Research On Cement Based Backfill Materials For Ground Source Heat Pump And Thermal Conductivity Prediction

In the context of vigorously developing ecological civilization construction and putting forward the goal of carbon neutrality and carbon peak,energy conservation and emission reduction in all walks of life is becoming more and more important.Ground source heat pump,as a clean,stable and efficient form,has a good application prospect.The gap between the drilling and the buried pipe needs to be filled with backfill materials,so as to enhance the heat transfer between the buried pipe and the surrounding rock and soil,and prevent groundwater pollution.There are two kinds of backfill materials:bentonite and cement-based.The cement-based material has better thermal conductivity and higher strength.Therefore,this paper studied the properties of cement-based backfill materials,developed some suitable backfill materials for application,and studied the prediction model of thermal conductivity for cement-based materials.The thermal conductivity is the most important parameter to determine the heat transfer efficiency of buried pipes.In order to explore the main influencing factors of the thermal conductivity,several thermal conductivity test methods were compared in this paper,and “constant power plane heat source method” was adopted.The experimental results shows that the thermal conductivity varies with the sand-cement ratio,watercement ratio,sand type and the amount of water reducer.According to the experimental results,four recommended ratios were optimized,and the conclusion was drawn that the thermal conductivity of materials decreases due to the "dispersion" and "air suction" of water reducer.In addition,the fluidity of materials is an important parameter to measure the performance of materials.The experiment also used the “trap-cone circular mold method” to test the fluidity of materials.It is concluded that the increase of sand-cement ratio and the decrease of water-cement ratio will decrease the fluidity,and the water reducing agent can enhance the fluidity.Porosity affects the strength and durability of the materials and thermal conductive performance,also has close relationship with permeability.to explore the cause of the coefficient of thermal conductivity changes and prevent high permeability,the “vacuum full water method” was adopted to test porosity of the material,it is concluded that the material porosity decreases with increasing ratio of sand,and increases with the increase of water cement ratio,The material porosity of common sand is higher than that of quartz sand,and the addition of water reducing agent increases the material porosity.In order to develop a numerical calculation method for the research on the thermal conductivity of cement-based materials and improve the research efficiency,a “solidliquid-gas three-phase thermal conductivity prediction model for cement-based materials”was established according to the structure of cement-based materials and the Fourier law of thermal conductivity.The model can predict the influence of aggregate content(sandcement ratio),water-cement ratio(porosity),saturation,thermal conductivity of singlephase components and other factors on thermal conductivity,and the prediction accuracy can reach 12%,covering a wide range of proportioning.In this paper,based on the prediction results and the fluidity and relative thermal conductivity,the “cement-based backfill material value area” and “cement-based backfill material applicable area” was determined,so that the thermal conductivity of the material can be directly found through the porosity and sand-cement ratio.The relationship between thermal conductivity and water-cement ratio and sand-cement ratio is also obtained through parameter fitting,so that the thermal conductivity can be obtained directly through the ratio.At the same time,the existing model was used to calculate the thermal conductivity,and compared with the model presented in this paper,it is found that the prediction result of applying the existing model has a large error.The prediction accuracy of the model in this paper is higher,and the parameter values of cement-based materials of different uses can be obtained through parameter identification,which is more flexible and has significant advantages in practical application.