Catalytic recombination assessment on carbon in dissociated shock tube flow

Owing to its superior chemical stability at high temperatures, carbon is attractive as a candidate for ablation thermal protection system (TPS) materials for hypersonic re-entry vehicles. The catalytic recombination behavior is crucial to the ablation process during atmospheric entry because it provokes considerable heat flux at the surface. In the present study, catalytic efficiency values for oxygen and nitrogen recombination on carbon material have been determined by reflected shock experiments that can replicate the flow behavior of a hypersonic stagnation-point blunt body. Dissociated oxygen and nitrogen atoms were adequately produced adjacent to the shock tube endwall. Typical thin-film gauges coated with carbon are used for the stagnation heat-transfer measurements at the endwall. Before and after the tests, the properties of carbon were investigated by scanning electron microscopy (SEM), atomic force microscopy (AFM), and X-ray photoelectron spectroscopy (XPS). Surface heat-transfer data were analyzed with catalytic heat-transfer theories. The role of recombination efficiency on the species mass fraction is also investigated by solving the governing equation for species conservation. The recombination efficiencies of carbon are found to be 0.0023 and 0.00075 for oxygen and nitrogen, respectively. It has been shown that the carbon surface is more catalytically active towards oxygen atoms than nitrogen atoms.