Effects of fuel bed structure on heat transfer mechanisms within and above porous fuel beds in quiescent flame spread scenarios

Background. Further understanding of the effect of fuel structure on underlying physical phenomena controlling flame spread is required given the lack of a coherent porous flame spread theory. Aims. To systematically investigate the effect of fuel structure on the heat transfer mechanisms within and above porous fuel beds. Methods. Radiant and total heat fluxes were measured in two extended series of laboratory-based quiescent flame spread experiments in pine needle beds across a range of structural conditions (various fuel loadings, bulk densities, and fuel depths). Key results. Peak radiant heat fluxes from the in-bed combustion region were greater than peak radiant heat fluxes from the above-bed flame front for all of the studied fuel conditions. However, the magnitude and duration of radiant heating from the above-bed flame increased with fuel loading (where bulk density was held constant and fuel depth allowed to vary). Conclusions. Our study highlighted the important role of fuel structure on heat transfer mechanisms, and the relevance of development of semi-empirical and simplified physics-based models. Implications. The interdependent effects of fuel bed properties on the underlying heat transfer mechanisms must be considered in the further development of coherent, flame spread theories.