Mechanisms of Coupled Vibrational Relaxation and Dissociation in Carbon Dioxide

A complete vibrational state-specific kinetic scheme describing dissociating carbon dioxide mixtures is proposed. CO₂ symmetric, bending and asymmetric vibrations and dissociation-recombination are strongly coupled through intermode vibrational energy transfers. Comparative study of state-resolved rate coefficients is carried out; the effect of different transitions may vary considerably with temperature. A nonequilibrium 1-D boundary layer flow typical to hypersonic planetary entry is studied in the state-to-state approach. To assess the sensitivity of fluid-dynamic variables and heat transfer to various vibrational transitions and chemical reactions, corresponding processes are successively included to the kinetic scheme. It is shown that vibrational-translational (VT) transitions in the symmetric and asymmetric modes do not alter the flow and can be neglected whereas the VT₂ exchange in the bending mode is the main channel of vibrational relaxation. Intermode vibrational exchanges affect the flow implicitly, through energy redistribution enhancing VT relaxation; the dominating role belongs to near-resonant transitions between symmetric and bending modes as well as between CO molecules and CO₂ asymmetric mode. Strong coupling between VT₂ relaxation and chemical reactions is emphasized. While vibrational distributions and average vibrational energy show strong dependence on the kinetic scheme, the heat flux is more sensitive to chemical reactions.