Study On Thermal Performance Of Cemented Paste Backfill In Mines Based On Non-uniform Sedimentary Characteristic

The geothermal energy as a renewable resource has the advantage of enormous potential and environmental friendliness,which is the future direction of energy development.Especially the deep geothermal energy,stored in deep mines,has a great development prospect.The reasonable utilization of it into the field of low-temperature power generation and space heating can alleviate the current energy shortage and high carbon emissions.The combined mine backfill-geothermal system is a new concept of mine thermal coordinated exploitation.By pre-laying ground buried pipes into the cemented paste backfill(CPB)in the goaf,it can provide clean thermal energy for the mining area on the ground.This method not only reduces the operation cost of the mining area but improves the sustainability of the mining industry with the realization of efficient exploitation of resources.The premise of the design of the ground source heat exchanger is to obtain the thermophysical properties of CPB accurately.Considering the thermal response test has been most widely used for the thermophysical properties,how to utilize this method properly plays an important role in the design of the mine backfill-geothermal system.Based on the mentioned above,this paper develops a full-scale three-dimensional unsteady numerical model of combined seepage with non-uniform stratification of CPB.The reliability of the model and user defined function(UDF)are verified by comparing with the experimental data of related literature,which indicates that the model can predict the complex thermal conductivity of CPB.Based on the UDF built in Fluent software,the thermal response test for the CPB considering the thermal properties stratification is simulated and the heat performance factors of the U-bend ground pipe heat exchanger in CPB are discussed.In this paper,the thermal response test under the different porosity,seepage velocity,and seepage time are simulated.The complex thermal conductivities of CPB are obtained through the mathematical model,and the influence of the above factors on the temperature field and complex thermal conductivities of CPB are analyzed.Finally,the heat performance of the horizontal U-tube heat exchanger buried in CPB is analyzed.The influence of Reynolds number and U-tube diameter on the system's thermal performance is investigated,and an integrated evaluation factor is proposed to evaluate it.The results show that since the internal stratification of CPB is not uniform,the complex thermal conductivity of CPB obtained by the thermal response test is more reasonable than the average value of the thermal conductivity of each layer.In the cases of vapor-saturated,with the increase of porosity,the complex thermal conductivity of CPB is reduced,and the heat exchange between CPB and the buried pipe is declined.Under the cases of seepage,the temperature field of CPB is shifted.When the seepage velocity is greater than 10-6 m/s,the slope of the fitting line is extremely small but the complex thermal conductivity is increased rapidly.However,if the hydraulic conductivity is too high,the identification results cannot be well fitted with the linear heat source model.Also,dimensionless evaluation criteria and integrated evaluation factor are used to evaluate the thermal and pressure performance of the U-bend ground heat exchanger buried in CPB.The integrated evaluation factor is 0.504,0.555,0.634,0.661 and 0.677 for Re=1500,Re=5000,Re=10000,Re=15000,and Re=20000,respectively.The integrated evaluation factor is 0.606,0.634 and 0.666 for different U-bend ground heat exchanger diameters of DN25,DN32,and DN40.It can be concluded that the better-integrated performance can be obtained by increasing the Re and the U-tube diameter.