Themes: Engineering sciences: fluid mechanics, energetics
Thesis location: Explosion and Fire Laboratory (LIE) - Cadarache (13)
Start: October 2021
Master's degree in energetics and transfers or materials
Knowledge in radiative transfers and modeling of porous media.
Age limit: 26 years old unless otherwise stated.
Several polymers encountered in nuclear facilities (PVC or compositions of ethylene-vinylacetate with mineral load for cables, polycarbonate of internal panels of Glove Boxes used in waste processing laboratories) form a porous residual when they are degraded under a thermal stress (pyrolysis phenomenon). Yet, predicting these materials pyrolysis requires a correct knowledge of their thermokinetic properties (Arrhenius laws, heats of pyrolysis), but also of their effective thermal (conductivity, radiative properties) and mass (permeability, diffusivity) properties, from the virgin state to their ultimate degradation state. In the case of strongly intumescent materials such as PVC and polycarbonate, the resulting residuals contain pores that can be observed with the naked eye, and the radiative transfer in the porous medium can be of the same order as the conductive transfer, in particular at high temperature. In this context, the present thesis aims at developing a methodology to evaluate the effective radiative properties of these materials at their different states of degradation, similarly with recent works on the effective thermal conductivity of the EVA-ATH cables. In this purpose, the work will be organized in the following manner. Firstly, the morphological properties of the studied materials will be characterized at different states of their degradation. Such an identification will be qualitative (bubbles, filaments, inclusions, etc.) and then quantitative (size distribution, etc.) using morphological characterization methods (tomography, SEM imaging). On this basis, a synthetic porous medium model will be tentatively developed, with morphologic properties similar with those of the real medium, but with continuously evolving properties all along the material degradation process. In a second phase, accounting for the optical properties of the constituents, (virgin material, char residuals), a choice will be made on the radiative modelling of the condensed phase (opaque or semi-transparent) in order to perform the direct numerical simulation of the radiative transfer in the material. In this purpose, a P’ Institute Monte-Carlo software will be developed and used. The results obtained on the actual material geometry and on the synthetic medium will be compared to verify the relevance of the model medium and then characterize an evolution law for the material radiative properties that can be used all along its degradation process. Finally, this evolution law will be introduced in the pyrolysis modulus of the CALIF3S-ISIS fire simulation software developed at IRSN. Comparisons between numerical simulations of the pyrolysis of these materials and reference experimental tests (cone calorimeter) will be therefore performed.