Applications of the DSMC Simulator SPARTA for Ablation Modeling

Speaker: Arnaud Borner, Postdoctoral Research Associate, University of Illinois Urbana-Champaign/NASA Ames Research Center
September 27, 2016

Presentation

Abstract

The direct simulation Monte Carlo (DSMC) simulator SPARTA is used to compute permeability of several TPS materials such as virgin FiberForm, oxidized FiberForm, Morgan carbon felt and Calcarb to high temperature gases. The actual porous geometry of the materials is digitized using X-ray micro-tomography and the code is modified to read in micro-tomography data as input geometries. Numerical results at various pressure and Knudsen number are compared with experimental data. The method confirms that the pressure dependence of gas permeabilities is well represented by the Klinkenberg formulation. The method is validated by showing close agreement between measurements of permeability from simulations and experimental investigations. It is found that the permeability is not a direct function of material porosity but strongly depends on the pore size distribution.

Additionally, a detailed molecular surface chemistry model which incorporates various standard surface reaction mechanisms is implemented in SPARTA. The different mechanisms considered in this model include adsorption, desorption, Langmuir-Hinshelwood and Eley-Rideal 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 by the Minton group at Montana State University are performed using SPARTA. The molecular beam experiments involve the bombardment of a relatively smooth vitreous carbon surface using a hyperthermal O/O2 beam to understand the product formation and the detailed mechanism of reactions and scattering at the surface. Commonly used values for the gas-surface interaction parameters are employed in the simulations along with a recently proposed finite-rate air-carbon surface reaction model. A comparison between the numerical flux distributions from the most comprehensive heritage models and experimental flux distributions of the scattered products show significant discrepancies in the temperature variation of the flux. The comprehensive results from this set of experiments are used to analyze and improve the existing surface chemistry models including both the finite-rate constants of the reactions and details of the gas-surface interaction/scattering.

Biography

Arnaud Borner is a postdoctoral research associate at the University of Illinois Urbana-Champaign under the supervision of Prof. Kelly Stephani, and a visiting scientist in the Computational Physics Branch under Nagi Mansour since 2014. Before NASA, Borner earned his M.S. and Ph.D. at Penn State under the supervision of Prof. Debbie Levin, and prior to that a M.S. equivalent at the Ecole Centrale de Lyon (France). His research interests are advanced kinetic particle methods such as the direct simulation Monte Carlo (DSMC) and Molecular Dynamics (MD), applied to ablation, space re-entry, electrospray propulsion and supersonic jets.