Research On Thermal Conductivity Model Of Aramid Nanofiber Aerogel Based On Fractal Theor

Aerogels are considered to be ideal thermal insulation materials because of their low density,high porosity,nano-scale pore size and other structural properties,which exhibit extremely low thermal conductivity.Aramid nanofiber aerogels(ANFAs)have excellent mechanical properties,flexibility,high thermal stability,good chemical stability,and flame retardancy,which make them more promising for extreme thermal insulation applications.Fractal geometry has been widely used to simulate the microstructure of porous materials,which can simplify the structure of the material to study its heat transfer process.In this thesis,the structural characteristics of ANFAs are simulated using fractal theory and their thermal conductivity models are further developed.The main work is as follows:Study of the thermal conductivity model of aramid nanofiber aerogel.For the microstructure of ANFAs,the Sierpinski carpet model(SC)and 17 modified Sierpinski carpet models(MSC)based on the number and arrangement of subsquares removed from SC were developed using fractal theory,and a total of 18 basic fractal units were used to model the microstructure of ANFAs.The thermal conductivity of ANFAs was obtained by combining series and parallel resistive networks,and the thermal conductivity of ANFAs was calculated by iterative method.Finally,the thermal conductivity calculated by the thermal conductivity model is analyzed with the experimental data for error,and the optimal basic fractal unit is selected for calculating the thermal conductivity of ANFAs.The results show that the thermal conductivity of ANFAs calculated by MSCb is the closest to the experimental results with an average error of only 6.935%,and the fractal dimension of MSCb is similar to that of ANFAs material,which further indicates that the model has the best effect in predicting the thermal conductivity of ANFAs and verifies the applicability of the thermal conductivity model.Study of thermal conductivity models of asymmetric structure aramid aerogels.To solve the shortcomings of ANFAs,asymmetric structure aramid aerogel membranes(ASAAMs)with dense skin structure were designed to effectively balance the mechanical properties and thermal insulation performance of the aerogels.To address the unique microstructure of ASAAMs,the hybrid fractal unit model(IFU)concept was used to model the internal and surface microstructures of ASAAMs by combining two fractal structures to model the thermal conductivity of ASAAMs.The model was then modified using the dynamic weighting method,and the modified thermal conductivity measurement model of ASAAMs matches the experimental data with an average error of only 2.58%.It is shown that the corrected model of thermal conductivity of ASAAMs can effectively predict the thermal conductivity of this material.It was found that the thermal conductivity measurement models for both aramid nanofiber aerogels and asymmetric structured aramid aerogels have high predictive performance and short computation time,and this work can reduce the economic cost of measuring the thermal conductivity of aerogel materials to a large extent,and it has some theoretical guidance value for the study of thermal insulation performance of other nanofiber aerogel materials.