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ŠUMARSKI LIST 1-2/2018 str. 79     <-- 79 -->        PDF

ON THE FIRE-SPREAD RATE INFLUENCE OF SOME FUEL BED PARAMETERS DERIVED FROM ROTHERMEL’S MODEL THERMAL ENERGY BALANCE
O UTJECAJU POJEDINIH PARAMETARA LOŽIŠTA IZVEDENIH IZ ROTHERMELOVOG MODELA RAVNOTEŽE TOPLINSKE ENERGIJE NA STOPU ŠIRENJA POŽARA
Carlos G. ROSSA*, Paulo M. FERNANDES
Summary
We analysed the role of some fuel bed properties on forest fire-spread rate based on the thermal energy balance upon which the well-known fire-spread rate model of Rothermel (1972) was developed, showing that neither fuel bed height, load or density directly influence the thermal energy balance. The influence of such parameters, often inferred from empirical descriptions of spread rate, must result from indirect effects on heat transfer mechanisms. The fraction of heat transferred from the flame to the unburned fuel depends mostly on fuel moisture content and is independent of spread rate and flame geometry. Because empirical models usually implicitly assume the underlying mechanisms of fire spread for describing fire behaviour, this study results can assist at idealizing and delineating future experiments and approaches.
Key words: fire behaviour, forest fires, combustion, heat transfer.
INTRODUCTION
UVOD
The ability to predict fire behaviour characteristics such as rate of spread, flame dimensions and heat release rate is central to effective forest fire management, from planning to operational decision-making. Rothermel (1972), following the formulation of Frandsen (1971), proposed a fire-spread model based on several empirical parameters determined from laboratory burn experiments. This model became the basis for well-known fire behaviour prediction systems and, although many fire-behaviour modelling studies were published since, no other has replaced it in terms of popularity. This is understandable because forest fire involves complex phenomena to which several fundamental subjects pertain, namely combustion, heat transfer, and fluid mechanics (Finney et al. 2015; Rossa et al. 2015), and finding a better compromise between a rigorous physical approach and the simplicity of empirical modelling is a difficult task.
Rothermel (1972) formulated rate of spread (R) based on the ratio between the net heat flux through the fuel bed (Q’’) and the amount of energy necessary for igniting the fuel, which depends on fuel bed density (ρb). A considerable number of empirical formulations for predicting R have