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.