Comparison Between Parallel and Competitive Reaction Schemes for the Pyrolysis of Cork/Phenolic Ablators

The evolution of the contemporary space economy is creating new challenges in the development of Thermal Protection Systems (TPS). The growing number of spacecraft in orbit, the renewed interest in human spaceflight, and advances in reusable components have increased the research focus on accurate material characterization for a dual function. TPS materials must ensure protection during atmospheric entry and complete demise during disposal. In both cases, reliable prediction of thermal degradation is essential to optimize design margins and ensure system safety. Cork is a promising material for TPS applications. Its cellular structure provides high porosity, low density, and excellent thermal insulation. These properties make cork an excellent candidate to replace widely used carbon/phenolic composites. However, TPS materials degrade through pyrolysis, a complex thermochemical process whose modelling is only partially understood. The challenge of developing a robust thermal degradation model for this phenomenon is increased by the limited literature available on cork. This is in contrast to the extensive studies conducted over the past decades on other types of biomass and carbon/phenolic composites. Previous studies have investigated pyrolysis modelling of carbon/phenolic composites at the von Karman Institute. In particular, the work of Torres-Herrador served as the primary reference for this work. Building on the developed methodology for extracting kinetic parameters, this thesis applies a similar approach to cork/phenolic composites, focusing on model fitting methods rather than isoconversional techniques. The objective was to propose multi-component and competitive reaction schemes to model the pyrolysis of cork/phenolic ablators. An accurate pyrolysis model is required to improve numerical simulations and reduce design margins. This work was divided into two phases. First, an experimental campaign based on Thermogravimetric Analysis (TGA) was conducted on two different carbon/phenolic ablators at three heating rates (5, 20, and 40 K/min). Second, parallel and competitive reaction models, based on Arrhenius-like kinetics, were proposed to describe the thermal degradation of each material. Kinetic parameters were identified using deterministic optimization to achieve the best fit with experimental data. The optimized parameters are relevant as input for material response codes such as the Porous Material Analysis Toolbox (PATO) developed by NASA Ames. Resulting simulations would greatly benefit accurate pyrolysis modelling to improve the fidelity of results. The experimental results highlighted differences in degradation behaviour between the two materials, despite the similarities in their constituents. It is known that parallel reaction schemes are not capable of adapting to heating rates relevant to TPS applications when optimised in the limited range of the presented data. This is caused by their inability to correctly represent the behaviour of several reactions, occurring simultaneously and in competition for the same reactant. Nonetheless, these models provided satisfactory agreement with TGA data in the analysed range. Developing a competitive reaction scheme for cork/phenolic composites proved to be more difficult, due to the complexity of the material and the lack of supporting literature. The results obtained are promising, but not yet sufficient to provide a fully reliable description of the underlying processes.