Experimental modelling of crown fire initiation in open and closed shrubland systems

Abstract

The transition of surface fire to live shrub crown fuels was studied through a simplified laboratory experiment using an open-topped wind tunnel. Respective surface and crown fuels used were excelsior (shredded Populus tremuloides wood) and live chamise (Adenostoma fasciculatum, including branches and foliage). A high crown fuel bulk density of 6.8kgm–3 with a low crown fuel base height of 0.20m was selected to ensure successful crown fire initiation. Diagnostics included flame height and surface fire evolution. Experimental results were compared with similar experiments performed in an open environment, in which the side walls of the wind tunnel were removed. The effect of varying wind speed in the range 0–1.8ms–1, representing a Froude number range of 0–1.1, on crown fire initiation was investigated. The suppression of lateral entrainment due to wind tunnel walls influenced surface fire behaviour. When wind speed increased from 1.5 to 1.8ms–1, the rate of spread of surface fire and surface fire depth increased from 5.5 to 12.0cms–1 and 0.61 to 1.02m. As a result, the residence time of convective heating significantly increased from 16.0 to 24.0s and the hot gas temperature at the crown base increased from 994 to 1141K. The change in surface fire characteristics significantly affected the convective energy transfer process. Thus, the net energy transfer to the crown fuel increased so the propensity for crown fire initiation increased. In contrast, increasing wind speed decreased the tendency for crown fuel initiation in an open environment because of the cooling effect from fresh air entrainment via the lateral sides of surface fire.