Coupled surface-volume pyrolysis effects of carbon-phenolic resin composites under hyperthermal non-equilibrium flows

The thermal protection system is essential for the safe and reliable flying of any high-speed aircraft. A carbon fiber-reinforced phenolic resin (PR) composite is one of the most important matrix candidates for ablative materials, and the microscopic understanding of the pyrolysis process of PR, however, still remains poor. The usual numerical approach is based on volumetric pyrolysis while neglecting the influence of external hyperthermal surroundings. A surface-volume coupled reactive molecular dynamics model is established in this work to simulate complicated heat/mechanics/chemistry multi-physical field coupled pyrolysis problems. The pyrolysis process of the carbon fiber-PR composite is investigated in the presence of vacuum and hyperthermal gas impacts and compared with the volumetric pyrolysis simulation only. The consideration of the surface-volume coupling reveals many unique features beyond the volumetric pyrolysis, which include the generation of different pyrolysis products and the dependence of the effective mass diffusivity of pyrolysis gases on impinging gases in the presence of the hyperthermal non-equilibrium environment. With the detailed revelation of the evolutions of PR solid phase and pyrolysis gaseous products, the work is of great help in improving the microscopic pyrolysis mechanisms, especially the "blowing gas effect,"a key phenomenon for improved understanding of the complicated hypersonic boundary layer flow.