Experimental investigation of the physical mechanisms governing the spread of wildfires
Frédéric Morandini A B and Xavier Silvani AA Laboratoire Sciences pour l’Environnement, UMR-CNRS (Unités Mixte Recherches-Centre National de la Recherche Scientifique) 6134, Université de Corse, Campus Grimaldi, F‐20250 Corte, France.
B Corresponding author. Email: frederic.morandini@univ‐corse.fr
International Journal of Wildland Fire 19(5) 570-582 https://doi.org/10.1071/WF08113
Submitted: 4 July 2008 Accepted: 12 November 2009 Published: 9 August 2010
Abstract
One of the objectives of the present study is to gain a deeper understanding of the heat transfer mechanisms that control the spread of wildfires. Five experimental fires were conducted in the field across plots of living vegetation. This study focussed on characterising heat transfer ahead of the flame front. The temperature and heat flux were measured at the top of the vegetation as the fire spread. The results showed the existence of two different fire spread regimes that were either dominated by radiation or governed by mixed radiant–convective heat transfer. For plume‐dominated fires, the flow strongly responds to the great buoyancy forces generated by the fire; this guides the fire plume upward. For wind‐driven fires, the flow is governed by inertial forces due to the wind, and the fire plume is greatly tilted towards unburned vegetation. The correlations of the temperature (ahead of the flame front) and wind velocity fluctuations change according to the fire regime. The longitudinal distributions of the radiant heat flux ahead of the fire front are also discussed. The data showed that neither the convective Froude number nor the Nelson convection number – used in the literature to predict fire spread regimes – reflect the observed behaviour of wind‐driven fires.
Additional keywords: field experiments, heat transfer, radiation, temperature.
Acknowledgements
This work was financially supported by the collectivité territoriale (territorial collectivity) of Corsica. Experiments 2–5 would not have been possible without the assistance of the French forestry, agriculture and firefighting services (Forestiers Sapeurs, Office National des Forêts, Direction Départementale de l’Agriculture et de la Forêt, Service Départemental d’Incendie et de Secours). We also thank our colleagues from CEREN (Centre d’Etude et de Recherche de l’Entente) for having invited us to their experimental campaign (experiment 1).
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Appendix 1
A preliminary study of the deposition of soot on the radiometer window was conducted in the laboratory using two radiation heat flux transducers with sapphire windows and fires from a forest fuel burner. The burner was made from a 50 cm‐diameter basket filled with 500 g of forest fuel (Pinus pinaster needles) and ignited using alcohol. Prior to each fire test, both radiometers were exposed to a radiant source of ∼60 kW m–2 in order to check for good agreement (Table A1). In the first test, one radiometer was exposed to a smoke plume ∼50 cm above the flame for 200 s. After exposure, the measurements of the heat flux gauge exposed to smoke were compared with those of the soot‐free reference radiometer. In the second test, the radiometer was exposed to the flame for 60 s, and the measurements obtained were compared with those of the reference radiometer. The attenuation of the gauge reading after smoke exposure was not significant. These results confirm that the deposition of soot on the gauge window can be considered negligible when the transducer is only exposed to smoke (in the preheating region, for instance). This is due to the well‐ventilated nature of the flame in the open in contrast with fire in an enclosure. In contrast, flame contact with the gauge deposited a significant quantity of soot on the window. In this case, the attenuation measured during this test was ∼25%; therefore, radiation measurement in the flaming region could no longer be considered. When the radiometer is used in a hostile flame environment, nitrogen purging is used to prevent soot deposition in order to keep the radiation‐transmitting window clean. The purge system requires the use of an inert gas that flows ahead of the window to prevent soot from depositing on it. Another alternative is the use of ellipsoidal radiometers with gas purging to prevent soot from entering their cavity.
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