Abstract
A one-dimensional numerical model and theoretical analysis involving both surface and in-depth radiative heat flux absorption are utilized to investigate the influence of their combination on ignition of PMMA (Polymethyl Methacrylate). Ignition time, transient temperature in a solid and optimized combination of these two absorption modes of black and clear PMMA are examined to understand the ignition mechanism. Based on the comparison, it is found that the selection of constant or variable thermal parameters of PMMA barely affects the ignition time of simulation results. The linearity between and heat flux does not exist anymore for high heat flux. Both analytical and numerical models underestimate the surface temperature and overestimate the temperature in a solid beneath the heat penetration layer for pure in-depth absorption. Unlike surface absorption circumstances, the peak value of temperature is in the vicinity of the surface but not on the surface for in-depth absorption. The numerical model predicts the ignition time better than the analytical model due to the more reasonable ignition criterion selected. The surface temperature increases with increasing incident heat flux. Furthermore, it also increases with the fraction of surface absorption and the radiative extinction coefficient for fixed heat flux. Finally, the combination is optimized by ignition time, temperature distribution in a solid and mass loss rate.
Highlights
Ignition of a solid combustible followed by thermal degradation under external incident heat flux determines the subsequent fire propagation, and has been investigated extensively in the last few decades because of its great importance
Delichatsios [28] modified the classical ignition theory by taking both absorption modes into account and provided some useful results. These studies provided exploratory investigation on effects of combination of surface and in-depth absorption on pyrolysis or ignition, numerous important issues remain unsolved, like methods on determination of optimized combination, influence of the char layer generated during thermal degradation, effects of spectral distribution of radiation on the radiative properties due to the non-grey spectral nature of polymers, variation of combination during pyrolysis caused by changing optical characteristics of surface etc
Even though the capability of the numerical model has been validated in Ref. [29], the effects of input parameters of PMMA on ignition time of simulation results need to be investigated
Summary
Ignition of a solid combustible followed by thermal degradation under external incident heat flux determines the subsequent fire propagation, and has been investigated extensively in the last few decades because of its great importance. Delichatsios [28] modified the classical ignition theory by taking both absorption modes into account and provided some useful results These studies provided exploratory investigation on effects of combination of surface and in-depth absorption on pyrolysis or ignition, numerous important issues remain unsolved, like methods on determination of optimized combination, influence of the char layer generated during thermal degradation, effects of spectral distribution of radiation on the radiative properties due to the non-grey spectral nature of polymers, variation of combination during pyrolysis caused by changing optical characteristics of surface etc. The effect of a coated black carbon surface layer on combination and ignition is studied
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