Rates of Spread of Some Wind-driven Fires

This paper shows that head fires spread in cribs and in controlled burns in gorse and heather (in the New Forest) at similar rates, R , given by where $ρ′b$ is the bulk density of the burnt fuel and U is the wind speed in m/s. Although the values of U were similar, the values of $ρ′b$ differed by more than an order of magnitude between the two types of fire. The constant of proportionality is roughly 0.07 kg/m ³ for the wildland fuels and 0.05 kg/m ³ for cribs. The similarity of the relationships is perhaps more significant than the agreement between the coefficients, which appears closer than it really is since the cribs were drier than the wildland fuels. Assuming a wind speed of 10 mile/h the constant for the Great Fire of London (1666) was also about 0.05 kg/m ³ . A comparison between the flames in the controlled burns and a partly theoretical correlation of flame lengths from burning cribs has been partly successful and the duration of flaming of a piece of fuel has been empirically correlated with moisture content, though the general effects of moisture on the rate of spread are not satisfactorily accounted for. The reason for this and other anomalies is probably the relatively few data for wildland head fires (ten in all) and the fact that their fuel composition was not uniform. The author uses many of the results of the work described in an earlier paper in Forestry (Thomas, 1967) to derive heat balances for head fires and to discuss the theoretical aspects of the above expression.