Savio Poovathingal

Large scale computational chemistry modeling of the oxidation of highly oriented pyrolytic graphite

Large scale molecular dynamics (MD) simulations are performed to study the oxidation of highly oriented pyrolytic graphite (HOPG) by hyperthermal atomic oxygen beam (5 eV). Simulations are performed using the ReaxFF classical reactive force field. We present here additional evidence that this method accurately reproduces ab initio derived energies relevant to HOPG oxidation. HOPG is modeled as multilayer graphene and etch-pit formation and evolution is directly simulated through a large number of sequential atomic oxygen collisions. The simulations predict that an oxygen coverage is first established that acts as a precursor to carbon-removal reactions, which ultimately etch wide but shallow pits, as observed in experiments. In quantitative agreement with experiment, the simulations predict the most abundant product species to be O2 (via recombination reactions), followed by CO2, with CO as the least abundant product species. Although recombination occurs all over the graphene sheet, the carbon-removal reactions occur only about the edges of the etch pit. Through isolated defect analysis on small graphene models as well as trajectory analysis performed directly on the predicted etch pit, the activation energies for the dominant reaction mechanisms leading to O2, CO2, and CO product species are determined to be 0.3, 0.52, and 0.67 eV, respectively. Overall, the qualitative and quantitative agreement between MD simulation and experiment is very promising. Thus, the MD simulation approach and C/H/O ReaxFF parametrization may be useful for simulating high-temperature gas interactions with graphitic materials where the microstructure is more complex than HOPG.

Molecular simulations of surface ablation using reaction probabilities from molecular beam experiments and realistic microstructure

Molecular simulations are performed of high temperature dissociated oxygen reacting with an idealized carbon-carbon composite material, where the microstructure is resolved. The Direct Simulation Monte Carlo (DSMC) method is used to simulate the convection and diffusion of reactants towards the microstructure and the transport of surface reaction products away from the microstructure. Simulations are performed with and without gas-phase chemical reactions in order to determine the relative importance of gas-surface reactions compared to gas-phase reactions next to the material surface. The simulations incorporate reaction probabilities for individual gas-surface collisions based on new reactive scattering data obtained in a molecular beam facility. The molecular beam experiments clearly indicate that a majority of surface reaction products were produced through thermal mechanisms. The experiments provide detailed data on the relative magnitude of O, O2, CO, and CO2 scattering from a representative material sample, made of vitreous carbon. For a gas-surface temperature of 800K, it is found from the simulations that despite CO being the dominant surface reaction product, a gas-phase exchange reaction forms significant CO2 within the microstructure region. The amount of CO2 production within the microstructure region is shown to be dependent on the local Knudsen number, based on the exposed microstructure height. Finally, preliminary simulations are performed for a real CarbonCarbon (C-C) surface. The surface topology is obtained through X-ray microtomography of an ablated C-C sample, which is triangulated and used directly within a DSMC simulation of the gas-surface interaction.

Molecular Simulation of Carbon Ablation Using Beam Experiments and Resolved Microstructure

Molecular simulations with a resolved surface microstructure were performed for high-temperature dissociated oxygen reacting with a carbon–carbon composite material. The direct simulation Monte Carlo method was used to simulate the convection and diffusion of reactants toward the microstructure and the transport of surface reaction products away from the microstructure. Simulations were performed with and without gas-phase chemical reactions to determine the relative importance of gas–surface reactions compared to gas-phase reactions next to the material surface. The simulations incorporated reaction probabilities for individual gas–surface collisions based on new reactive scattering data obtained from molecular beam experiments. The experiments provide detailed dynamical data on the scattering of O, O2, CO, and CO2 from a representative material sample, made of vitreous carbon. These experiments indicate that a majority of surface reaction products were produced through thermal mechanisms. For a gas–surf...

Finite-Rate Oxidation Model for Carbon Surfaces from Molecular Beam Experiments

An oxidation model for carbon surfaces has been developed where the gas–surface reaction mechanisms and corresponding rate parameters are based solely on observations from recent molecular beam exp...

