Thermal-Chemical Coupling Analysis Of Low Density Carbon/Phenolic Composites Under Ablation Condition

In recent years, in order to adapt to the trend of lightweight and low cost aircraft and the demand for deep space exploration, aerospace industry puts forward urgent demand on lightweight ablative materials. As a new generation low density carbon/phenolic composites, PICA(Phenolic Impregnated Carbon Ablator)possesses very low density(about 1/5 of traditional carbon/phenolic composites), exhibits excellent thermal protection, thermal insulation and ablation resistant capability, which were successfully applied to thermal protection structure of and interplanetary exploration vehicle in the atmospheric reentry repeatedly. Under the effect of aerodynamic heat, PICA undergoes intense physicochemical reaction and even ablation recession, such as the pyrolysis reaction of resin matrix, pyrolysis gas transport in material, ablation recession of surface and so on. Therefore, it is necessary to study pyrolysis mechanism of the phenolic resin matrix and ablation mechanism of low density carbon/phenolic composite, and then establish material’s thermal response analysis model to predict material thermal protection performances, which is of great importance for the selection of thermal protection materials and the design and evaluation of thermal protection structure.Firstly, thermo-gravimetric experiments of low density carbon/phenolic composites and phenolic resin matrix were conducted, respectively. Based on DTG curves, thermal decomposition kinetic model was obtained by peak analysis method. The mass loss mechanism and energy loss mechanism of the low density carbon/phenolic composite in the aero-dynamic environment were analyzed.Secondly, based on decomposition kinetic model and ablation mechanism analysis, macroscopic material thermal-chemical coupling governing equations, containing mass conservation equation, momentum conservation equation and energy conservation equation, were deduced. The temperature-pressure coupling equation for the material under the effect of heat flow was established to characterize the physical chemistry process, where the material pyrolysis and pyrolysis gas flow were taken into accounted.Thirdly, ablation experiment of low density carbon/phenolic composite under unilateral radiation heat flux was conducted. Digital image correlation system combining with thermocouple temperature measurement system and the oxyacetylene radiation flux system were used to accurately measure the specimen displacement field distribution, temperature history of the points with different distances from heating end and ablation rate. The specimen morphology and microstructures before and after ablation experiment were observed, compared and analyzed.Finally, macroscopic material thermal-chemical coupling governing equations of low density carbon/phenolic were analyzed by using multi-physics field coupling analysis software COMSOL-Multiphysics. Temperature field distribution of low density carbon/phenolic composites during the process of heating was obtained, which were in good agreement with the experimental values. Meanwhile, decomposition degree, pyrolysis gas pressure field distribution and pyrolysis gas velocity were also obtained. The proportions of all endothermic mechanisms in the total heat absorption were analyzed, which could lay a theory foundation for ablation performance evaluation for the low density carbon/phenolic composite further.