| In order to deeply implement the goal of "carbon peak,carbon neutral" and create an ecological civilization society,the buried tube heat exchanger,as an efficient geothermal energy utilization device,has been vigorously promoted in China.Engineering application results show that in recent years,some buried nest of tubes of the overall thermal efficiency in present the downward trend year by year,unable to meet the energy needs of the air conditioning system for a long time,to establish the accurate buried nest of tubes long-term heat transfer model and predict the heat transfer characteristics and evolution law of formation temperature field and the complex hydrological conditions include the forecast model,is the prerequisite of buried nest of tubes in thermal efficiency for a long time.Therefore,the paper carried out research on the evolution law of long-term heat transfer and optimization design of buried pipe under stratification and seepage conditions.The research results have important theoretical and engineering significance to improve the long-term heat transfer efficiency of buried pipe group and reduce the engineering cost.In this paper,theoretical analysis,in situ test,model test and numerical analysis are used to study the heat transfer mechanism and optimal design of buried pipe group.Through theoretical analysis,the heat transfer law of buried pipe in complex stratum is revealed.The main research contents and results are as follows:1)The fluid energy equation and heat conduction equation were solved jointly,the borehole wall temperature and fluid inlet and outlet temperature were taken as intermediate variables,and the temperature field inside and outside the borehole was coupled with the temperature of the heating water tank.The three-dimensional transient heat transfer model of the buried tube was constructed,and its rationality was verified by comparing with the experimental results.The model can comprehensively consider the influence of the structure of buried pipe,groundwater seepage and cold and heat load on its long-term heat transfer performance.Combining the model with monte Carlo algorithm,the effects of rock mass inhomogeneity,temperature gradient,fluid inlet direction and inlet temperature on the heat transfer efficiency of buried pipe are studied.2)In order to use clay and waste steel slag to prepare high thermal conductivity backfill materials for buried pipes,the effects of mixing ratio,gradation,dry density and moisture content of steel slag clay on the thermal conductivity of the mixture were studied,and the test results were analyzed from the aspects of mineral composition,microstructure and liquid bridge effect.The results show that the optimal thermal conductivity configuration scheme is "30% clay +70% steel slag".The influence of moisture content and dry density on the thermal conductivity of clay-steel slag mixed backfilling is greater than that of gradation.Therefore,the control of moisture content and dry density should be emphasized in the backfilling process.3)In order to study the internal structural characteristics of buried pipe and the influence of lateral rock mass on its heat transfer,a single pipe prototype(vertical)and a single pipe model(horizontal)test bench were set up respectively.The effects of flow velocity,heating power,cross-section size of buried pipe and inner pipe thermal conductivity on inlet and outlet temperature and fluid pressure of buried pipe are studied in the prototype test.The results show that the increase of fluid flow velocity has little effect on heat transfer power.As heating power increases,heat transfer power of buried pipe increases obviously.The larger the diameter ratio of inner and outer coaxial tube,the lower the heat transfer efficiency;The better adiabatic performance of buried pipe,the higher heat transfer efficiency;The prediction model of fluid velocity on pipe pressure is proposed.The model test results of large buried pipe with 18 m length show that the heat transfer efficiency of buried pipe increases linearly with the increase of thermal conductivity of rock and soil mass.Evaluation of thermal conductivity of layered rock mass by means of homogenizing thermal conductivity will underestimate the heat transfer efficiency of buried pipes.The heat transfer efficiency is the highest when the excellent heat transfer layer is located at the bottom of buried pipe.4)Based on the similarity theory,a single buried pipe seepage test device with a similarity ratio of 10:1 was designed to systematically study the influence of soil seepage condition,cold and heat load and rock material properties on the long-term heat transfer characteristics of single buried pipe.The results show that: The thermal conductivity of sand in dry state,saturated state,flow rate of 0.2mm/s,0.5mm/s and0.6mm/s are 0.17W/m·K,0.32W/m·K,0.81W/m·K,0.85W/m·K and 1.03 W/m·K,respectively.The long-term relative thermal conductivity of heat transfer is0.10W/m·K,0.45W/m·K,1.55W/m·K,2.89 W/m·K and 3.03 W/m·K,respectively.The thermal conductivity of sand and rubber are 0.77W/m·K and 0.09 W/m·K respectively under saturated condition,and 1.12W/m·K and 0.16 W/m·K respectively under seepage condition.Under the condition of non-uniform heat and cold load,the seepage formation has no obvious heat accumulation.5)In order to study the effect of groundwater seepage and cold and heat load on the heat transfer characteristics of buried pipes,a test device was designed with the similarity ratio of 10:1.The results show that "heat accumulation" exists in the non-seepage stratum,and the heat accumulation coefficients of buried pipes in the outer and inner circles are 0.15 and 0.27 respectively.Under the condition of groundwater seepage,the distribution characteristics of formation temperature field are less affected by the number of cycles,and the specific degree of influence is related to the seepage velocity.Based on the model test results of buried pipe group,the corresponding two-dimensional numerical analysis model is established and the rationality of the model is verified.Based on the model,the influence of the layout shape of buried pipes on the heat transfer efficiency of buried pipes and the formation temperature field is studied,and the "quincunx" optimal layout scheme is proposed.6)Through the secondary development of COMSOL numerical software,the three-dimensional transient heat transfer model of the casing buried pipe group was constructed,and the spatial layout characteristics of the buried pipe group were optimized by using this model.The results show that in the formation without seepage,the heat transfer power decreases by 6.8%,2.2% and 4.5% in the case of through-length arrangement,external length arrangement and internal length arrangement respectively.The decrease of heat transfer efficiency is related to the phenomenon of "heat accumulation" of rock and soil mass,and the external layout can better alleviate the phenomenon of heat accumulation of buried pipe group.In the seepage formation,the heat transfer efficiency of the front length and rear length of buried pipe group is always higher than that of the general length,and the heat transfer efficiency is increased by 7.6% and 6.5% respectively.The front-length arrangement can improve the heat exchange efficiency of buried pipe group most efficiently.According to the characteristics of formation seepage and the uniformity of cold and heat load,a three-dimensional optimization layout scheme of casing buried pipe group is proposedFigure [167] Table [24] Reference [178]... |
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