Jean Lachaud

Multiscale Approach to Ablation Modeling of Phenolic Impregnated Carbon Ablators

A multiscale approach is used to model and analyze the ablation of porous materials. Models are developed for the oxidation of a carbon preform and of the char layer of two Phenolic Impregnated Carbon Ablators (PICA) with the same chemical composition, but with different structures. Oxygen diffusion through the pores of the materials and in depth oxidation and mass loss are first modeled at microscopic scale. The microscopic model is then averaged in set of partial differential equations describing the macroscopic behavior of the material. Microscopic and macroscopic approaches are applied with progressive degrees of complexity to gain a comprehensive understanding of the ablation process. Porous medium ablation is found to occur in a zone of the char layer, called ablation zone, whose thickness is a decreasing function of the Thiele number. The studied PICA materials are shown to display different ablation behaviors, a fact that is not captured by current models that are based on chemical composition only. Applied to Stardust’s PICA, the models explain and reproduce the unexpected drop in density measured in the char layer during Stardust post-flight analyses [Stackpoole, 2008].

Validation of a volume-averaged fiber-scale model for the oxidation of a carbon-fiber preform

The oxidation of FiberForm, an industrial carbon-fiber preform, has been studied in an oxidation reactor. The microscopic oxidation behavior of the fibers has been analyzed by scanning electron microscopy. The carbon fibers ablate showing progressive reduction of their diameter. The overall material recession occurs when the fibers are consumed. A reaction/diusion -convection competition is shown to drive the oxidation process and control the depth of oxidation. A fiber-scale model is proposed for the prediction of carbonfiber preform oxidation. A macroscopic model is derived by volume-averaging the microscopic model and a porous-medium formulation is used to model mass transport in the preform. The proposed model has been implemented in a Carbon Oxidation Analysis Code based on OpenFOAM (COACO). Using inverse analysis, it was possible to estimate the intrinsic fiber reactivity and then validate the model. The reactivity obtained is surprisingly high compared to literature data. This is explained by the fact that the carbon fibers contain traces of calcium and potassium, which are known to be catalysts for oxidation. They progressively accumulate at the surface in the form of combustion residues.

Microscopic scale simulation of the ablation of fibrous materials

This slide presentation reviews the ablation by oxidation of carbon-fiber preforms impregnated in carbonized phenolic-formaldehyde matrix is modeled at microscopic scale. Direct numerical simulations show that the matrix ablates in volume leaving the carbon fibers exposed. This is due to the fact that the reactivity of carbonized phenolics is higher than the reactivity of carbon fibers. After the matrix is depleted, the fibers ablate showing progressive reduction of their diameter. The overall material recession occurs when the fibers are consumed. Two materials with the same carbon-fiber preform, density and chemical composition, but with different matrix distributions are studied. These studies show that at moderate temperatures (< 1000K) the microstructure of the material influences its recession rate; a fact that is not captured by current models that are based on chemical composition only. Surprisingly, the response of these impregnated-fiber materials is weakly dependent on the microstructure at very high temperatures (e.g., Stardust peak heating conditions: 3360K).

High-Fidelity Charring Ablator Thermal Response Model

Low-density carbon/phenolic is a class of ablative materials that is attractive for space exploration missions that use blunt bodies where weight and performance of the material are of primary importance, but shape preservation is not critical. We consider a relatively simple class, PICA, that consists of carbon fibers impregnated with phenolic as the matrix. A new formulation for models of the response of this class of materials to high-enthalpy environments is summarized. The new formulation consists of conservation equations for species, mass, and energy in porous media. The velocity is obtained using Darcy’s law with the pressure obtained so that mass is conserved. Pyrolysis of the matrix is modeled using a discrete number of progress variables representing the decomposition reaction stages. Each decomposition reaction produces its own set of species. The one-dimensional equations are solved by discretizing in space using a second-order staggered mesh on a moving grid, and an implicit dual time step scheme is used to advance the solution in time. The Charring Ablator Thermal response (CAT) code that implements the formulation is tightly coupled to a chemistry code that enables handling equilibrium as well as finite rate chemistry. It was thoroughly verified against analytical solutions and comparisons of results to results using different numerical methods and techniques. Sample verification cases are summarized showing excellent accuracy. Sample material response cases are presented showing the capability of the code. We have established that models of this type of ablative materials are highly sensitive to the energy and chemistry balance at the gas surface interface and that these balances are significantly related to material performance. Unfortunately, highquality data for the decomposition of phenolic is missing and is needed to enable validation of the formulation.

