Browsing by Author "Martins, Diana Isabel Ribeiro"
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- Simulation of heat transfer of carbon fibres felts and microstructure effects on thermal conductivity of carbon/phenolic ablatorsPublication . Martins, Diana Isabel Ribeiro; Gamboa, Pedro Vieira; Magin, ThierryAtmospheric entry is one of the most critical phases of space exploration missions. Spacecraft are equipped with Thermal Protection Systems (TPS) to ensure the structure’s integrity from the encountered intense heat fluxes. In recent years, lightweight materials such as carbon/phenolic ablators have become the preferred option as TPS for atmospheric entry. These materials are made of a carbon fibre preform impregnated with a phenolic resin and combine high porosity, lightweight, low density and low effective thermal conductivity. Accurate modeling of the thermal response of TPS requires adequate characterization of thermophysical properties. Effective thermal conductivity of carbon/phenolic ablators is one of the most significant factors of heat transfer towards the interior of TPS. With increased computational capabilities, numerical simulations of materials at microscale resolution have become more affordable, minimizing the need for expensive experimental campaigns. This can be achieved with the Porous Microstructure Analysis software (PuMA) and its Python version pumapy, developed at NASA Ames Research Center. The main objective of this work is to analyse the effect of the microstructure and the different intrinsic properties on the effective thermal conductivity. The first step consisted in developing a numerical model for CALCARB CBCF 18-2000, the carbon preform of the material in the study, ZURAM, by artificially generating a transverse isotropic material with a normal distribution in the Through-Thickness (TT) and a uniform distribution in the In-Plane (IP) that was later verified and validated to assess the influence of microstructure. In addition, the charred and virgin ZURAM material was analyzed by generating a coaxial cylinder resembling a uniform coating on the fibre. To stuy the virgin carbon/phenolic, it must be considered that the material comprises fibres, gas and phenolic resin. A new synthetic material generator has been implemented in PuMA, which combines the previous code developed for CALCARB with a generation of uniformly distributed voxels in the domain that mimics the gas trapped inside the material. In the latter, the medium represents the phenolic resin. Both codes were later compared with available experimental data. Although there is a fair agreement between the behaviour of the simulated and measured conductivities, the disparity can reside in the fact that some of the properties are not known such as the fibres’ intrinsic conductivity at the experiments’ temperature. Even though further developments are required, this work provides important information about the effect of different components and geometrical features of the microscopic properties of porous ablative materials on their macroscopic model evaluation.