A general thermal thickness model and a heating rate related ignition criterion for combustible solids

The objective of this research is to investigate the ignition behavior of combustible solid materials in fire. Three key tasks were conducted for it: (1) a new theoretical temperature profile prediction model, referred as General Thermal Thickness (GTT) model, was proposed and then validated by the experimental data from E-glass/polyester composite panels at different heat fluxes; (2) a new ignition criterion (the Heating Rate-related Ignition Temperature (HRIT) criterion) was proposed and its accuracy under various heat fluxes was shown by comparing with experimental data of different materials; (3) uncertainty and sensitivity analyses were conducted on a new integrated ignition prediction model (i.e. the GTT combined with the HRIT ignition criterion), to investigate the variations in ignition and identify key affecting factors.;A simplified heat transfer model was constructed and solved in order to theoretically predict the temperature profile of the GTT combustible solid materials subjected to one-sided heating. The theoretical solution of the GTT model was validated by experimental data from intermediate-scale calorimeter fire tests of E-glass fiber reinforced polyester composite panels at three heat flux levels. The GTT model was also verified by results from finite element modeling predictions. Compared with the classical theoretical models (such as thermally thick (TTK) and thermally thin (TTN) models), the GTT model is more accurate and it is valid through the whole range of thermal thickness.;Since existing ignition criteria cannot handle the variations of external heat flux and surrounding environmental conditions, a new ignition criterion referred as Heating Rate-related Ignition Temperature (HRIT) criterion, was proposed, developed and validated. In the new HRIT ignition criterion both the surface temperature and its increasing rate are used to determine ignition. The accuracy of the HRIT ignition criterion under different external heat fluxes was validated by the piloted ignition data of a thermoplastic material (Black PMMA), a thermoset composite material (E-glass fiber reinforced polyester composite) and a cellulosic material (Red Oak) subjected to different external heat fluxes. The adaptability of the HRIT criterion in different surrounding environmental conditions was also discussed.;Both local and global sensitivity and uncertainty analyses were performed to understand the variations and identify important factors affecting the ignition process. First, the local sensitivity analysis was applied to the GTT model and the HRIT criterion separately. Then a Monte Carlo analysis using the Latin Hypercube Sampling method was performed on the integrated ignition prediction model (the GTT model combined with HRIT ignition criterion), yielding the global sensitivity coefficients (or important index) and uncertainty ranges of the ignition.;In summary, unlike classical temperature profile prediction models such as thermally thick or thermally thin models whose applications are restricted by thermal thickness, the new GTT model proposed in this research is valid within the whole range of thermal thickness. The challenge of the existing ignition criteria handling varying heat fluxes and surrounding environment was resolved by the new HRIT ignition criterion, and the accuracy of the HRIT criterion was validated by experimental data from three materials. The important affecting factors and uncertainties during ignition process were identified in this research through the uncertainty and sensitivity study, which can serve as a guideline for fire safety design.