The effects of surface-chemistry processes of a graphite sample exposed to a subsonic high-enthalpy nitrogen flow are investigated using a coupled computational fluid-dynamics/surface-chemistry model. The results obtained are assessed for the accuracy of the model using experimental data from tests conducted in a 30 kW inductively coupled plasma torch facility at the University of Vermont. Significant discrepancies are observed between the computational and experimental results. Therefore, a study is performed to determine sensitivities of flow and surface parameters to variations in testing input conditions, as well as physical modeling parameters. Measurements of the absolute number density are required to draw firm conclusions about the surface-chemistry models, as well as the surface reactions involved.Nomenclature C k = concentration of gas species k, mol∕m 3 D k = diffusion coefficient of species k, m 2 ∕s E ad = energy barrier for adsorption, J∕mol E ER = energy barrier for Eley-Rideal recombination, J∕mol h = species enthalpy, J∕kg K = total number of species K g = total numbers of gas species K nb = number of species in bulk phase nb K ns;na = number of species in active site set na on surface phase ns k B = Boltzmann constant k fi , k bi = forward and backward reaction rates for reaction i M k = molar weight of species k, kg∕mol _ m b = mass blowing rate due to surface reactions, kg∕m 2 ∕s N = total number of phases N nb = number of bulk species N g , N s , N b = number of gas, surface, and bulk phases N ns;a = number of active site sets in surface phase ns N R = number of surface reactions P = pressure, Pa q = total heat flux, W∕m 2 q conv = convective heat flux, W∕m 2 q diff = diffusive heat flux, W∕m 2 R u = universal gas constant, J∕mol∕K r i;ns = reaction flux of reaction i on surface phase ns, mol∕m 2 ∕s S, S 0 = sticking coefficient T = translational-rotational temperature, K ν ki = net stoichiometric coefficient for species k in reaction i ν s = sum of the stoichiometric coefficients of all surface reactants ν k = thermal speed of gas-phase species k, m∕s ν 0 ki = reactant stoichiometric coefficient for species k in reaction i ν 0 0 ki = product stoichiometric coefficient for species k in reaction i _ w k = production rate of species k in all reactions, mol∕m 2 ∕s _ w ki = production rate of species k in reaction i, mol∕m 2 ∕s, Y = mass fraction Γ k = impingement flux of gas species k, m 2 ∕s γ = reaction efficiency θ ns;k = fraction of active sites occupied by species k on surface phase ns ρ = density, kg∕m 3 σ = Stefan-Boltzmann constant, W∕m 2 ∕K −4 Φ ns = active site density on surface phase ns, mol∕m 2 Φ ns;k = concentration of species k on surface phase ns, mol∕m 2 χ k = mole fraction of species k χ nb;k = mole fraction of bulk species k in bulk phase nb Subscripts b = bulk phase e = empty site g = gas phase na = number of active sites nb = number of bulk phases ns = number of surface phases s = surface phase tr = translational-rotational energy mode ve = vibrational-electronic energy mode...
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.
The aerothermal heating of a Thermal Protection System (TPS) is significantly affected by the physical and chemical interactions that occur between the planetary entry vehicle surface and the hypersonic atmospheric gas. To study these processes, a gas-surface interaction model is used in the present numerical analysis that accounts for surface catalytic reactions and surface participating reactions. The study examines the effects of gas-surface interactions for graphite exposed to high enthalpy reacting nitrogen flow. The processes analyzed are the catalytic recombination of nitrogen atoms to molecules at the surface and the carbon nitridation reaction where nitrogen atoms react with the surface carbon to form gaseous CN. The results obtained using a computational fluid dynamic (CFD) code are assessed using data from experimental tests conducted in a 30 kW Inductively Coupled Plasma (ICP) Torch Facility at the University of Vermont. The species concentration gradient in the boundary layer and heat flux transferred to the surface are strongly affected by the surface reactions. The rate of carbon mass removal due to carbon nitridation is also calculated and compared to the measured value.
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