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Numerical modelling of pyrolysis in normal and reduced oxygen concentration

Ahmed Kacem has defended his​ thesis on 30th May 2016 in IUSTI, Marseille (France).

Document type > *Mémoire/HDR/Thesis

Keywords >

Research Unit > IRSN/PSN-RES/SA2I/LIE

Authors > KACEM Ahmed

Publication Date > 30/05/2016

Summary

The predictive capability of computational fluid dynamics (CFD) fire models depends on the accuracy with which the source term due to fuel pyrolysis can be determined. The pyrolysis rate is a key parameter controlling fire behavior, which in turn drives the heat feedback from the flame to the fuel surface. In the present study an in-depth pyrolysis model of a semi-transparent solid fuel (here, clear polymethyl methacrylate or PMMA) with spectrally-resolved radiation and a moving gas/solid interface was coupled with the CFD code ISIS of the IRSN which included turbulence, combustion and radiation for the gas phase. A combined genetic algorithm/pyrolysis model was used with Cone Calorimeter data from a pure pyrolysis experiment to estimate a unique set of kinetic parameters for PMMA pyrolysis. In order to validate the coupled model, ambient air flaming experiments were conducted on square slabs of PMMA with side lengths of 10, 20 and 40 cm.

From measurements at the center of the slab, it was found that i) for any sample size, the experimental regression rate becomes almost constant with time, and ii) although the radiative and total heat transfers increase significantly with the sample size, the radiative contribution to the total heat flux remains almost constant (~80%). Coupled model results show a fairly good agreement with the literature and with current measurements of the heat fluxes, gas temperature and regressing surface rate at the center of the slabs. Discrepancies between predicted and measured total pyrolysis rate are observed, which result from the underestimation of the flame heat flux feedback at the edges of the slab, as confirmed by the comparison between predicted and observed topography of burned samples. Predicted flame heights based on a threshold temperature criterion were found to be close to those deduced from the correlation of Heskestad. Finally, in order to predict the pyrolysis of PMMA under reduced ambient oxygen concentration, a two-step chemical reaction and a flammability diagram for flame extinction was used. Model results are compared with data obtained in the experimental facility CADUCEE of the IRSN using PMMA slabs with side length of 20cm for ambient oxygen concentrations of 18.2 and 19.5%. Data show that the total mass loss rate and, to a lesser extent flame temperature, decrease with the oxygen concentration, which is well reproduced by the model.
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