Numerical Modeling of Surface Chemistry Processes for Hypersonic Entry Environments.

Abstract

The high temperatures on a hypersonic vehicle surface caused by heat loads encountered during entry through a planetary atmosphere require a reliable Thermal Protection System (TPS) that makes a good understanding of the physical and chemical processes essential for its design. TPS is a single point of failure system as the prolonged exposure to high temperature can cause the TPS materials to fail. Catalycity of an ablative TPS material and surface-participating reactions that lead to surface recession are key factors that impact the heating of the vehicle surface.

The major objective of this dissertation is to investigate surface chemistry processes (e.g. catalysis, nitridation) using coupled CFD-surface chemistry models. Another objective is to assess surface chemistry models using experimental data. The numerical simulations in this work are conducted using the computational fluid dynamics (CFD) code LeMANS developed at the University of Michigan. The investigation is performed using a finite rate surface chemistry model that incorporates the effects of surface catalysis as well as surface participating reactions. Experimental data for flow and surface properties from tests conducted in the 30 kW Inductively Coupled Plasma Torch (ICP) Facility at the University of Vermont are used for the evaluations of the computations for different surface chemistry processes.

The effects of surface chemistry processes of a graphite sample exposed to a subsonic high-enthalpy nitrogen flow are investigated. The processes studied are the recombination of nitrogen atoms to molecules at the surface due to catalysis, and carbon nitridation where nitrogen atoms react with the surface carbon to form gaseous CN.

The results show a good agreement of the computations with all experimental measurements if all the flow, surface and material physics are included in the simulations. It is shown that the loss of nitrogen atoms observed in the experiment is caused by a combined effect of nitrogen recombination due to surface catalysis and the carbon nitridation reaction. It is revealed that true validation of the surface chemistry models requires absolute number density measurements. It is also determined that validation of such simulations requires better characterization of the power absorbed by the plasma in the ICP torch.