Atomistic to Macroscopic Modeling and Validation for Gas-Surface Interactions
Start Date
2-3-2011 11:55 AM
Description
The high-temperature shock layer generated by a hypersonic vehicle results in the dissociation of free-stream diatomic and polyatomic molecules. These reactive atomic species may diffuse through the boundary layer and recombine back into molecules on the vehicle surface (catalysis) or actually react with and remove material from the surface (ablation). Efforts are underway to develop a finite-rate gas-surface-interaction model, incorporated as a CFD boundary condition, in order to reduce the uncertainty in predicted heat flux associated with these phenomena.
An important aspect of the finite-rate model is that it consists of a set of one-step (elementary) gas-surface chemical reaction mechanisms with parameterized rates. Unlike prior models parameterized by overall recombination coefficients or overall oxidation rates, for example, the parameters required by the new model have a quantitative link to individual surface chemistry mechanisms. Although such parameters may be partially informed by existing experimental data and chemistry theory, the uncertainty is quite large. A suggested approach is to first apply uncertainty analysis to a given finite-rate model to determine the dominant mechanisms (and associated parameters) that contribute to the predicted heat flux. Followed by targeted computational chemistry simulations and experimental measurements to reduce the uncertainty in specific parameters of interest.
Techniques for simulating molecular beam experiments to determine parameters such as sticking coefficients, reaction efficiencies, scattering angles, and energy accommodation coefficients, will be presented. Techniques for predicting surface coverage at various (ri, p, T) conditions will be presented and compared to low-energy electron diffraction (LEED) and temperature-programmed desorption (TPD) experiments, as well as scanning tunneling microscopy (STM) images. Finally, techniques for simulating activation energies for E-R/L-H recombination and oxidation reactions will be presented.
Current results are for well-characterized model systems (Pt(111), silica-quartz). Extension to simple carbon surface ablators under flight-relevant conditions will be discussed along with associated challenges.
Atomistic to Macroscopic Modeling and Validation for Gas-Surface Interactions
The high-temperature shock layer generated by a hypersonic vehicle results in the dissociation of free-stream diatomic and polyatomic molecules. These reactive atomic species may diffuse through the boundary layer and recombine back into molecules on the vehicle surface (catalysis) or actually react with and remove material from the surface (ablation). Efforts are underway to develop a finite-rate gas-surface-interaction model, incorporated as a CFD boundary condition, in order to reduce the uncertainty in predicted heat flux associated with these phenomena.
An important aspect of the finite-rate model is that it consists of a set of one-step (elementary) gas-surface chemical reaction mechanisms with parameterized rates. Unlike prior models parameterized by overall recombination coefficients or overall oxidation rates, for example, the parameters required by the new model have a quantitative link to individual surface chemistry mechanisms. Although such parameters may be partially informed by existing experimental data and chemistry theory, the uncertainty is quite large. A suggested approach is to first apply uncertainty analysis to a given finite-rate model to determine the dominant mechanisms (and associated parameters) that contribute to the predicted heat flux. Followed by targeted computational chemistry simulations and experimental measurements to reduce the uncertainty in specific parameters of interest.
Techniques for simulating molecular beam experiments to determine parameters such as sticking coefficients, reaction efficiencies, scattering angles, and energy accommodation coefficients, will be presented. Techniques for predicting surface coverage at various (ri, p, T) conditions will be presented and compared to low-energy electron diffraction (LEED) and temperature-programmed desorption (TPD) experiments, as well as scanning tunneling microscopy (STM) images. Finally, techniques for simulating activation energies for E-R/L-H recombination and oxidation reactions will be presented.
Current results are for well-characterized model systems (Pt(111), silica-quartz). Extension to simple carbon surface ablators under flight-relevant conditions will be discussed along with associated challenges.