Measurement of pyrolysis products from phenolic polymer thermal decomposition

Batch pyrolysis of phenolic polymer was performed using a step-wise heating procedure in a 50 K increment from room temperature up to 1250 K. A phenolic-polymer sample of 50 mg was loaded in a reactor assembly speci cally designed and built for this study. The mass loss was measured after each 50 K step and the production of gas-phase species was quanti ed using gas-chromatography techniques. The overall mass loss reached about 35%. Water was found to be the dominant product below 800 K. Yields of permanent gases such as hydrogen, methane, carbon monoxide, and carbon dioxide increased with temperature up to 900 K and then decreased at higher temperatures. The yields of light hydrocarbons, such as C2 to C4 hydrocarbons, increased with reaction temperature up to 1000 K and dropped subsequently. Yields of aromatic products, including benzene, toluene, and xylene, were signi cant between 700 and 850 K. The quantitative molar production of species versus temperature is made available for the development of detailed phenolicpolymer pyrolysis models.

Flow-Tube Oxidation Experiments on the Carbon Preform of PICA

Oxidation experiments on the carbon preform of a phenolic-impregnated carbon ablator were performed in the NASA Ames flow-tube reactor facility, at temperatures between 700 and 1300 K, under dry air gas at pressures between 10 and 10 Pa. Mass loss, volumetric recession and density changes were measured at different test conditions. An analysis of the diffusion/reaction competition within the porous material, based on the Thiele number, allowed us to identify low temperature and low pressure conditions to be dominated by in-depth volume oxidation. Experiments above 1000 K were found at transition conditions, where diffusion and reaction occur at similar scales. The microscopic oxidation behavior of the fibers was characterized by scanning electron microscopy and energy dispersive xray analysis. The material was found to oxidize at specific sites forming a pitting pattern distributed over the fibers’ surface. Calciumand oxygen-rich residues from the oxidation reactions were observed at several locations.

Effects of Water Phase Change on the Material Response of Low-Density Carbon-Phenolic Ablators

1 Research Assistant, Department of Mechanical Engineering; ali.omidy2@uky.edu 2 Post Doctoral Research Associate, Department of Mechanical Engineering; francesco.panerai@uky.edu 3 Scientist, Silicon Valley Initiative. Senior Member AIAA; jlachaud@ucsc.edu 4 Chief Scientist for Modeling and Simulation, TN Division. Associate Fellow AIAA; nagi.n.mansour@nasa.gov 5 Assistant Professor, Department of Mechanical Engineering. Associate Fellow AIAA; alexandre.martin@uky.edu

Experimental and Numerical Study of Carbon Fiber Oxidation

The oxidation at high Knudsen number of FiberForm R ©, the matrix material of NASA’s Phenolic Impregnated Carbon Ablator, is investigated both experimentally and numerically. The experimental setup consists of a quartz tube through a clamshell heater. Mass loss and recession of carbon preform samples are measured at temperatures between 700 and 1300 K and pressures around 2000 Pa. A volume average fiber-scale oxidation model is used to model the setup and extract the effective reactivity of the material. New values for carbon fiber reactivity are suggested and discussed.

Quantitative determination of species production from the pyrolysis of the Phenolic Impregnated Carbon Ablator (PICA)

Experiments to quantitatively determine detailed species production from the pyrolysis of Phenolic Impregnated Carbon Ablator (PICA) were performed using a reactor assembly adapted from a previous study on phenol-formaldehyde resin decomposition. A step-wise heating procedure that used a 50 K increment from room temperature up to 1250 K was employed for the experiments. The mass loss was measured after each 50 K step for PICA samples with an initial mass of 100 mg. Species production from the pyrolysis process was quantified using state-of-the-art gas-chromatography techniques. Compared to the more traditional mass spectroscopy techniques, gas chromatography allows to measure all species, from hydrogen to large aromatics. The quantitative molar production of species versus temperature is reported in this work. The species product from PICA pyrolysis are quite different from the species obtained in a previous study on a resole type phenolic resin pyrolysis. This suggests that characterizations need to be carried out for all variations of phenolic-matrix based ablators.

Identification of microscale ablative properties of C/C composites using inverse simulation

The objective of this work is to obtain, by an inverse approach, the reactivity of microscale components of ablative C/C composites. A model has been built for micro-scale ablation and a has been numerically implemented using a VOF method. The application of the approach is illustrated by the identification of reactivities from recession data and SEM observation of the ablated composite surface.