Some Aspects of the Growth and Spread of Fire in the Open

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Abstract
This paper describes some recent experimental and theoretical work on the growth and spread of fire in the open and discusses some examples of field data in terms of the theoretical calculations presented. The lengths of flames from laboratory fires have been related to the size and rate of burning of the fuel by formulae derived from a simplified dimensional analysis. The effects of a wind blowing across a long fuel bed on the length and orientation of flames are also described. The scaling laws for flame height suggest that in the horizontal spread of fire, heat transfer from the flames above the fuel bed is important primarily with shallow fuel beds. It is suggested that the main effect of a wind on crib fires is aerodynamic. The wind deflects the advancing fire front from the vertical, but perpendicular to this deflected front the rate of spread of fire, at least for cribs, is roughly the same as in still air. However, a theory of spread allowing for heat transfer through the fuel bed and radiation from the flames predicts that there can be a stable ‘fast‘ spread as well as a ‘slow’ spread. In ‘fast’ spread the flames are thick and control the spread. In ‘slow’ spread radiant heat transfer from the burning zone is usually responsible for the spread. The flames are thin and of low emissivity. The most important factors determining the rate of ‘slow’ spread R are pb the bulk density of the fuel bed and ø the deflection from the vertical of the front of burning fuel which varies with wind speed. Rpb cos ø is approximately constant over a wide range of conditions with an order of magnitude of 5–10 mg cm −2 s −1 .

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