Effect of Fuel Bed Structure on the Controlling Heat Transfer Mechanisms in Quiescent Porous Flame Spread

Abstract

The increasing importance of prescribed fire use has led to an increased focus on the development of modelling tools suited to conditions typical of prescribed fire scenarios. An improved understanding of flame spread through porous surface fuels represents an important part of these efforts. In the lower wind speed conditions typical of many prescribed burns, the role of fuel structure may be of greater importance than in highly wind-aided flame spread scenarios. The porous nature of wildland fuel beds complicates efforts to apply existing, solid surface theories for flame spread in low or quiescent wind conditions as radiation, convection and conduction may all occur within the porous fuel in addition to flame heating. An important first step in the development of any flame spread theory is the identification of the dominant heat transfer mechanisms but for wildland fuels the effect of fuel structure on the relative importance of different heating mechanisms must be considered. To investigate the role of fuel structure we therefore present a series of laboratory-based flame spread experiments conducted in pine needle fuel beds with various structural properties. The fuel loading and bulk density were independently varied by controlling the fuel bed height with water-cooled heat flux gauges used to measure the (radiant and total) heat flux from both the above-bed flame and the in-bed combustion region. A single dimensionless parameter (ασδ), incorporating the fuel bed porosity (α), fuel element surface-to-volume ratio (σ), and fuel bed height (δ), was used to describe the overall fuel bed structure. The heat flux measurements highlighted the dominant role of in-bed heating across all of the studied fuel conditions although the magnitude of above-bed flame heating increased with increasing fuel loading. Heat fluxes from the in-bed combustion region exceeded those from the above-bed flame region with the magnitude of the peak (radiant and total) heat flux at each measurement location generally increasing with increasing ασδ across the studied range (ασδ=49 to 399). However, the effect of fuel loading was also apparent with a positive relationship also observed between fuel loading and flame height. The experimentally observed effective heating distances also varied with bulk density and fuel loading and were used to evaluate the use of a thermal modelling approach incorporating the bulk structural properties of the porous fuel bed. Comparison with experimental observations of spread rate indicated a maximum variation in predicted spread rate of 29 % where only radiative transfer from the in-bed combustion region was considered, with closer agreement at lower ασδ values. Where the contributions of both the in-bed and above-bed heat transfer mechanisms were considered, the need to incorporate additional heat loss terms into this thermal model were apparent. This study therefore emphasises the important role of porous fuel structure on the in-bed heat transfer and assesses suitable, physically meaningful structural descriptors. The experiments presented in this study will also provide a valuable dataset for future model development efforts incorporating measurements of fire behaviour and underlying physical phenomena across a wide range of structural conditions.