| Ultra-lightweight carbon-phenolic composite is a novel kind of heat shielding and thermal insulation unified ablation resistant charring composite material, aimed at catering the technical requirements of thermal protection system applied to lightweight deep space exploration reentrant vehicles.Such material is fabricated by padding carbonaceous 3D network skeleton with nanostructured phenolic resin aerogel, fiber surface is coated with polymer resin thin film with uniform thickness to form a rime like structure. Material properties of two subtypes, namely ultra-lightweight carbon bonded carbon fiber / phenolic resin aerogel composite and ultra-lightweight carbon fiber needle felt / phenolic resin aerogel composite,are adjustable according to needs. Meanwhile,pressure filtration or fiber weaving technique impels composites to be transverse isotropy in terms of both structure and properties.The composites show outstanding ablation resistance property under typical conditions of Arc-jet stagnation ablation tests. High temperature surface radiation and heat blockage by mass injection may markedly weaken aerodynamic heating effect. On the other hand, marvelous thermal insulation performance is proved by thermophysical properties measurements and thermal analysis experiments.Pyrolysis of phenolic aerogel and low thermal conductivity through thickness efficaciously prevent further transmission of energy.Physical model of ablation stratification phenomenon can be described by four layers, respectively ablative, char, pyrolytic and virgin. Under the couple effects of surface and volumetric ablation, complicated processes of energy transmission and mass/momentum transportation happens external and internal.Based on thermal protection mechanism analysis, an intrinsic mathematical model describing ablation behavior is established, which gives priority to solid phases and includes influences of complex factors such as pyrolytic reaction and mass injection. Thermogravimetric analysis shows that thermochemical reactions of phenolic aerogel is consistent with the theory of three stages pyrolysis. Using Gaussian peak differentiating and imitating method as well as peak analysis,multistage pyrolysis kinetic model based on multiple generalized Arrhenius equations is obtained. And there comes a calculation model describing the variation of the material properties parameters over time and temperature by adopting pyrolysis ratio as an interpolation coefficient. Moreover, a flow model of pyrolysis gases in porous media is established based on the conservation of mass and momentum. Net heat flow transferred to the inside is calculated by surface energy balance equation and the thermal response equation of the control body is solved subsequently by the assumption of thermodynamic equilibrium.Based on the mathematical control equation derived. full-scale multi-dimensional model is mathematical modeling by COMSOL-Multiphysics and is solved transiently in a multi-field strong coupling way. The material, geometry and boundary nonlinear effects are considered, and surface recession is characterized by deformed geometry technique. Numerical simulation results in comparison with the arc tunnel experiments show both regularity consistency of response and reasonable relative error. This, in turn, validates the validity of mathematical model and indicates high fidelity of simulation solution. In the post-data processing, massive line-diagrams and cloud-graphics of property field variables and physical field variables are presented. The analysis of primary and secondary factors in thermal protection is to further supporting material selection optimization and system architecture design. |
