The ignition of green and preburn wood by radiation

Abstract

The ignition of green and pre-burn wood by radiation has been examined. Preliminary heating for extended periods at low temperatures produces charring and ignition under certain circumstances, but it also reduces density and depletes some of the volatiles, leading to an increase in the minimum density required for ignition as compared to the unexposed green wood. To predict the piloted ignition time of wood and pre-burn wood in Cone Calorimeter, one-dimensional solid phase heat conduction model was developed to correlate ignition time with thermophysical properties. Using a critical surface temperature criterion, the conduction model allowed for variable thermophysical properties. Linear interpolation between the property values of the virgin material and char was used to derive thermophysical properties of the charring solid matrix, while that of wood and char were assumed to be constant. 25mm thick oven dry green wood and 20mm thick pre-burn wood of Nyatoh hardwood were exposed horizontally in Cone Calorimeter to seek the minimum critical heat flux. The preburn wood specimens were exposed to 250°C in oven until a 50% weight reduction was achieved prior to testing in Cone Calorimeter. The results showed that for green wood, the present correlation has been fitted for much longer time (up to one-hour). The experiments have been able to yield a critical heat flux of 10kW/m2 for piloted ignition after being exposed for more than two hours. For pre-burn wood, the correlation covered a shorter period (less than an hour); the effect of thermal inertia on flaming ignition was significant and responsible for high surface ignition temperatures, as shown by thermocouple-measured values and simulated temperatures in thermal model developed in this study adopting finite volume approach using FLUENT 6.2. Theoretical and experimental correlations illustrated the discrepancies at the lower heating rates and extended time regimes.