Computation of Surface Catalysis for Graphite Exposed to High-Enthalpy Nitrogen Flow

Abstract

The high temperatures on a hypersonic vehicle surface caused by heat loads encountered during (re-)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. Surface catalysis is a crucial chemical process that directly impacts aerothermal heating of the vehicle TPS. To study the effects of this process, a binary catalytic atom recombination model is implemented in a computational fluid dynamics (CFD) code. The study examines the effects of surface catalysis for graphite exposed to high enthalpy nitrogen flow. As expected, surface catalysis strongly affects the boundary layer gradients of temperature and species concentration, and heat transfer to the surface. A fully catalytic surface causes the heat flux to increase by a factor of approximately 3.5. The physical accuracy of the model is assessed using data from experimental tests conducted in the Inductively Coupled Plasma (ICP) Torch Facility at the University of Vermont. Comparisons are presented of computed results with measured experimental data for translational temperature and relative nitrogen atom number density in the flow in front of the test article. NomenclatureD 12 binary diffusion coefficient between species 1 and 2 [m 2 /s] J sp species diffusion flux [kg/m 2 /s] M mass flux [kg/m 2 /s] N sp number of species in the mixture [dimensionless number] T translational-rotational temperature [K] Y mass fraction [dimensionless number] h sp species enthalpy [J/kg] k B Boltzmann constant k w wall catalytic speed [m/s] m particle mass [kg] q total heat flux [W/m 2 ] q conv convective heat flux [W/m 2 ] q dif f diffusive heat flux [W/m 2 ] γ wall catalytic efficiency [dimensionless number] κ thermal conductivity [W/m/K] ρ density [kg/m 3 ] subscripts w wall value tr translational-rotational energy mode ve vibrational-electronic energy mode ∞ reference freestream conditions sp species value * Graduate Student, Student Member AIAA. † James E. Knott Professor, Fellow AIAA.

Numerical Analysis of Surface Chemistry in High-Enthalpy Flows

“…It is concluded that the nonuniform inlet profile does not significantly affect the solution. The results for the study are discussed in [18,46]. An assumption of chemical equilibrium of the nitrogen gas mixture at the ICP torch exit is used to calculate its composition in these simulations, as experimental data at the exit are not available.…”

Computational Modeling of Gas-Surface Interactions for High-Enthalpy Reacting Flows

“…The exit of the ICP torch is the inlet to the test chamber of the facility flow where the sample is tested. Previous studies 8,9 examined the effects of surface catalysis and the nature of the flowfield in the ICP torch facility for graphite exposed to high-enthalpy nitrogen flow. In Ref.…”

“…In Ref. 8, the surface catalysis effects were accounted for by using a simplified catalytic atom recombination model where the recombination of nitrogen atoms to molecules due to catalysis was investigated. It was shown that the temperature in the boundary layer is not affected by surface catalysis whereas it strongly affects the boundary layer gradients of species concentration, and heat transfer to the surface.…”