Impinging Flames

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Physical modelling of forest fire and compartment fire


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Laboratory of Thermofluids, Combustion
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  Physical modelling of forest fire behaviour

Experimental work has been carried out in the frame of two EU projects: “Fire Star: a decision support system for fuel management and fire hazard reduction in Mediterranean wildland - urban interfaces” http://www.eufirestar.org/ (contract number: EVG1-CT-2001-00041), and “EUFIRELAB: Euro-Mediterranean Wildland Fire Laboratory, a ‘wall-less’ Laboratory for Wildland Fire Sciences and Technologies in the Euro-Mediterranean Region” http://www.eufirelab.org/ (contract number EVR1-CT-2002-40028).

The objectives of the experiments carried out at IST were to gather data on the propagation of wind driven fire fronts on fuel beds of Pinus halepensis needles. Furthermore, fire transition from the fuel bed to upper layers separated by different vertical gaps was to be investigated as a function of wind velocity. Fire transition across gaps in the fuel bed was to be investigated, as well.

The large wind tunnel at IST was prepared for wind driven experimental fires with Pinus halepensis. During the first campaign some difficulties were encountered that hindered the running of the experiments. Most of those difficulties were related to the poor quality of the needles. Indeed, in Portugal there are no extensive stands of Pinus halepensis, and these trees are usually found together with other vegetation, making it very difficult to collect “clean” samples of needles. Eventually it was decided to collect the needles in Spain. This was done by a team of CREAF - Unitat d'Ecologia from the Facultat de Ciencies, Universitat Autonoma de Barcelona. The quality of these needles allowed, finally, to progress with the experiments.

A device was designed and built to allow setting a second layer of fuel above the fuel bed. Every experimental run was video recorded. The images were digitised and analysed using appropriate software, providing data on flame angle, flame height, and rate of spread, as well as qualitative information on flame front shape.

At the time this text is being written, all experimental runs for the study of the flame front propagation were finished. However, the analysis is not yet completed. Therefore, only some qualitatively remarks will be made here. The experimental runs on the horizontal and vertical transition of fire across gaps are still being run.

An extensive set of similar experiments on flame front propagation had been carried out in the frame of another project (Efaistos) with needles of Pinus pinaster. A number of important differences were immediately clear (but, as mentioned, has yet to be quantified):
  • except for high wind velocity in wind driven fires, flame propagation is more difficult and much more dependent on the quality (cleanness) of the needles’ sample;

  • for no wind situation the flame front geometry is much more irregular, with a clear loss of the symmetry found for wind driven or backing fire propagation (for Pinus pinaster fuel beds this symmetry was always clear, even for the no wind situation);

  • except for high wind velocity in wind driven fires, flames are smaller and the flame front is not necessarily continuous;

  • in spite of low rates of spread (hence, high resident time) the combustion of the needles is not complete. In many cases some needles are completely burned (but the ashes clearly show the needles geometry), but others are not. The amount of carbon in the latter is enough to burn them afterwards with a clearly defined flame.
All these points show the necessity of further research in the field of flame propagation on forest fuel beds.

During 2004 the group at IST was also responsible, in the frame of EUFIRELAB project, for the coordination and final writing of deliverable D-07-08: “Data acquisition and logging systems: general presentation”, which can be seen at http://www.eufirelab.org/unit.php?unit=7.