Glowing and flaming autoignition of wood exposed to coupled convective and radiative heating

Abstract

Autoignition of solid fuels driven by coupled convective and radiative heating is frequently encountered in fires, whereas its controlling mechanism is rarely revealed. To challenge this issue, autoignition tests of 5 and 15 mm thick cylindrical wood samples were conducted using a newly developed apparatus enabling coupled convective and radiative heating. The total heat flux imposed on wood was maintained at 25 kW/m2, while the fractions of the two heating components changed as the airflow velocity and temperature varied in the ranges of 0–1.51 m/s and 298–473.3 K. Glowing ignition occurred in all scenarios except for one case due to enhanced heat loss and declined radiative heating. Flaming ignition was observed in all conditions except for 298 K airflow temperature. Four stages were identified during flaming combustion: a preheating and deformation stage, a glowing combustion stage, a coupled glowing and flaming combustion stage, and an extinction stage. Compared to 5 mm samples, the surface temperature rise of 15 mm wood was significantly delayed due to its thermally thick nature. By fitting experimental data, a new formula was proposed to correlate glowing ignition temperature with radiative heat flux, airflow velocity, and airflow temperature. Both glowing and flaming ignition times were affected by airflow and radiative heating in a complex manner, and their values to the power of −0.5 approximately linearly depended on radiative heat flux. Damköhler number and nondimensional heat loss ratio were employed to quantitively analyze the flaming ignition propensity. Meanwhile, an ignitability trend map identifying the flaming and non-flaming ignition regimes was drawn.