Influence of Carbon Nitridation in a Nonequilibrium Finite-Rate Ablation Model

Thermal protection system design for atmospheric re-entry vehicles remains to be a challenging and complex problem. There have been several attempts to model surface chemical kinetics but there is no universally supported model among investigators. Recent advances in computational modeling of air-carbon interactions now consider competing finite-rate reactions on a limited number of available surface sites. One of the most advanced kinetic models is due to Zhluktov and Abe. However, the Zhluktov and Abe model only describes the oxidation and sublimation of carbon and has no nitridation mechanism. The following study develops a modification to the Zhluktov and Abe air-carbon interaction model that accounts for all three reaction mechanisms. The study examines two possible paths that represents carbon nitridation as a direct or a surface participating reaction. The augmented surface reaction model is assessed in a representative blunt body re-entry flow and compared against the well-known Park model. It is shown that the implemented direct nitridation mechanism has the most significant impact on predicted surface mass fluxes and species mass fractions. There are notable differences between the Park and direct nitridation mechanisms, particularly at carbon sublimation surface temperatures. More detailed measurements of the carbon nitridation reaction at higher surface temperatures are required to further validate and improve the rate parameters derived in this study.