ABSTRACT
A one-dimensional numerical model describing the phenomena occurring up to ignition for a solid fuel is used to investigate the failure of the classical ignition theory at heat fluxes above 70 kW/m2. The model is deliberately simple. Comparison with a very large data set of experimental measurements of time to piloted ignition for black PMMA samples shows that model predictions agree well for heat fluxes from 20 to 200 kW/m2. The model predicts well the measurements at high heat fluxes only when in-depth radiation absorption is included. This is in agreement with recently published work of Jiang et al. [8]. The study also shows that the failure of classical ignition theory cannot be explained neither by changes in the reaction scheme, temperature dependency of material properties, combination of the heat losses in a single heat transfer coefficient nor changes of parameter values, such as the critical mass flux, Arrhenius constants or convective heat transfer coefficient. Predictions, including the in-depth radiation absorption of temperature profiles, also agree well with measurements. The model shows that the effect of a black carbon layer on the sample does not cancel the in-depth radiation absorption but increases the effective in-depth radiation absorption coefficient.
Keywords: Ignition, Pyrolysis model, Carbon black, In-depth radiation absorption.