Computational Chemistry Modelling of the Oxidation of Highly Oriented Pyrolitic Graphite (HOPG)

At these high temperatures, dissociated oxygen atoms (O) strike the TPS surface leading to several possible gas-surface chemical reactions. Specifically, the oxygen atom could chemically bond to the surface, it could recombine with another adsorbed oxygen and leave the surface as a molecule (O2), or the impinging atom could ’oxidize’ the carbon surface resulting in products such as CO and CO2 leaving the surface and being injected into the boundary layer. Such oxidation reactions result in the recession of the surface (surface ablation). Currently, much uncertainty exists in both the dominant reactions themselves as well as the rates of these reactions. The mechanisms and rates are required as input into state-of-the-art CFD simulations of hypersonic flows. Experimental determination of the chemical mechanisms themselves under extreme hypersonic conditions is difficult and often they must be inferred from macroscopic observations such as heat flux and surface recession measurements. However, as the field of computational chemistry continues to advance in step with advances in computational power, fundamental chemistry studies may be able to provide insight into the chemical mechanisms and associated rates for such surface ablation processes. www.boeing.com

Effect of Microstructure on Carbon-based Surface Ablators using DSMC

The effect of protruding microstructures/granules from non-porous carbon ablators in a high-speed flowfield is studied to characterize the concentration of oxidation products in the boundary layer and to understand the effect of microstructure on macroscopic surface reaction rates. Numerical simulations are performed using Direct Simulation Monte Carlo (DSMC), a stochastic particle-based technique able to accurately model the flow at small length scales including gas-surface interactions occurring at the micro-scale. Two different studies are performed; convection-diffusion within a boundary layer and also an isolated diffusion study. For both cases, gas-phase reactions are neglected, however, probabilities of surface reaction are applied to each particle-surface collision, and are referred to as microscopic probabilities. Both studies reveal that for high microscopic probabilities, the concentration of oxidation products in the boundary layer is not sensitive to changes in microstructure or reaction probabilities, and thus, the surface reactions are limited only by the diffusion of reactants towards the surface from the boundary layer. For low microscopic probabilities, the surface reactions clearly become reaction limited where changes in the microstructure surface area and reaction probabilities significantly affect the concentration of oxidation products in the boundary layer. It is further observed that an increase in total surface area due to the microstructure does not correspond to an equivalent increase in the net rate of surface reactions, due to the interplay between diffusion and surface reaction within the microstructure.

Nonequilibrium flow through porous thermal protection materials, Part I: Numerical methods

Abstract Numerical methods are developed to simulate high temperature gas flow and coupled surface reactions, relevant to porous thermal protection systems used by hypersonic vehicles. Due to the non-continuum nature of these flows, the direct simulation Monte Carlo (DSMC) method is used, and the computational complexity of the simulations presents a number of unique challenges. Strategies for parallel partitioning, interprocessor communication, complex microstructure geometry representation, cutcell procedures, and parallel file input/output are presented and tested. Algorithms and data structures are developed for a microstructure generation tool called FiberGen that enables realistic microstructures to be constructed based on targeted fiber radius, orientation, and overall porosity, with user defined variations about these values. The data structures and algorithms associated with FiberGen are robust and efficient enough to enable DSMC simulations where the microstructure geometry changes to directly simulate ablation problems. Subsonic boundary conditions are described and validated, and a number of example solutions are presented. The example problems demonstrate the difference between surface ablation and in-depth volume ablation regimes for porous TPS materials.

Nonequilibrium flow through porous thermal protection materials, Part II: Oxidation and pyrolysis

Abstract Micro scale simulations are performed of flow through porous (pyrolyzing) thermal protection system (TPS) materials using the direct simulation Monte Carlo (DSMC) method. DSMC results for permeability are validated with computational fluid dynamics (CFD) calculations and theory, for simple porous geometries under continuum flow conditions. An artificial fiber-microstructure generation code FiberGen is used to create triangulated surface geometry representative of FiberForm® (FiberForm) material. DSMC results for permeability of FiberForm are validated for a range of pressures (transitional flow conditions) and agree with experimental measurements. Numerical uncertainty is determined to be within 2% if sufficiently large portions of the microstructure are included in the computation. However, small variations in fiber size and angle bias can combine to give + 30 % uncertainty when comparing with experimental permeability data. X-ray microtomography scans of FiberForm are used to create microstructure geometry for incorporation within DSMC simulations of coupled oxygen diffusion and gas-surface chemistry in the presence of a blowing pyrolysis gas. In-depth penetration of atomic oxygen is limited to 0.2 – 0.4 mm for the range of Knudsen number and pyrolysis gas conditions studied.