The effect of solar insolation on the burning rate of shallow fuel beds

Solar insolation has been observed to affect fire intensity. Most if not all wildland firefighters have heard stories of or personally observed nearly instantaneous reductions in fire intensity when the sun is obscured and conversely the increase in fire intensity when the vegetation and fuels are suddenly exposed to sunlight after the passage of a cloud. However, little if any quantitative work can be found exploring this effect. A set of experiments was conducted to explore this phenomenon. They consisted of exposing a nominally 0.4m wide by 2.4m long fuel bed to two different levels of irradiation generated by halogen lamps and igniting one end of the fuel bed at the same time as the lights were turned on. The irradiation effect was characterized by measuring the time required for the flame to fully burn down the 2.4m length of the fuel bed. Approximate surface incident flux levels were 400 and 800 W/m2 (0.4 and 0.8 solar constants respectively). The data indicate that at the lower irradiation level, no significant difference between the irradiated and nonirradiated case was detectable. However at the 0.8 solar constant level, the irradiated bed burned as much as 40% faster than the nonirradiated bed. Fuel moisture and mass loss measurements do not suggest that short term drying of the fuel is a significant factor. Flow measurements indicate that the irradiation induces a local increase in buoyancy driven turbulence at the fuel bed surface. Additional experiments are being planned to further characterize this phenomenon and the physical mechanisms driving the increased fire intensity.