and built to conduct heat transfer experiments of a single-tube heat exchanger under constant heat flow load,and obtain the temperature response characteristics of the circulating fluid of the heat exchanger and rock mass.By comparing the measured data with the numerical calculation results,the reliability of the numerical calculation method can be ensured.(2)Using COMSOL software non-isothermal pipeline flow module and solid heat transfer module,a three-dimensional single-tube heat transfer numerical model has been established to analyze how the different mine stratum conditions(primary ground temperature,rock thermal conductivity and constant pressure heat capacity),structural design factors of heat exchange tubes(drilling radius,drilling depth,heat exchange tube type,branch tube spacing and thermal conductivity of filling material),operating parameters(water injection temperature and water injection rate)and so on,affect the heat transfer performance of single-tube heat exchanger,and draw their respective sensitivity on the heat transfer performance.This provides a reference for setting the location,structure and operating parameters of the actual engineering mine geothermal development.(3)A heat transfer model of the tube group considering mutual interference has been established to study the influence of different drilling angles,arrangement methods and spacing on the overall heat transfer performance.The results give recommended values for reasonable drilling angles and spacing,and show the arrangement of the fork row is more conducive to the system heat exchange,which has certain significance for the design of the underground heat exchanger system in mines.According to the analysis of the impact on different factors,after selecting the main control factors that affect the heat exchange efficiency,using the multi-element orthogonal regressive analysis,the prediction formula of the heat exchange performance index has been obtained and verified,which can provide references for actual engineering.(4)A mine in the north has been taken as an engineering case,after designing the corresponding operation plan,simulating its heat transfer process in a single operation cycle,and analyzing the heat exchange performance indicators and the change rule of the temperature field of the mine rock moss in different periods,the ground-source heat pump system shows good heat exchange effect,which verifies the feasibility of its application.The results show that the cold and hot joint mining mode can reach the system stable state faster,the heat exchange efficiency is 83.3%of the first heat exchange season,which is 2.4 times that of the single heat extraction mode,and has good long-term stability.