Pyrolysis and spontaneous ignition of wood under time-dependent heat flux

Abstract

This work investigates experimentally and theoretically the effect of time-dependent incident heat flux (HF), which is more reasonable in fire-like environment, on thermal degradation process of wet pine wood. A feedback method was utilized to generate a time-dependent heat flux by controlling the output power of radiative heater. Both quadratic and linear heat fluxes were studied in this study. Comparison between measured heat fluxes and designed values indicates that the method provides high accuracy. Measurements of temperature distribution at different depths of material, ignition time and mass loss rate were implemented in the tests to examine the effects of time-dependent heat fluxes. Additionally, analytical model and numerical model were developed to predict the pyrolysis behaviors, and good agreement exists between the experimental and simulational results. Results showed that the heat penetration layer is restricted to a thinner depth for HF with higher increasing rate. A linear relationship was found between ignition time and HF parameters, which is also validated by experimental data and reexamined by constant heat flux circumstance. Mass loss rate was affected significantly by the changed heat flux compared with constant scenario. Furthermore, critical mass flux, which keeps almost unchanged, can be employed as ignition criterion due to the fact that the ignition temperature increases with increasing heat flux, which also certifies the conclusions of other researchers.