An Investigation of Oxygen Availability in Spreading Fires

Wildfire spread models that couple physical transport and chemical kinetics sometimes simplify or neglect gas-phase pyrolysis product oxidation chemistry. However, empirical evidence suggests that oxygen (O-2) is available for gas-phase and solid-phase combustion within the flaming reaction zone. This study addresses outstanding questions of O-2 availability by directly measuring O-2 concentrations near fuel surfaces within spreading fuel bed fires for the first time. Temporally and spatially resolved O-2 concentrations within laboratory fires were investigated using uniform fuel beds of medium density fiberboard (MDF) and cardboard (CB) combs at various packing ratios (beta) and angles. Minimum O-2 concentration reached approximately 2-5 mol% for most fuel beds of higher beta (> 0.010), while some fuel beds of low beta (<= 0.010) exhibited higher minimum O-2 concentrations up to 8 mol%. Flame rate of spread, residence time, and minimum O-2 were found to vary with beta and were influenced by bed angle within a given beta. Thermogravimetric analysis was used to assess O-2 availability impacts on flaming and smoldering fuel consumption rates. Negligible impacts were observed for initially unburned fuels at ambient O-2 concentrations of <= 10 mol%, but O-2 availability had significant impacts on char oxidation rates at all tested concentrations. A one-dimensional diffusion model of O-2 availability at char surfaces for near-particle O-2 concentrations tested and measured in TGA and fuel bed experiments revealed a strong dependence of O-2 availability on mass flux from the particle. The results of this work provide new insights regarding O-2 availability and inert assumptions for fire spread models. [GRAPHICS] .