Research And Development Of TCPTU Test Module And Study Of Soil Thermal And Physical Properties Evaluation Method

With the development of underground energy engineering,ground source heat pump,energy pile and other technologies have been developed to a large extent.Ground source heat pump(GSHP)is an efficient and environmentally friendly energy saving technology that USES geothermal energy in the shallow soil on the earth surface as a cold(heat)source to realize building heating in winter and cooling in summer.The traditional ground-source heat pump technology with buried pipes has the problems of large area(underground space)and high cost,and its application in urban areas and economically developed areas is often limited.As a new building energy saving technology improved based on the traditional geothermal pump technology,the energy pile places the heat exchange pipeline of the traditional ground source heat pump in the building pile foundation,and realizes the heat exchange between the pile and the surrounding rock and soil by using the temperature difference between the circulating heat transfer medium in the pipeline and the soil mass around the pile.The thermal physical parameters of soil layer reflect the response ability when the object is subjected to thermal load,and the accurate acquisition of the thermal conductivity of soil layer is the prerequisite for the rationality of underground energy engineering.The testing methods of soil thermal property parameters mainly include laboratory test and in situ test.Laboratory experiments such as probe test,T-TDR(thermal time-delay reflectometry)test and hot plate test is mainly when measuring the sample temperature change over time,heat conduction coefficient of soil is derived by measuring the temperature change,the indoor experiment with quick and convenient,and low test cost,but its test precision co.at the same time do not reflect of other mechanical properties of soil in situ.Field in-situ test,such as thermal response test,is based on the field environment of soil,which can comprehensively analyze the influence of soil,groundwater and other factors,and has great guiding significance for the development of underground energy engineering,but there are problems such as long time and low accuracy.In this context,researcher designed and developed thermal piezocone penetration testing(TCPTU)based on the principle of transient line heat source method combined with CPTU,aiming at the lack of efficient and accurate test and number of domestic soil heat conductivity test methods.The reliability of the test method was verified through field test at the Jiu long lake energypile test site and comparative analysis with indoor thermal probe test results,and the engineering practice process of the test method was summarized.Secondly,the process of heat transfer and test of TCPTU probe in real soil was simulated and restored by multi-physical field coupling finite element analysis software,so as to analyze and verify the reliability of TCPTU testing principle and the accuracy of test results.In addition,the design requirements again according to the model test,with independent research and development of southeast university TCPTU test equipment,equipment in accordance with the model of independent research and development of southeast university slot for typical clay,silty clay and sandy soil indoor experimental research on the model,and indoor thermal probe test,study the heat transfer coefficient in the change rule of different dry density and moisture content,test and verify the reliability of TCPTU probe test,combined with the model tank experiments and indoor thermal probe test results,set up by the soil dry density and moisture content of soil factors to predict the heat transfer coefficient of the forecasting model.In order to make the TCPTU test method for geothermal energy development and energy engineering has important practical significance and guiding significance.The main research contents and conclusions of this paper are as follows:(1)the existing domestic and foreign soil thermal property testing equipment and testing principles are described.The advantages and disadvantages and applicable scope of several widely used testing methods are summarized in combination with the literature.Constructive Suggestions are put forward through the analysis and summary of several testing methods.In addition,several heat conduction CPTU test methods applied in foreign literatures were summarized,and the design difficulties,advantages and disadvantages,and application scope of each method were analyzed.Then,the TCPTU probe design idea of independent research and development was compared with the domestic situation.(2)in this paper,the independent research and development of southeast university TCPTU probe test principle and the calculation method of transient line heat source method,and through the line heat source method to establish TCPTU soil test method calculating model of research and development to test the heat transfer coefficient of TCPTU probe,according to many field test condition optimization of heat source and the detector material selection,arrangement of sensors and the change of wire rod,satisfy TCPTU probe experiment and engineering requirements.(3)based on finite element analysis software COMSOL more physical field coupling,fullyconsidering the real soil TCPTU probe heat transfer in the test process,respectively using solid heat transfer model and the porous medium heat transfer model for heat transfer TCPTU probe material itself as well as the soil test,to simulate the heat transfer process simulation calculation by Lagrangian mesh mesh free method was carried out on the heat transfer of soil,IDA algorithm by solving implicit time calculation of transient study reduction tests of soil and the temperature of the probe sensor change process with time,and compare the different period to test the soil temperature change rule,The simulated thermal conductivity was obtained by solving the temperature change curve of the sensor through the TCPTU test and calculation method,and the reliability of the TCPTU probe test method and the accuracy of the test results under ideal conditions were summarized.(4)Use of independent research and development of heat conduction CPTU probe(TCPTU)and thermal probe for typical clay,silty clay and clay model test has carried on the heat transfer coefficient,and southeast university in its nine dragons energy field,on-site testing,in situ test hall TCPTU testing method is validated by the test results of the reliability and efficiency,and analysis its heat transfer coefficient under the condition of dry density and moisture content change of change rule.The analysis results show that under the condition of fixed dry density,the soil thermal conductivity increases with the increase of moisture content when the moisture content is lower than the optimal moisture content.Under the condition of fixed moisture content,the heat conduction coefficient increases with the increase of dry density,and the change is not obvious when the moisture content is close to the optimal moisture content.Based on the above experimental results and data,a prediction model of soil thermal conductivity was established by using various factors such as soil dry density and moisture content,and the reliability of the model was verified by other measured values in literature.