Critical mass flux for flaming ignition of dead, dry wood as a function of external radiant heat flux

Extreme weather often contributes to crown fires, where the fire spreads from one tree crown to the next as a series of piloted ignitions. An important aspect in predicting crown fires is understanding the ignition of fuel particles. The ignition criterion considered in this work is the critical mass flux criterion – that a sufficient amount of pyrolysis gases must be generated for a diffusion flame to be established above the surface. An apparatus was built to measure the critical mass flux for sustained flaming ignition of woody materials for varying environmental conditions (heat flux and external oxidizer flow velocity). This paper reports the results of measured critical mass fluxes for dead, dry poplar under a constant flow velocity of 1 m/s under a range of radiant heat flux levels from 20 kW/m to 50 kW/m. The critical mass flux is seen to agree qualitatively with those in the literature (around 1-2 g/ms) and to increase with increasing levels of heat flux. Future work will explore the changes in critical mass flux with flow velocity, fuel moisture content, species, and live fuels.