Experimental and Numerical Study of Carbon Fiber Oxidation

The oxidation at high Knudsen number of FiberForm® , the matrix material of NASA\u27s 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

Federated Learning Aggregation: New Robust Algorithms with Guarantees

Federated Learning has been recently proposed for distributed model training at the edge. The principle of this approach is to aggregate models learned on distributed clients to obtain a new more general "average" model (FedAvg). The resulting model is then redistributed to clients for further training. To date, the most popular federated learning algorithm uses coordinate-wise averaging of the model parameters for aggregation. In this paper, we carry out a complete general mathematical convergence analysis to evaluate aggregation strategies in a federated learning framework. From this, we derive novel aggregation algorithms which are able to modify their model architecture by differentiating client contributions according to the value of their losses. Moreover, we go beyond the assumptions introduced in theory, by evaluating the performance of these strategies and by comparing them with the one of FedAvg in classification tasks in both the IID and the Non-IID framework without additional hypothesis

Effects of structural changes of new organophosphorus cationic exchangers on a solvent extraction of cobalt, nickel and manganese from acidic chloride media

The effects of structural changes of organophosphorus cationic exchangers on metal-distribution curves (extraction efficiency vs. pH) have been investigated for the recovery of cobalt(II), nickel(II) and manganese(II) from acidic chloride solutions. By using alkyl groups with different branching and hydrophobicity and by including oxygen atoms in the hydrophobic chains of the cationic exchangers, it has been shown that the change of the chemical structure in the hydrophobic chains plays an important role in the extraction behaviours and causes more specifically a shift of the extraction curves to higher or lower pH depending on pKa values of extracting agents (evaluated by using QSPR calculations), distribution constants of metal–ligand complexes between organic and aqueous phases and the formation constant of the metal–ligand complexes. Based on the present results, interest in the new cationic exchangers synthesized in the present work for cobalt(II), nickel(II) and manganese(II) separation is also discussed

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 ow-tube reactor facility, at temperatures between 700 and 1300 K, under dry air gas at pressures between 103 and 104 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 x- ray analysis. The material was found to oxidize at specific sites forming a pitting pattern distributed over the fibers\u27 surface. Calcium- and oxygen-rich residues from the oxidation reactions were observed at several locations

Analysis of Fibrous Felts for Flexible Ablators Using Synchrotron Hard X-Ray Micro-Tomography

We analyzed the material properties of low-density felts that are used as substrates for new-generation flexible and conformal carbon/phenolic ablators, and compared them with those of a rigid carbon fiber preform that is used to manufacture rigid carbon/phenolic ablators. Micro-tomography measurements were obtained using synchrotron X-rays, allowing the characterization of the materials microstructure at the scale of the fibers. Using the tomography voxels as computational grids, we computed tortuosity and room temperature conductivity. In addition we performed micro-scale simulations of the oxidation of carbon fibers using a random walk model for oxygen diffusion and a probability law to model surface reactions

Code-to-Code Comparison, and Material Response Modeling of Stardust and MSL using PATO and FIAT

This report provides a code-to-code comparison between PATO, a recently developed high fidelity material response code, and FIAT, NASA's legacy code for ablation response modeling. The goal is to demonstrates that FIAT and PATO generate the same results when using the same models. Test cases of increasing complexity are used, from both arc-jet testing and flight experiment. When using the exact same physical models, material properties and boundary conditions, the two codes give results that are within 2% of errors. The minor discrepancy is attributed to the inclusion of the gas phase heat capacity (cp) in the energy equation in PATO, and not in FIAT

A New Approach To Light-Weight Ablators Analysis: From Micro-Tomography Measurements to Statistical Analysis and Modeling

The morphology characteristics and ablation behavior of a highly porous carbon fiber preform are studied using a combined experimental/numerical approach. Morphological characterization of the three-dimensional structure of the material is performed by hard X-rays synchrotron micro-tomography at the Advanced Light Source of Lawrence Berkeley National Laboratory. The resulting micro-tomography voxels are used to compute geometrical properties of the carbon preform, like porosity, specific surface area and tortuosity, that are otherwise indirectly measured through experimental techniques. The reconstructed volumes are used to build a computational grid for numerical simulations of the fibers\u27 ablation. By modeling the diffusion of oxygen through the porous medium using Lagrangian methods, and the oxidation at the carbon fibers\u27 surface using a reactivity model, the ablation of the carbon fibers are simulated for a range of Thiele numbers. It is shown that in the diffusion limited regime (large Thiele number), the ablation of the fibers occurs at the surface of the material. In the reaction limited regime (low Thiele number), the oxygen penetrates into the fibers, resulting in volumetric ablation and high material spallation

Preliminary Numerical and Experimental Analysis of the Spallation Phenomenon

The spallation phenomenon was studied through numerical analysis using a coupled Lagrangian particle tracking code and a hypersonic aerothermodynamics computational fluid dynamics solver. The results show that carbon emission from spalled particles results in a significant modification of the gas composition of the post shock layer. Preliminary results from a test-campaign at the NASA Langley HYMETS facility are presented. Using an automated image processing of high-speed images, two-dimensional velocity vectors of the spalled particles were calculated. In a 30 second test at 100 W/cm2 of cold-wall heat-flux, more than 1300 particles were detected, with an average velocity of 102 m/s, and most frequent observed velocity of 60 m/s