Modeling Air-SiO2 Surface Catalysis under Hypersonic Conditions with ReaxFF Molecular Dynamics
Start Date
1-3-2011 8:00 AM
End Date
3-3-2011 12:30 PM
Description
Accurate characterization of the aerothermal heating on vehicles traveling at hypersonic velocities is essential to their design and capabilities. One significant contribution to the heating can come from the heterogeneous recombination of species dissociated in the shock layer on the thermal protection system of the vehicle. The goal of this work is to develop a surface catalysis model for air on SiO2, which is a significant component in both ablative and reusable heat shields. Our initial efforts focus on identifying and describing the mechanisms for the recombination of atomic oxygen on quartz-SiO2. These mechanisms can be incorporated into Computational Fluid Dynamics simulations in the form of a Finite Rate Catalytic (FRC) Wall Boundary Condition to accurately describe the chemical reactions and heating on the surface of a vehicle. To accomplish these goals, we perform reactive molecular dynamics simulations using the ReaxFF potential, which naturally allows bond formation and breaking to occur during the course of a molecular dynamics simulation. Silica surfaces are populated with a gas of atomic oxygen at various temperatures and pressures using a flux boundary condition. Once populated, recombination coefficients and the rates of individual reactions can be measured by counting events. The measured recombination coefficients have an exponential trend with temperature, with an activation energy in reasonable agreement with experimental results. These simulations are used to identify several possible pathways for recombination, which include the Eley-Rideal and Langmuir Hinshelwood mechanisms. Individual reaction rates from the FRC model are then investigated using single-collision molecular dynamics simulations.
Modeling Air-SiO2 Surface Catalysis under Hypersonic Conditions with ReaxFF Molecular Dynamics
Accurate characterization of the aerothermal heating on vehicles traveling at hypersonic velocities is essential to their design and capabilities. One significant contribution to the heating can come from the heterogeneous recombination of species dissociated in the shock layer on the thermal protection system of the vehicle. The goal of this work is to develop a surface catalysis model for air on SiO2, which is a significant component in both ablative and reusable heat shields. Our initial efforts focus on identifying and describing the mechanisms for the recombination of atomic oxygen on quartz-SiO2. These mechanisms can be incorporated into Computational Fluid Dynamics simulations in the form of a Finite Rate Catalytic (FRC) Wall Boundary Condition to accurately describe the chemical reactions and heating on the surface of a vehicle. To accomplish these goals, we perform reactive molecular dynamics simulations using the ReaxFF potential, which naturally allows bond formation and breaking to occur during the course of a molecular dynamics simulation. Silica surfaces are populated with a gas of atomic oxygen at various temperatures and pressures using a flux boundary condition. Once populated, recombination coefficients and the rates of individual reactions can be measured by counting events. The measured recombination coefficients have an exponential trend with temperature, with an activation energy in reasonable agreement with experimental results. These simulations are used to identify several possible pathways for recombination, which include the Eley-Rideal and Langmuir Hinshelwood mechanisms. Individual reaction rates from the FRC model are then investigated using single-collision molecular dynamics simulations.