Experimental investigation of ablation and pyrolysis processes of carbon-phenolic ablators in atmospheric entry plasmas

We study the ablation and transient pyrolysis outgassing of the carbon-phenolic ablators AQ61 and Asterm in air and nitrogen plasmas. We investigate their resistance to high heating conditions, and characterize gas-surface interaction phenomena, including the interaction of the pyrolysis gases with the hot plasma flow. The experiments were carried out in the Plasmatron facility of the von Karman institute for Fluid Dynamics. The aero-thermodynamic environment of atmospheric entry in the boundary layer of a test object was selected with surface temperatures between 1900 K and 2800 K, and test chamber pressures of 15 hPa, 100 hPa, and 200 hPa. Those conditions led to recession rates between 39 mu m/s and 83 mu m/s in air plasmas. Micrographs revealed oxidation of the char layer and carbon fibers. Carbon deposition in the form of soot was observed on samples tested in nitrogen, contrary to air ablation where charred resin was not found at the surface. We propose an approach to estimate the temporally resolved pyrolysis outgassing rate, based on the emission signature of pyrolysis products and the volume change of the sample. The temporal recession rate was obtained from high-speed camera imaging. This enabled evaluation of the surface recession as a function of the pyrolysis outgassing rate, which was then compared to numerical estimates predicted by thermochemical equilibrium tables. The thermochemical equilibrium model generally underpredicted experimental recession rates, particularly at low pressure (15 hPa). Stronger mechanical failure of the material was ruled out as experiments at the same test conditions in nitrogen plasmas did not show any significant recession. Micrographs did not indicate internal oxidation of the material, neither was spallation observed during the low pressure experiments. (C) 2016 Elsevier Ltd. All rights reserved.