Mechanisms of Coupled Vibrational Relaxation and Dissociation in Carbon Dioxide.

Abstract

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.