Model Development and Experimental Validation of Reactive Gas and Pyrolysis Product Interactions with Hot Carbon Chars

Ablative thermal protection system (TPS) materials are required for the extreme heating encountered during hypersonic entry into the Martian and Outer Planet atmospheres as well as for manned and sample-return missions into the terrestrial atmosphere. This proposal addresses the Topic 1 – Advanced Thermal Protection Materials Modeling. We propose to provide high-quality experimental data to support the development of high-fidelity ablation models that include internal reactions and the chemical evolution of pyrolysis gases within hot carbon chars. Our primary objectives are to: (i) perform experiments with well-controlled test conditions and quantified uncertainties on all relevant test variables; (ii) conduct these experiments in a configuration that can be directly simulated by the modeling community using the porous media reactive-flow formulations under development for high-fidelity ablation codes, and (iii) use the data in a high fidelity coupled Fluid Dynamics and Material Response framework to extract parameters and validate models. We propose a 3-year research program to accomplish our objectives. We will investigate both internal oxidation reactions in partially dissociated air flows and the chemical evolution of representative pyrolysis gas mixtures by heterogeneous surface reactions with a hot char. Experiments will be performed in a furnace-heated, flow tube configuration, using a porous carbon-fiber tile material (FiberFormÆ) as a char stimulant. We will regulate in- put mass flow rates, measure absolute and differential pressures across the porous specimen, and monitor sample temperatures. We will use a microwave discharge and chemical titration techniques to produce and quantify partially dissociated air flows and calibrated gas mixtures representative of incipient phenolic pyrolysis products to study the chemical evolution of pyrolysis gases. Gas compositions will be monitored as a function of temperature, pressure, and flow rate after they exit the hot carbon char simulant using a combination of advanced mass spectroscopic techniques. Changes in char density, permeability, and microstructure due to oxidative attack or carbon deposition by coking reactions will be documented and correlated with gas composition measurements. The data obtained through the experiments will be used in a numerical modeling frame- work that is been designed to model detailed coupling problems that involved free flow problem and porous media. The framework couples a Computational Fluid Dynamics code as well as a Material response code through detailed mass, momentum and energy balance, which ensures that the detailed fixed and transport phenomenon at the interface are cap- tured. The non-reacting experimental results will be used to validate the coupling scheme, as well as calculate the macroscopic parameters such as permeability, needed in porous modeling zone. Using a volume-averaged technique for the gas-surface interactions inside the porous chars, a chemistry model will be put assemble. The data will be used to chose and calibrate the kinetic rates, for the specific problem. The outcome of this research effort will be benchmark data sets that can be used by modelers to construct and validate internal reaction and pyrolysis gas evolution models for high-fidelity ablation codes. The longer term payoff for NASA will be a more sophisticated suite of thermal response codes for ablative TPS sizing.