Investigation of the role of bulk properties and in-bed structure in the flow regime of buoyancy-dominated flame spread in porous fuel beds

In a quiescent atmosphere, the flame spread process in porous fuels is controlled to a large degree by the fuel bed structure, fuel loading and bulk density, and fuel moisture content. Previous studies have shown that increases in flame spread rate, fire intensity and burning rate are observed with independent increases in fuel loading or decreases in bulk density, however neither of these parameters adequately describe the physical processes that control flame spread. A series of laboratory-based, flame spread experiments involving fuel beds of differing fuel loading and structure were conducted in the absence of wind and slope effects and with consistent fuel conditioning. Changes in fuel bed structure are shown to change the observed fire behavior in both the flaming phase and the smouldering region behind the flame front, while also influencing the physical mechanisms contributing to flame spread. Bulk density and fuel loading were shown to independently affect the physical mechanisms both above (buoyant flow regime) and within (in-bed flow, gas phase temperature) the fuel bed. Increases in buoyant flow velocity were observed with increases in fuel loading, along with increases in the maximum in-bed entrainment induced towards the approaching flame front. To fully understand the complex interlinking of these flow regimes and their role in quiescent flame spread, physically linked parameters to describe the internal fuel bed structure must be developed.