Thermal Response Analysis And High Frequency Plasma Wind Tunnel Test Evaluation Of New Low-density Ablative Thermal Protection Materials

A new round of research for deep space exploration represented by Lunar exploration and Mars exploration has been raised.Manned or sample return and interplanetary entry have put forward more severe requirements for thermal protection system of probe.The new generation of ablative thermal protection materials represented by Phenolic Impregnated Carbon Ablator（PICA）in the United States have been successfully applied in many spacecrafts.China also has developed a variety of low-density ablative thermal protection materials,which also have finished the demo validation.Test and Evaluation（T&E）is always the most critical part to develop the thermal protection system.New material system,service environment and task requirements have brought new challenges to T&E.Ground wind tunnel tests are the main means of T&E for thermal protection material performances,in which high frequency plasma wind tunnel is one of the ideal means of T&E of ablation performances for low-density thermal protective materials because of its long running time,strong simulation ability and stable gas environment.In this dissertation,a type of low-density ablative thermal protection materials self-developed by China were focused.The environment characteristics,such as low heat flux,high enthalpy and low stagnation point pressure,on the windward surface of the deep space exploration return capsule were also fully considered.After reasonably characterizing thermophysical properties of materials,the material surface ablation and macro thermochemical response analysis models were established.The numerical simulation could effectively guide the design of high-frequency plasma wind tunnel test.Some material ablation tests in the wind tunnel under the air and carbon dioxide working medium were conducted,which were used to comprehensively evaluate the ablation performances of the new low-density ablation thermal protection materials.This study can provide technical support for developing efficient and low-cost test and evaluation methods,and fully understanding the performances of new thermal protection materials.Based on the differences of reinforcement fiber type,fabric form and apperant density of the materials,this paper mainly focuses on three kinds of new low-density ablative thermal protection materials:needle punched carbon fiber/quartz fiber felt/modified phenolic（NQF/PR-Si OC）material,chopped carbon fiber skeleton/modified phenolic（CBCF/PR-Si OC）material and chopped carbon fiber/modified phenolic（CF/PR-Si OC）material.Firstly,for NQF/PR-Si OC material,the thermogravimetry/differential thermal analysis,Fourier transform infrared spectroscopy,and mass spectrometry analysis experiments were carried out,the pyrolysis products and the pyrolysis kinetic parameters of the material were determined.Based on the Micro-CT observation of the micro structure characteristics of the material,a multi-scale prediction model of the thermal conductivity of the material was established.Combined with the experimental results,the effects of different pyrolysis degrees on the thermophysical properties of NQF/PR-Si OC materials were obtained.Secondly,based on the high frequency plasma wind tunnel testing principle and parameter acquisition method,the constant heat flux surface model of the new low-density ablative thermal protection material was designed as the wind tunnel test model.The problem of large fluctuation of the measured data caused by the charge in the flow field was also solved.Several high frequency plasma wind tunnel ablation tests with low,mediem and high heat flux in the air working medium of CBCF/PR-Si OC material were carried out.Then,the surface ablation model of the silicon-rich new low-density ablative thermal protection material was established.The surface temperature and surface ablation recession rate of the material under different heat flux conditions were determined.The influence of the change of material constituents on the surface ablation response of the material was also obtained.Combined with the thermophysical properties of the material and the pyrolysis kinetic equation,the macro thermochemical ablation model of the material was established.The finite element weak forms of the multi-field coupling equations were derived.The thermochemical responses of the model under tunnel testing conditions were solved by the finite element method,which were also verified in comparison of the ablation test results.Finally,the effects of heat flux,recovery enthalpy and pressure on the ablation performances of the new low-density ablative thermal protection materials were calculated and analyzed by using the surface ablation model,which effectively guided the design of high-frequency plasma wind tunnel test.The ablation properties of new low-density ablative thermal protection materials in air and carbon dioxide working medium were tested in high frequency plasma wind tunnel.It was found that the NQF/PR-Si OC material had the best comprehensive performances,and the CF/PR-Si OC material had the worst thermal resistance and insulation performances.The CBCF/PR-Si OC material could generate a large amount of Si O₂ in carbon dioxide medium,which could effectively improve the thermal resistance insulation ability of the material.The ablation mechanisms of the materials in different medium were also revealed by the comparative analysis of the ablation processes and ablation morphologies of the materials.Through analyzing the thermophysical properties,modeling thermochemical response analysis and studying the ablation test of high-frequency plasma wind tunnel for new low-density ablative thermal protection materials,this study can provide the technical support for the comprehensive analysis and evaluation of the ablation performances of these materials.