Vegetative fuel flammability
To date, no comprehensive methodology for assessing vegetative fuel flammability has been established. Wildfire behaviour depends on three main components: Fuel – Environment – Topography. Thus, understanding fuel flammability is a paramount aspect in assessing wildfire behaviour as a whole. Flammability of porous media, such as is considered for vegetative fuels, is complex and depends on heat and mass transfer mechanisms. These mechanisms are impacted by many factors such as particle/bulk properties and external conditions. Our approach is to apply existing fire science knowledge that originated in the build environment to the wildfire research field. This allows us to explore the heat and mass transfer mechanisms and define the flammability of vegetative fuels.
The behaviour of a wildfire is commonly assessed by the spread rate and fireline intensity. Significant modelling activities are ongoing and can take (semi-)empirical or physics-based approaches. Physics-based modelling (CFD) is applied in order to understand the physical phenomena that drive wildfire, but require validation from experimentation. This is an immense challenge for the research community, since it requires field work that is expensive (cost and labour) but also involves a great risk. Our approach includes development of a suite of measurement equipment and techniques for field campaigns. The data obtained from field experiments (spread rate and intensity maps) is than used for validation exercised for physics-based models (e.g. FDS). Furthermore, this data is also used to link wildfire behaviour and firebrand generation (see below).
Firebrands are a leading cause of ignition of structures in the Wildland-Urban Interface (WUI). Current mitigation efforts revolve about assessing the vulnerabilities of typical structures and reducing potential hazards, i.e. removing combustible material around the structure and preventing firebrands from penetrating the exterior shell of the structure. This is a crucial step in reducing the risk posed by firebrands. Two main issues are currently linked to the lack of fundamental understanding: (1) the ignition mechanism of structural material by firebrands and (2) firebrand generation (firebrand flux) from wildfire. Our approach is to develop a methodology to determine firebrand fluxes and link these to fuel conditions and wildfire behaviour (field scale). Output from this method are firebrand characteristics (type, size, mass) and firebrand flux related to wildfire behaviour and separation to source. The field data is then used to inform bench scale experimentation to determine the heat flux from firebrand accumulations and ignition of structural material.
It is a common strategy to use constant wildfire emission factors in atmospheric modelling. However, it has been discussed by the wildfire community in recent years that constant factors are likely a large source of uncertainty. This is due to the fact that emissions (gaseous or particulate) strongly depend on the combustion behaviour, e.g. flaming and/or smouldering, which is in turn impacted by particle/bulk properties and environmental factors. Our approach is to conduct bench scale experimentation to assess the emissions of vegetation/biomass under well controlled combustion conditions and explore dominant drivers (e.g. heating rate, oxidizer flow rate, bulk density, etc.).