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Hernandez, N., Fuentes, A., Reszka, P., & Fernandez-Pello, A. C. (2019). Piloted ignition delay times on optically thin PMMA cylinders. Proc. Combust. Inst., 37(3), 3993–4000.
Abstract: The theory to predict ignition of solid fuels exposed to incident radiant heat fluxes has permitted to obtain simple correlations of the ignition delay time with the incident heat flux which are useful in practical engineering applications. However, the theory was developed under the assumption that radiation does not penetrate into the solid phase. In the case of semi-transparent solids, where the penetration of radiation plays an important role in the heating and subsequent ignition of the fuel, the predictions of the classical ignition theory are not applicable. A new theory for the piloted ignition of optically thin cylindrical fuels has been developed. The theory uses an integral method and an approximation of the radiative transfer equation within the solid to predict the heating of an inert solid. An exact and an approximate analytical solution are obtained. The predictions are compared with piloted ignition experiments of clear PMMA cylinders. The results indicate that for opticallythin media, the heating and ignition are not sensible to the thermal conductivity of the solid, they are highly dependent on the in-depth absorption coefficient. Using the approximate solution, the correlation 1/t(ig) proportional to (q)over dot(inc)'' was established. This correlation is adequate for engineering applications, and allows the estimation of effective properties of the solid fuel. The form of the correlation that was obtained is due to the integral method used in the solution of the heat equation, and does not imply that the semi-transparent solid behaves like a thermally thin material. The approximate solution presented in this article constitutes a useful tool for pencil-and-paper calculations and is an advancement in the understanding of solid-phase ignition processes. (C) 2018 The Combustion Institute. Published by Elsevier Inc. All rights reserved.
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Reszka, P., Cruz, J. J., Valdivia, J., Gonzalez, F., Rivera, J., Carvajal, C., et al. (2020). Ignition delay times of live and dead pinus radiata needles. Fire Saf. J., 112, 7 pp.
Abstract: There are still many open questions related to the fire behavior of live and dead wildland fuels and their senescence process. We have physically and biochemically studied live and dead pinus radiata needles, their aging process, and their fire behavior using a systematic aging procedure which allows to characterize the evolution of the fuel moisture content and the photosynthetic pigments over time, and to determine the period of time after sample collection in which specimens can be considered to be alive. Results show that pine needles stay alive for up to 12 h after collection if they remain attached to the twigs. The influence of senescence on spontaneous ignition was tested on two bench-scale devices, the I-FIT and the SCALA, under discontinuous and continuous configurations, respectively. Live pine needles showed larger critical heat fluxes than dead needles, while dead and re-hydrated samples have increased critical heat fluxes for greater moisture contents. Experimental results were interpreted with thermal models based on a two-phase description of the fuel layer. We established a correlation of the form 1/t(ig)proportional to q(inc)" for both ignition configurations, which is adequate for engineering applications and allows the estimation of effective properties for wildland fuel beds.
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