Rate of spread of free-burning fires in woody fuels in a wind tunnel

We describe the results of 357 experimental fires conducted in an environmentally controlled large wind tunnel. The fires were burned over a range of particle sizes, fuel bed depths, packing ratios, moisture contents and windspeeds. We find that spread rate decreases with moisture content in a way which depends on the fuel type and diameter. It decreases as the square root of the packing ratio. Fuel bed depth has little effect on spread rate, and fuel diameter has significant effect only for diameters above 1 mm. The relationship between rate of spread and windspeed is virtually linear. We develop a predictive model for rate of spread based on energy transfer considerations and the laboratory results. Other laboratory-based models for spread rate are compared with our model, and tested against the laboratory data. The other models have forms similar to ours, but do not predict our data well. Our model predicts well the spread rates for fires burned in windspeeds below 3 m/sec in other laboratories. The scale of the experiments and the similarity of the dependence on windspeed to that found in the held indicate that a field model may be developed from the laboratory model with relatively few modifications.