James B. Scoggins

Development of a Reduced Kinetic Mechanism for PICA Pyrolysis Products

A Nonequilibrium Ablation and Pyrolysis model is presented along with the development of a detailed kinetic mechanism for the decomposition of pyrolysis products for carbon-phenolic thermal protection systems. The detailed mechanism consists of 49 species and 136 reactions. A simple procedure for kinetic mechanism reduction is outlined and then used to form a reduced model that is comprised of 19 species and 23 reactions. Results are obtained using the 19 species model for a 5 cm slab of PICA-like material heated to 1644 K. A more comprehensive and more fundamentally base mechanism reduction procedure is then employed to arrive at a reduced model which applies over the entire parameter space of the pyrolysis gas. The resulting reduced model consists of 19 species and 18 reactions. The benets and problems associated with each mechanism reduction approach are also discussed.

Coupled Flow, Radiation, and Ablation Simulations of Atmospheric Entry Vehicles using the Hybrid Statistical Narrow Band Model

A model for coupled flow, radiation, and ablation calculations along the stagnation lines of atmospheric entry vehicles is developed to study the effects these coupled phenomena have on each other as well as the predicted quantities of interest to vehicle designers. The flow model is based on the two-temperature, multicomponent, reacting Navier-Stokes equations coupled to radiative heat and photochemistry source terms and reduced to onedimension using the dimensionally reduced Navier-Stokes approximation. The radiative source terms are computed using the hybrid statistical narrow band model developed at the EM2C laboratory at Ecole Centrale Paris. This model has previously been shown to accurately produce radiative properties at significantly reduced computational cost when compared to line-by-line calculations for uncoupled flows without ablation. In this work, the hybrid statistical narrow band model is coupled to the one-dimensional stagnation line flow solver with ablation. Ablation is treated through a simple steady-state ablation boundary condition with finite-rate heterogenous reactions capable of simulating ablation products blowing into the boundary layer. The model is first used to simulate the effect of carbonaceous species in a typical boundary layer environment. In particular, it is shown that a boundary layer contaminated with a relatively small amount of ablation and pyrolysis products can have significantly lower transmissivities than a boundary layer with pure air. Finally, the peak heating point for the Apollo 4 command module is analyzed using each of the four possible coupling strategies: flow, flow and ablation, flow and radiation, and fully coupled. It is shown that for this case, radiation coupling is the dominant phenomena due to a relatively small carbon yield in the boundary layer. In addition, comparison of cumulative intensities with the radiometer measurement made during the flight shows excellent agreement and confirms the assumption that the radiometer cavity has little effect on the measured intensity due to the low amount of carbon present in the cavity.

Multi-Group Reductions of LTE Air Plasma Radiative Transfer in Cylindrical Geometries

Air plasma radiation in Local Thermodynamic Equilibrium (LTE) within cylindrical geometries is studied with an application towards modeling the radiative transfer inside arc-constrictors, a central component of constricted-arc arc jets. A detailed database of spectral absorption coefficients for LTE air is formulated using the NEQAIR code developed at NASA Ames Research Center. The database stores calculated absorption coefficients for 1,051,755 wavelengths between 0.04 m and 200 m over a wide temperature (500K to 15 000K) and pressure (0.1 atm to 10.0 atm) range. The multi-group method for spectral reduction is studied by generating a range of reductions including pure binning and banding reductions from the detailed absorption coefficient database. The accuracy of each reduction is compared to line-by-line calculations for cylindrical temperature profiles resembling typical profiles found in arc-constrictors. It is found that a reduction of only 1000 groups is sufficient to accurately model the LTE air radiation over a large temperature and pressure range. In addition to the reduction comparison, the cylindrical-slab formulation is compared with the finite-volume method for the numerical integration of the radiative flux inside cylinders with varying length. It is determined that cylindrical-slabs can be used to accurately model most arc-constrictors due to their high length to radius ratios.

Crossed contributions to electron and heavy-particle transport fluxes for magnetized plasmas in the continuum regime

We propose a unified fluid model for multicomponent plasmas in thermal nonequilibrium accounting for the influence of the electromagnetic field. In a previous work, this model was derived from kinetic theory based on a generalized Chapman-Enskog perturbative solution of the Boltzmann equation, scaled using the ratio of electron to heavy-particle masses. Anisotropic transport properties were derived in terms of bracket integrals. In this work, explicit expressions for asymptotic solutions of the transport properties are derived using a spectral Galerkin projection supplied with Laguerre-Sonine polynomial basis functions, and we analyze the crossed contributions to electron and heavy particle mass and energy fluxes, known as the Kolesnikov effect.