Study On Heat Extraction Characteristics Of Underground Coaxial Heat Exchangers For Medium And Deep Geothermal

The medium-deep geothermal underground heat exchange system is a new type of heat pump heating technology,compared wigh shallow buried tube heat exchange system,it has small footprint and high efficiency and energy saving,it is important for achieving clean heating.The core of the heat pump system is the downhole heat exchange device on the heat source side.In order to ensure the high efficiency and stability of the downhole heat transfer,a coaxial casing type downhole heat exchanger is generally used.In this paper,the heat transfer mechanism between a single mediumdeep geothermal downhole coaxial heat exchanger and underground rock and soil is studied,and the heat transfer characteristics are simulated.The parameter values required for the simulation study were obtained by collating the geological characteristics and the distribution of the underground geotechnical distribution obtained from the exploration of the medium-deep geothermal wells.The overall structure of the geothermal well and its geometrical dimensions are introduced.The temperature field and temperature gradient of the underground rock and soil are obtained by the heat transfer experiment of the deep geothermal downhole coaxial heat exchanger in the field.Using the heat transfer analysis method of the traditional shallow underground vertical buried pipe,the downhole heat exchange area is divided into two areas,the well hole and the well hole,for analysis and research,and the difference between the external input and the inward and out is obtained.The temperature distribution of the annular cavity fluid and the inner tube fluid in the flow mode,but this conventional heat transfer analysis method assumes that the temperature of the wellbore wall is consistent in the well depth direction,ignoring the axial gradient of the middle and deep wells.The calculation results will produce a certain error.Aiming at the complex heat transfer process of the geothermal heat exchanger in the middle and deep geothermal wells,this paper uses a numerical model based on finite difference to simulate the equation.In the numerical model,the equation is discretized by the ADI method and chased by the chasing method.Solving is greatly shortened the calculation time.In the numerical model,the geothermal gradient of the underground rock and soil,the backfill material,and the heat capacity of the pipe wall and fluid are considered.The simulation results are compared with the results of the field heat transfer experiments,showing that the calculation accuracy is high.In this paper,the heat transfer performance of the mid-deep geothermal downhole coaxial heat exchanger is studied by the numerical model.The factors affecting the heat transfer performance are analyzed.Some conclusions about optimizing the heat transfer performance of the heat exchanger are obtained.The research results provide a theoretical basis for in-depth analysis of the heat transfer mechanism of the deep-soil geothermal heat exchanger.