Thermal Conductivity Test Method For Materials With Temperature-dependent Thermal Conductivity Or Continuous Phase Change Based On Back Analysis

The test of thermal conductivity of materials is of great significance in all aspects of production and life,engineering construction,energy conservation and environmental protection.At present,most of the existing thermal conductivity test methods are developed for the constant thermal conductivity,and the materials whose thermal conductivity changes with temperature in actual engineering can be seen everywhere.The test of thermal conductivity of such materials has important engineering significance.The material whose thermal conductivity changes with temperature can be divided into temperature-changing thermal conductivity material and continuous phase change material.The difference between the two is that there is no latent heat of phase change in the former.Firstly,by reading and arranging the literature,summarizing the existing thermal conductivity test methods and their limitations,and understanding the analytical solutions of the one-dimensional unsteady thermal and thermal conduction problems of temperature dependent thermal conductivity materials and continuous phase change materials,master the matlab software.Secondly,based on the back analysis,two test methods for the thermal conductivity of the temperature-dependent thermal conductivity material are proposed.Firstly,the material property parameters are approximated by constant elements to find the analytical solution of the heat conduction process of the material in the semi-infinite region.This paper refers to the Tao solution.Based on the solution,two methods for measuring the thermal conductivity of temperature-dependent thermal conductivity materials are proposed.In the first method,the surface heat flux and temperature of the material need to be measured,and the nonlinear equation obtained by the back analysis is directly used to obtain the thermal conductivity of each constant element.Method 1 has no iterative process and the application is relatively simple.In the second method,it is necessary to measure the temperature change at two points,and the Levenberg-Marquardt method is used to determine the thermal conductivity of the material.For the temperature-dependent thermal conductivity materials,five types of thermal conductivity were considered respectively,and the above two methods were used to calculate the accuracy of the two methods.The calculation results show that the error of the two methods in testing the thermal conductivity of the temperature-dependent thermal conductivity material is within 7%.The measurement error of surface heat flow and temperature during the experiment,the form of thermal conductivity with temperature and the number of sub-intervals when the constant element is approximated have an effect on the test results.Then,based on the back analysis,two test methods for the thermal conductivity of continuous phase change materials are proposed.The analytical solution of the heat conduction process of the material in the semi-infinite region is found.This paper refers to the solution.Based on the solution,two methods for measuring the thermal conductivity of the continuous phase change material are proposed.In the first method,the surface heat flux and temperature of the material need to be measured,and the nonlinear equation obtained by the back analysis is directly used to obtain the thermal conductivity of each constant element.In the second method,it is necessary to measure the temperature change at two points,and the Levenberg-Marquardt method is used to determine the thermal conductivity of the material.For the continuous phase change materials,five types of thermal conductivity were considered respectively,and the above two methods were used to calculate the accuracy of the two methods.The calculation results show that the error calculation results of the two methods in testing the thermal conductivity of continuous phase change materials show that the two methods are feasible when testing the thermal conductivity of temperature-dependent thermal conductivity materials,and the error is within 8%.Finally,the thermal conductivity of silty clay in Xuzhou area was tested,and the thermal conductivity of silty clay at different temperatures in three initial water content states was obtained.The results show that at the same temperature,the higher the initial moisture content,the greater the thermal conductivity of the soil sample.For the same soil sample,the thermal conductivity is greater at negative temperature than at positive temperature.