Study On Heat Extraction Characteristics Of Vertical Deep-buried U-bend Pipe Heat Exchangers For Middle-Deep Geothermal

Middle-Deep U-bend ground heat exchanger is an innovative attempt of ground source heat pump heating technology.Compared with the shallow ground heat exchanger,the middle-deep U-bend heat exchanger has the characteristics of high heat flux,stability and high temperature.In view of the problems of shallow buried pipe heat exchanger,such as large land area,unsuitable for severe cold area,low energy efficiency,poor economy and so on,the numerical heat transfer model of middle-deep U-bend ground heat exchanger is established,and the model is verified based on the experimental data.The heat transfer characteristics of middle-deep U-bend ground heat exchanger for geothermal are studied.First of all,the thermal physical parameters of rock-soil of middle-deep U-bend ground heat exchanger and the field experiment of middle-deep geothermal heat exchange system are discussed.Based on the actual heating project of middle-deep geothermal,the geological condition data obtained from U-bend geothermal well exploration are sorted out,and the initial parameter values required for application in the early stage of the study are obtained.After the completion of well,the initial temperature distribution of rock-soil is measured by using distributed optical fiber temperature measurement system,and the geothermal gradient at each depth is obtained.Through the heat exchange experiment of middle-deep U-bend ground heat exchanger,the temperature distribution of rock-soil,the water temperature of geothermal well and the heat of geothermal well after 720 h operation of the heat extraction system of middle-deep U-bend ground pipe heat exchanger are recorded.Secondly,as an unsaturated porous medium,the effective thermal conductivity of rock-soil is the key parameter to characterize the porous medium,so it is very important to accurately measure and predict the effective thermal conductivity.In this paper,based on the fractal geometry theory,a mathematical model of effective thermal conductivity of medium and deep unsaturated rock and soil porous media is established.The influence of geometric parameters such as porosity,soild phase fractal dimension,fluid phase fractal dimension,tortuosity fractal dimension and saturation on effective thermal conductivity of middle-deep unsaturated rock-soil is considered.The simulated data are verified by using the pulished experimental data.Finally,according to the heat transfer process of middle-deep U-bend ground heat exchanger,combined with the effective thermal conductivity,a numerical heat transfer model of U-bend ground heat exchanger based on the fully implicit finite difference method is established.This method greatly reduces the calculation time and ensures sufficient calculation accuracy.The influence of local rock-soil temperature gradient and lithology change on heat transfer performance are token into account.In this paper,in order to verify the stability of the mathematical model,the data of 480h-720 h operation of geothermal wells are used to verify the simulation data,and the maximum error is only 7.09%.The results show that the mathematical model is accurate.Based on the numerical heat transfer model of middle-deep U-bend geothermal well heat exchanger,the heat transfer characteristics of U-bend geothermal well heat exchanger used in middle-deep layers was studies in this paper,the influence of key factors on its heat transfer performance was analyzed and discussed,and the corresponding conclusion of optimizing the heat transfer performance of middle-deep geothermal well heat exchanger was obtained.The research results can provide support for the optimization design of heat extraction system of middle-deep buried tube heat exchanger.