Arnaud Borner

Detailed DSMC surface chemistry modeling of the oxidation of carbon-based ablators

This work employs a recently developed gas-surface interaction model (referred to herein as PSMM) constructed from molecular beam experimental data for use with the direct simulation Monte Carlo (DSMC) method. While recent models have been proposed to produce macroscopic rates consistent with the experimental measurements for use in CFD solvers, this work aims to reproduce the microscopic details (including angular distributions and time-of-flight distributions) obtained from the experimental data for modeling gas-surface interactions in DSMC. The different mechanisms considered for the PSMM model include adsorption, desorption, surface participating and direct impact mechanisms. The microscopic data of probabilities and characteristic frequencies for each type of reaction are obtained from the macroscopic parameters of reaction rate constants and sticking coefficients. Numerical simulations closely resembling a recent set of molecular beam experiments were performed using this model within DSMC, and the ...

DSMC Study of Carbon Fiber Oxidation in Ablative Thermal Protection Systems

Carbon fiber preforms, used as thermal protection materials for planetary entry systems, decompose via oxidation under hypersonic aerothermal conditions. The oxidation process is driven by two competing time-scales: the diffusion of reactants within the porous medium and the reaction speed at the surface of the fibers. A model to simulate this microscale process is proposed to be developed, tailored to the carbon preforms used to build lightweight carbon/phenolic ablators. The use of Direct Simulation Monte Carlo (DSMC) methods enables computation in the rarefied regime using advanced chemistry models for the gas/surface interactions at the fibers’ surface. To enable high fidelity simulations of the fibers decomposition, digitized computational grids are obtained from X-ray microtomography of real material. The recession due to oxidation is tracked using a marchingcube algorithm for surface discretization. Gas-phase and gas-surface chemistry near the surface are coupled and handled by the DSMC. Test cases of oxidation of a cylindrical carbon fiber embedded in a carbon matrix in ∗Postdoctoral Research Associate, Advanced Supercomputing Division, Mail Stop 258-5, NASA Ames Research Center, AIAA Member. †Postdoctoral Scholar, Department of Mechanical Engineering, 261 Ralph G. Anderson Building. Visiting Scientist, Thermal Protection Materials Branch, Mail Stop 234-1, NASA Ames Research Center, Moffett Field, CA, 94035, AIAA Senior Member. ‡Fundamental Modeling and Simulation Branch Chief, Advanced Supercomputing Division, Mail Stop 258-5, AIAA Associate Fellow.

Development of a Detailed Surface Chemistry Framework in DSMC

A generalized finite-rate surface chemistry framework incorporating a comprehensive list of reaction mechanisms is developed and implemented into the direct simulation Monte Carlo (DSMC) solver SPARTA. The various mechanisms include adsorption, desorption, Eley-Rideal (ER), and several types of Langmuir-Hinshelwood (LH) mechanisms. The approach is to stochastically model the various competing reactions occurring on a set of active sites. Both gas-surface (e.g., adsorption, ER) and pure-surface (e.g., desorption) reaction mechanisms are incorporated, and the framework also includes catalytic or surface altering mechanisms involving the participation of the bulk-phase species (e.g., bulk carbon atoms). Marschall and MacLean developed a general formulation in which multiple phases and surface sites are used and a similar convention is adopted in the current work. Expressions for the microscopic parameters of reaction probabilities (for gas-surface reactions) and frequencies (for pure-surface reactions) that are required for DSMC are derived from the surface properties and macroscopic parameters such as rate constants, sticking coefficients, etc. The energy and angular distributions of the products are specified according to the reaction type and input parameters. This framework also presents physically consistent procedures to accurately compute the reaction probabilities and frequencies in the case of multiple reactions. The result is a modeling tool with a wide variety of surface reactions characterized via user-specified reaction rate constants, surface properties and parameters.