The effect of fuel structure and wind speed on ignition behaviour and fire spread of wildland fuels under firebrand exposure

Firebrands are recognized as a major source of wildland fuel ignition and a critical driver of fire spread in wildland–urban interface (WUI). This study experimentally examines the ignition behaviour of two widely present vegetative fuel beds in the WUI, pine needle and eucalyptus, when exposed to glowing firebrands under both no wind and wind conditions. Key ignition parameters, including fuel consumption rate, rate of spread, and flame development, were evaluated. Pine needle beds consistently exhibited more intense burning behavior than eucalyptus, with higher fuel consumption rates, faster fire spread, and greater flame heights. For the same fuel load, the average peak values of mass loss rate, rate of spread, and flame height in pine needle fuel beds were approximately 4.5, 1.5, and 1.85 times greater, respectively, than in eucalyptus. Increasing fuel load resulted in increased mass loss and flame height by factors of approximately 1.7 and 1.3 times, respectively, while reducing the rate of spread to about 0.9 times. A notable flame separation phenomenon was also observed during spot fire, where the flame front detached and subsequently created two flame zones. These findings highlight the importance of fuel structure in determining ignition intensity and fire spread under firebrand exposure. • Fuel type, load, and wind govern ignition and fire propagation dynamics • Pine needle beds burn faster with higher consumption rates, and greater flames • Once spot ignition was initiated in the fuel, it leads to sustained fire propagation • Fires in fuel beds with lower loads spread faster but produce smaller flames • Spot fires cause flame separation, forming two flame zones