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Title: Computational study of material fire behaviour under transient irradiation
Author: Vermesi, Izabella
ISNI:       0000 0004 7223 4447
Awarding Body: Imperial College London
Current Institution: Imperial College London
Date of Award: 2018
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The majority of material flammability studies in fire science literature consider a constant external irradiation exposure, due to the ease of quantifying the boundary condition. This thesis uses a different approach, namely a transient external irradiation, either in the form of parabolic pulses or convex parabolas followed by constant irradiation periods. This allows for a more general study of pyrolysis and ignition. We use a combination of computational modelling and experimental analysis to look at materials with different responses and pyrolysis characteristics. The materials considered are a translucent polymer (Polymethylmethacrylate), a softwood species (spruce) and an engineered wood product (fiberboard). We evaluate the validity of solid-phase ignition criteria, which have been previously established using constant irradiation. We find that for PMMA the critical mass loss rate is the only ignition criteria capable of indicating ignition. However, for a more conservative approach, we propose using a dual threshold which offers the earliest time to ignition. This dual threshold states that before a polymer reaches a mass loss rate of 3 g/m2s and a surface temperature of 305 C, ignition will not occur. For wood, we find that no current criterion can indicate autoignition, as all thermal indicators are in the same ranges both in cases with or without ignition. The pyrolysis of wood and fiberboard is influenced by the moisture content. Accounting for moisture content, thus adding a drying step in the reaction scheme, is necessary for capturing initial mass loss rate. While for constant irradiation, we observe the drying to influence the surface temperature, for transient irradiation the effect of moisture content on surface temperature is negligible. It is, however, important for predicting in-depth temperatures. We also confirm previous findings that the non-uniform density profile of fiberboard is important when predicting mass loss rate, because of the higher external density and the lower internal density. Whereas for wood, in-depth absorption is negligible, for clear PMMA, it has a great effect on temperature predictions. Prior to ignition it is essential to account for in-depth absorption. However, after ignition, which coincided with the end of the external exposure in many scenarios, absorption occurs at the surface. This is due to the soot on the surface and to the different wavelength of the flame compared to the transient exposure. Therefore a combination of in-depth and surface absorption has to be applied to obtain good predictions. Considering these findings a better understanding of pyrolysis is obtained under a more general external heat flux exposure for solid materials of relevance to fire science.
Supervisor: Rein, Guillermo Sponsor: Not available
Qualification Name: Thesis (Ph.D.) Qualification Level: Doctoral