Graphite Ablation and Radiation on Interaction with Hypervelocity Earth-Entry Flows

Abstract

The ablating species from the thermal protection systems upon atmospheric entry interact with the atomic/molecular gas species in the shock layer and form new products that affect total radiation. In this work, the ablation of carbon was experimentally studied by resistively preheating graphite strips to representative reentry wall temperatures and exposing them to hypervelocity Earth-entry flow conditions generated in the X2 expansion tube facility at The University of Queensland, Australia. The graphite strips were mounted on a compression wedge and preheated to wall temperatures ranging from 1000 to 3000 K. The flowfield and interaction processes were optically diagnosed using a high frame-rate video camera, two-dimensional filtered imaging, and ultraviolet emission spectroscopy. The distribution of cyanogen (CN) violet band emission was investigated at different slices in the flowfield. Experimentally recorded CN emissions were compared with the spectra simulated using Specair to estimate CN number densities and transrotational and vibro-electronic temperatures at various spatial locations with a two-temperature model. The ablating flowfield was also compared with no-ablation cases by using an unheated steel strip instead of heated graphite. The results in the expansion region are possible indications of non-Boltzmann population distribution in CN energy levels.