Experiment On Coupled Heat Transfer And Thermal Property Identification Of Nanocomposite Insulation At High Temperature

The typical nanocomposite thermal insulation is made up of silica aerogel,infrared opacifier and fiber reinforcement,featuring nanoscale pore structure,high porosity,low thermal conductivity,low apparent density,and strong attenuation behavior of infrared radiation at high temperature.As a new lightweight and efficient high temperature thermal insulation,the nanocomposite thermal insulation has been found excellent potential applications in high temperature engineering fields.For example,it can be used as high temperture thermal insulation of hypersonic vehicle thermal insulation systems.In high temperature applications,heat transfer in nanocomposite thermal insulation involves the transient coupling of multi-scale conduction,thermal radiation in multi component micro and nano solid composite structure,and gaseous conduction,the heat transport mechanism is complex.In addition to the micro scale based method on investigating heat transfer mechanism,the continuous scale based numerical simulations and experimental investigations can be used to master heat transfer mechanism and characteristics,to obtain basic high temperature thermal properties,and to predict accurately the thermal insulation performance as well as the influencing factors,thus can provide guidance and fundamental support on the process of developing new material and designing of the material based thermal structures.This dissertation mainly focuses on the problems of predicting high temperature trnasient coupled heat transfer characteristics,and of acquisiting high temperature conductive and radiative properties of nanocomposite thermal insulation,the following reasearches concerning continuous medium level based coupled heat transfer mechanism analysis,high temperature experimental measurements,and retrieval of thermal properties of nanocomposite thermal insulation were investigated.Based on the basis knowledge of nanocomposite thermal insulation,a coupled conductive and radiative heat transfer model for absorbing,emitting and scattering medium was built.The Monte Carlo method(MCM),the two flux approximation,the P1 approximation,and the Rosseland diffusion approximation were seperately employed to solve radiative transfer of the medium.The finite volume method was employed to solve the energy equation,the corresponding computation codes were developed.The radiative properties of nanocomposite thermal insualtion were predicted on the basis of the microstructure and composition of the material,as well as their volume fractions by employing the Lorenz Mie theory.In combination of the estimated thermal conductivity,the coupled heat transfer mechanism,as well as the strong radiation attenuation characteristics of the material were preliminarily analyzed.The applicability of various radiation models was investigated from the accuracy and the efficiency point of view,the two flux approximation method was found more suitable for solving radiative transfer in nanocomposite thermal insulation.In order to get high temperature conductive and radiative properties of nanocomposite thermal insulation,a multi parameter identification model on the basis of experimentally measured transient temperature data was developed.Based on the genetic algorithm(GA)optimization,two types of inverse methods separately used to retrieve temperature dependent effective thermal conductivity,and to retrieve simultaneously the true thermal conductivity and radiative properties were proposed,and the corresponding codes were developed.The identification models,the inverse methods,and the codes were firstly validated numerically by retrieving the effective thermal conductivity,the true thermal conductivity and radiative properties of typical thermal insulation material.Then,based on high temperature thermal characteristics of four types of glassmelts with different iron content,measured from actual experiments for temperature ranging from 1373 K to 1823 K,the thermal conductivities and absorption coefficients of the glassmelts were retrieved.By comparing and analyzing the retrieved data with those reported in the literatures,the reliability of the proposed inverse method for retrieving conductive and radiative properties,as well as the corresponding codes were further validated experimentally.A mathematical analysis technique based on the Cramér-Rao lower bound(CRB)method was presented for estimating a priori the uncertainties of the conductive and radiative parameters to be retrieved.Unlike the traditional methods,the method takes into account not only the random error of the measured temperature data,but also the uncertainties of the known model parameters(such as the geometry and the known physical properties).Thus it can give a complete analysis of the uncertainties of the retrieved parameters.By combining the CRB analysis method with actual experiments,the uncertainties of the conductive and radiative properties of nanocomposite thermal insulation were estimated,and the error contributions of the error sources were determined.Furthermore,the optimal sensor positions of the high temperature experiments were designed based on the CRB method.An experimental apparatus for measuring transient heat transfer characteristics of nanocomposite thermal insulation was designed.This apparatus allows to measure temperatures at various positions,and to measure total heat flux as well as infrared thermogram at the rear face of various materials including nanocomposite thermal insulation,optical window materials,porous materials,ceramics and other solid materials,at gas pressure from near vacuum to 200 k Pa,the surface radiation source can achieve a high temperature up to 2300 K.Based on analyzing combined heat transfer in multidimensional nanocomposite thermal insulation specimen,in combination of temperature data measured from thermocouples installed at various positions,thus validated one-dimensional heat transfer characteristic of the specimen measurement region in the high temperature experiments.By using the experimental apparatus,the transient temperatures at various positions of the nanocomposite thermal insulation specimen with porosity of about 83%,and the steady-state heat flux through the specimen thickness direction were measured at nitrogen pressure of 0.01 Pa~100 k Pa,and temperature between 290 K and 1190 K.Based on the measured transient thermal characteristics and the inverse method,the temperature and gas pressure dependent effective thermal conductivity,the true thermal conductivity,the Rosseland mean transport extinction coefficient and scattering albedo were retrieved.The retrieved conductive and radiative properties were used to predict thermal behaviors of nanocomposite thermal insulation,the numerical predictions were compared with experimental data and very good agreement was observed,the results validated the reliability of the retrieved data.Furthermore,by considering the errors of thermal properties,the model parameter errors,and their associated error bounds,the MC method was employed to determine randomly the computation parameters of combined conduction and radiation simulation.By performing thousands of simulations and by analyzing the results from a statistical way,the thermal insulation performance of nanocomposite thermal insulation specimen was statistically evaluated.Through the present study,two methods based separately on theoretical analysis and experimental research,were established to investigate high temperature coupled heat transfer in nanocomposite thermal insulation.A series of temperature and gas pressure dependent conductive and radiative properties of nanocomposite thermal insulation were obtained,and the thermal insulation performance evaluation was achieved.The achivements can provide theoretical basis and technical support for understanding high temperature coupled heat transfer mechanism,predicting transient thermal insulation performance,and designing reliable thermal insulation structures.