Modellierung reaktiver Prozesse auf Siliziumkarbid-Oberflächen in verdünnten Nichtgleichgewichts-Luftströmungen

Drei Ansatze zur Berechnung der Wechselwirkungen zwischen Luft im thermischen und chemischen Nichtgleichgewicht und Siliziumkarbid bzw. Siliziumdioxidoberflachen werden vorgestellt und diskutiert. Siliziumdioxid ist ein Oxidationsprodukt der Oxidation von Siliziumkarbid durch Sauerstoff und kann auf der Oberflache angelagert werden. Dort wirkt es als Diffusionsbarriere und verzogert damit das Fortschreiten der Oxidation. Abhangig vom Gasdruck bildet sich bei hohen Temperaturen verstarkt gasformiges Siliziummonoxid. In Kombination mit Hochenthalpiestromungen im chemischen Nichtgleichgewicht hat der Abbau des Siliziumdioxid eine um Grosenordnungen beschleunigte Oberflachenerosion in Verbindung mit einer Oberflachentemperaturerhohung von mehreren 100 K zur Folge. Die Verbindung katalytischer Prozesse mit Reaktionen zu Bildung und Abbau von Siliziumdioxid bildet ein Reaktionsgleichungssystem aus 110 einzelnen Reaktionsschritten und ermoglicht es, den Gaszustand an der Oberflache, die Erosion sowie die Temperatur zu berechnen. Durch Kopplung mit dem im Rahmen des Sonderforschungsbereichs 259 "Hochtemperaturprobleme ruckkehrfahiger Raumtransportsysteme" entwickelten Navier-Stokes Verfahren URANUS, das Luftstromungen im thermochemischen Nichtgleichgewicht genau und effizient berechnen kann, ist es moglich, die Oberflachenbelastung eines Raumfahrzeugs, dessen Hitzeschutzsystem auf Siliziumkarbid oder Siliziumdioxid basiert, bei der Ruckkehr zur Erde zu berechnen. Eine einheitliche Schnittstelle fur die reaktiven Oberflachenmodelle bildet die kinetische Randflussbehandlung. Diese gestattet es, die Gultigkeit des Simulationsverfahrens in den Ubergangsbereich zwischen Kontinuumsstromung und freier Molekularbewegung auszuweiten, so dass Belastungen der Luvseite eines Ruckkehrfahrzeugs bis in etwa 100 km Hohe vorhergesagt werden konnen. Demonstriert wird dies am Beispiel des US-Shuttle Orbiters sowie des experimentellen Wiedereintrittsfahrzeugs MIRKA. Three different approaches have been developed for the modelling of the interactions between air in thermal and chemical nonequilibrium and surfaces. The models for silicon carbide and silicon dioxide will be explained and discussed in detail. Silicon dioxide is one of the reaction products formed between oxygen and silicon carbide. The accumulation of silicon dioxide may form a surface layer acting as a diffusion barrier which reduces further oxidation. With decreasing pressure and increasing temperature, the formation probability of gaseous silicon monoxide increases. In high enthalpy flows in thermochemical nonequilibrium the decomposition of the silicon dioxide layer leads to an increase of erosion rate by orders of magnitude combined with a rise of surface temperature by several 100 K. Taking catalytic processes as well as formation and decomposition of silicon dioxide into account, a surface reaction scheme arises which consists of 110 elementary reaction steps. Hence, gas state, erosion rate as well as temperature can be computed. The surface reaction scheme is coupled with the advanced nonequilibrium Navier-Stokes Code URANUS (Upwind Relaxation Algorithm for Nonequilibrium Flows of the University of Stuttgart) which was developed within the collaborative research center 259 "High Temperature Problems of Re-Entry Vehicles". The URANUS code allows for an accurate and efficient computation of air flows in thermochemical nonequilibrium. By coupling the gas-surface interaction models with URANUS, the thermal and mechanical loads of thermal protection systems based on silicon carbide or on silicon dioxide can be simulated. A common interface to all surface models is realized by the gas-kinetic flux spitting at the surface. The flux based boundary conditions allow for temperature and velocity slip at the surface hereby extending the validity of the code into the transition regime. Hence, surface loads at the windward side of a re-entry vehicle can be determined up to an altitude of approximately 100 km. The abilities of the surface models are demonstrated for the re-entry of the US-Space Shuttle Orbiter as well as for the experimental MIRKA vehicle.

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