| With the gradual progress of energy conservation and emission reduction globally,there is currently a trend towards vigorously developing and utilizing renewable energy.As clean and renewable energy,geothermal energy in middle-deep layers features wide distribution and large reserves.Using closed deep-buried ground heat exchangers(DGHEs)for building heating is a new technology of utilizing middle-deep geothermal energy,and there are many problems to be solved in the theory and application of DGHEs.In this dissertation,experimental,numerical and analytical methods are used to study the theory and technology on heat transfer through DGHEs.A field experimental platform with controllable buried pipe inlet temperature and heat transfer rate is designed and built,a numerical and analytical model for heat transfer in buried pipes is developed,the heat transfer characteristics of buried pipes in long-year operation are studied and the operating parameters for heat transfer in buried pipes are optimized based on the numerical method.In the field experiments of DGHEs,the field experimental platform for heat transfer in DGHEs is designed and built based on a coaxial-type DGHE(C-DGHE)with a depth of 2539 m and a U-type DGHE(U-DGHE)with a depth of 2781 m in Xi’an.Based on the control system,heat pump unit,cooling tower,plate heat exchanger,electric heater,etc.in the experimental platform,fine adjustment of water inlet temperature and heat transfer rate of buried pipes is realized,and multiple sets of experimental conditions with constant buried pipe inlet temperature and heat transfer rate are completed.The experimental results reflect the real characteristics of short-term heat transfer of buried pipes and provide an important basis for establishing the calculation model for heat transfer in buried pipes.In the study on the numerical model for heat transfer in DGHEs,a full-scale dynamic numerical calculation model of coupled inner and outer pipe heat transfer is established based on the field experiments of DGHEs and the heat transfer mechanism.The established numerical model focuses on the stability of the mesh and the reliability of the calculation results for the coupling of radial small-scale inside the pipe,radial large-scale outside the pipe and axial largescale.By writing UDF files and the TUI language that can be read into ANSYS Fluent,the personalized boundary assignment and fully automatic calculation of the long-year heat transfer in buried pipes with an annual cycle are realized.In the study of analytical model for heat transfer in DGHEs,an analytical model for the reliable solution of heat transfer in buried pipes is established based on the theory of infinite line source model and the logarithmic mean temperature difference.Taking into account the layered lithology and temperature gradient distribution of the ground around the buried pipe,the analytical model solves the unsteady heat transfer of the layered ground around the buried pipe and the axial transfer of temperature in the heat transfer process between the buried pipe and the ground.In this dissertation,the heat transfer characteristics of DGHEs in long-year operation are studied based on the established numerical model,and the operating parameters are optimized by applying multi-factor analysis and other methods.The research contents and conclusions are as follows:(1)The heat transfer stability of buried pipes operating for 50 years with annual heating cycle is studied,and the ground temperature response is analyzed.The study shows that the heat transfer decay of buried pipes operating for long years mainly occurs in the first 5 years,and the annual decay rate in the 5th year is less than 3%.(2)The heat transfer performance of C-DGHE and U-DGHE for 5 years of continuous operation is analyzed to quantify the advantages and disadvantages of the heat transfer performance of the two types of buried pipes.The study shows that there is a critical flow rate GC for the heat transfer rate per meter of CDGHE and U-DGHE at the same depth,the heat transfer efficiency of C-DGHE is better when the flow rate is less than GC,and otherwise that of the U-DGHE is better.The value for GC is8.63 kg/s in the case of the present study.(3)A multi-factor orthogonal simulation condition is designed to analyze the heat transfer performance of buried pipes under different influencing factors and corresponding levels,and level ranking and weight analysis methods are proposed to determine the optimal level of factors and their contribution rate to the heat transfer performance of buried pipes.When the heat transfer rate per meter of buried pipe,performance coefficient of heat pump unit,and heat transfer decay rate are used as evaluation parameters for heat transfer performance.The contribution rates of the factors with the same weight of each evaluation parameter are obtained in the order of ground thermal conductivity,buried pipe depth,inlet water temperature,ground temperature gradient and water flow rate in buried pipes.(4)The heat transfer characteristics of DGHE under different dynamic load heating modes and different multi-pipe layouts are studied,and load optimization strategies and multi-pipe layouts are proposed to improve the building heating potential of buried pipes.The study shows that the heating area of buried pipes is significantly increased when heating is performed in the mode of not guaranteeing the number of heating days of water inlet temperature or load peak shaving.In the case of multi-pipe heat transfer,the plug pipe group arrangement with equilateral triangle as its basic unit is superior to the unstaggered pipe group arrangement with straight line as its basic unit.The study aims to provide theoretical and technical support for improving the solution method of heat transfer in DGHEs,guiding the reliable design and operation of heat transfer system of buried pipes,analyzing and optimizing the influencing factors of heat transfer in buried pipes,improving the heat transfer potential of buried pipes,etc. |
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