| Thermal protection system is crucial to the safety of the spacecraft since the spacecraft is subjected to severe aerodynamic heating when flying in the earth atmosphere.Polymer matrix composites have the advantages of low thermal conductivity,low density and heat absorption during pyrolysis.When polymer matrix composites are subjected to aerodynamic heating,various physical and chemical reactions take place at the material surface and in-depth material.Up to now,the thermal response model of polymer matrix composites has not considered the influence of air pressure and heating rate on the thermal response.Moreover,there is still no effective method to measure the high-temperature thermophysical properties of the pyrolysis layer of the composites.Therefore,there are technical bottlenecks in the optimal design of spacecraft thermal protection system,which needs to be solved urgently.Based on the theories of heat transfer,fluid mechanics,physical chemistry,material testing,etc.Researches on the pyrolysis layer model and high-temperature thermophysical properties measurement of polymer matrix composites are conducted in this thesis.A pyrolysis layer model with pressure is presented,the accumulation and backflow of pyrolysis gases are found,and proportions of different thermal mechanisms of the polymer matrix composites are given.Firstly,based on the theories of heat transfer,fluid mechanics and physical chemistry,the energy conservation equations considering pressure of the virgin layer,the pyrolysis layer and the char layer in polymer matrix composites are deduced combined with the continuity equation and the momentum conservation equation.The relevant mathematical model of polymer matrix composites is formed by combining the the deduced energy conservation equations,the gas state equation and the Darcy equation.Secondly,the mathematical model is discretized by the second-order central difference scheme and the first-order forward difference scheme,and the implicit difference equations are obtained and programmed in the MATLAB.Finally,the thermal response of TACOT material is simulated by using the program,and the numerical results are in good agreement with the experimental data.In addition,the thermal response of phenolic impregnated carbon ablator(PICA)under different ambient pressures and the thermal response of PICA with different mass fractions of resin are conducted by using the program.Numerical results indicate that:(1)Ambient pressure affects the pyrolysis gases flow state in-depth PICA materials.As the ambient pressure exceeds a critical value,the pyrolysis gases appear to accumulate and backflow inside the material apparently.(2)The thermal protection mechanism of PICA materials mainly consists of heat absorbed by material heat capacity,surface radiation and thermal blockage effect of pyrolysis gases.But the thermal blockage effect of pyrolysis gases weakens as time increases.Besides,the thermal blockage effect of pyrolysis gases is enhanced with the increase of resin mass fraction in the material.The pyrolysis rate formula of PICA materials under variable heating rates is obtained,and the pyrolysis layer model with heating rate and pressure is given.The influence of heating rate on the thermal response of the polymer matrix composites is revealed,and numerical results is helpful to reduce the abundant design of thermal protection system of hypersonic vehicles.Pyrolysis process of polymer matrix composites is a complex process involving high temperature,phase change and a variety of physical and chemical reactions.Heating rate can affect the pyrolysis rate of resin inside the material and then affect the temperature distribution,the pressure distribution and the pyrolysis gases flow state.In order to study the influence of heating rate on the thermal response of polymer matrix composites,the pyrolysis rate formula of polymer matrix composites under different heating rates is obtained based on the thermogravimetric analysis results and the Levenberg-Marquardt algorithm.Later,the obtained pyrolysis rate formula is substituted into the energy conservation equation of the pyrolysis layer model to incorporate pressure and heating rate,and then the model is discretized and solved numerically.The thermal response of PICA is predicted.Later,based on the pyrolysis layer model with pressure and heating rate,the thermal response of PICA material under the aerothermal environment of the Stardust vehicle is calculated.Numerical results show that when considering the air pressure and the heating rate,the bondline temperature of PICA material starts to rise earlier due to the backflow of pyrolysis gases.But the heat dissipated by the thermal blockage effect of pyrolysis gas on the material surface decreases accordingly,which leads to the increase of surface temperature.Meanwhile,the heat dissipated by surface radiation increases.The temperature rise of the material bondline decreases significantly.A set of testing system is developed,and the carbon/phenolic composite specimens’ high-temperature thermophysical parameters are measured by using the established testing system.The pyrolysis layer of polymer matrix composites is a very complex and constantly changing solid-gas two-phase region,and the thermophysical properties of the pyrolysis layer of polymer matrix composites are difficult to measure.To figure out the technical bottleneck,a set of testing system is developed,which is composed of the test instrument and the parameter identification codes.Besides,the carbon/phenolic composite specimens are prepared by the vacuum impregnation process,and their hightemperature thermophysical parameters are measured by using the established testing system.Experimental results illustrate that the testing system can effectively measure the high-temperature thermophysical parameters of the carbon/phenolic composites.With the increase of temperature,the thermal conductivity and the specific heat capacity increase gradually,but the thermal diffusion coefficient declines slightly.When the temperature continues to increase to the start pyrolysis temperature,there is an obvious turning point in the curves of the thermal conductivity,the specific heat capacity and the thermal diffusion coefficient,but all the three curves show a nonlinear upward trend. |
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