A large variety of graphitic materials have been tested in an arc heated air stream at a surface pressure of 4.3 atm and a nominal surface temperature of 3925 K. Included were commercial and developmental grades of artificial graphites, both two and three dimensional carbon-carbon composites, composites seeded with refractory compounds, and several special materials such as pyrolytic graphite, mesophase graphite, glassy carbon, and natural graphite. ATJ graphite was used as a control material. Except for the seeded materials which had poor performance, the mass loss rate for all the man-made graphitic materials fell within the range of 17 per cent less to 30 per cent more than the rate for ATJ. Thus it is concluded that wide variations in constituents, processing, fabrication and structure have relatively little effect on the ablation performance of graphitic materials, at least under the conditions of the present tests. Particulate mass loss was observed for all the materials tested and is the dominant mechanism for mass removal at the present test conditions. It is suggested that this mechanism results from physical failure, primarily by compressive thermal stress.

The ablation of graphitic materials in the sublimation regime

La necessite de proteger les vehicules de rentree atmospherique a conduit a la selection des materiaux carbones pour les applications les plus extremes [1, 2]. Au vu du cout prohibitif des essais en conditions reelles [1], la simulation permet de predire et comprendre le comportement du materiau lors de la rentree atmospherique. Les modeles de l’ablation du carbone n’ont cesse de se complexifier [1, 3, 4, 5, 6]. Cependant, ils ne donnent pas de descriptions fines de la surface du materiau. C’est pour remedier a cela que plusieurs etudes ont ete lancees au Laboratoire des Composites Thermostructuraux (LCTS) [7, 8]. Le Plasmatron du VKI est le moyen a haut flux de chaleur et a haute vitesse utilise pour cette these [9, 10]. Dans cette etude, des echantillons sont testes sous des flux de chaleur allant jusqu’a 2800 kW/m2. Ces essais sur le composite 3D carbone/carbone (Cf/C) ont mene a des resultats tres interessants sur la morphologie du composite lors de l’ablation. Le four a image d’arc est le moyen a tres haut flux de chaleur et a convection nulle utilise au cours de cette these. Des flux de chaleurs de 10 MW m-2 ont pu etre appliques a des echantillons de 3D Cf/C. Les observations MEB couplees a la reconstruction numerique du four ont permis de noter les differences entre l’oxydation et la sublimation du composite. Un code de diffusion/ablation est developpe depuis plusieurs annees au LCTS [7, 11, 12, 13, 14] : AMA. Il permet de simuler l’ablation d’un materiau en carbone. Ce code a ete valide par J. Lachaud sans prise en compte de l’ecoulement [7]. Un nouveau couplage avec le code Open-Foam [15] a ete developpe et valide. Le code AMA a ete utilise afin de determiner les parametres influencant la rugosite du 3D Cf/C sous un ecoulement laminaire.

Ablation de matériaux carbonés sous très haut flux : étude multiphysique de l'interaction matériau/écoulement

Comparison of graphite ablation experiment results in the diffusion-controlled oxidation and sublimation regimes with results of an equilibrium chemistry, film coefficient ablation analysis. Mass transfer and energy transfer effects are considered. Tests were conducted in an arcjet facility at convective heating rates of 600 to 800 W/sq cm, radiative heating rates up to 2900 W/sq cm, with test specimen surface pressures of 0.06, 0.1, and 0.3 atm in an air stream. The experimental and analytical mass loss and surface temperature results agreed well when the carbon vapor thermodynamic properties from the JANAF tables are used in the analysis.

R. M. Wakefield

Papers

A study of graphite ablation in combined convective and radiative heating.