Numerical simulation of chemical vapor infiltration of propylene into C/C composites with reduced multi-step kinetic models

A parallel-consecutive reaction model of chemistry and kinetics is proposed to simulate homogeneous gas-phase reactions of propylene pyrolysis in CVI processes. An improved bipore model is also suggested to describe the changes of the pore topology with densification. The competition between the heterogeneous reactions of pyrolytic carbon deposition and the homogeneous reactions is analyzed by a numerical simulation method. Numerical simulation shows that continuous higher density region occurs early in a certain depth of the substrate, which blocks precursor transport into the deeper region. Changing processing parameters can alter when and where the continuous higher density region takes place. Inside-out densification is an inherent characteristic for CVI processes, while premature surface crusting is an apparent phenomenon. According to the concentration ration between C₂H_x and C₆H_y, the textures of pyrolytic carbon are successfully predicted. The present model is validated by comparing predicted with observed densities.