FIRE PROPAGATION IN VERTICAL STICK ARRAYS - THE EFFECTS OF WIND

A simple geometrical model of fire spread through arrays of vertically mounted fuel elements performs well in the absence of wind. The theor y assumes that an adjacent fuel element ignites when the flame from th e previous fuel element moves downward sufficiently that its temperatu re isotherm corresponding to the temperature of ignition intersects th e top of the adjacent fuel element. This simple geometrical model is e xtended to incorporate the effects of wind, and its predictions are co mpared to wind tunnel observations of burning arrays. The model perfor ms well at low wind speeds, but underestimates the wind speed at which the flame makes contact with adjacent fuel elements. The reason for t his underestimate is likely to arise because of a weakness in one or m ore of the assumptions concerning, (1) the laminar nature of the flame , (2) the constancy of the flame height as the wind increases, or (3) the existence of a constant ignition temperature. The most significant finding is that this simple conceptual theory indicates that the rate of spread of a fire front as a result of wind is unlikely to be a sim ple function such as a power-law or an exponential, but is likely to b e the solution to a set of differential equations that can be approxim ated by such simple functions over a portion of their range.