Development of a numerical and experimental framework to understand and predict the burning dynamics of porous fuel beds
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Date
10/07/2017Author
El Houssami, Mohamad
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Abstract
Understanding the burning behaviour of litter fuels is essential before developing
a complete understanding of wildfire spread. The challenge of predicting the
fire behaviour of such fuels arises from their porous nature and from the strong
coupling of the physico-chemical complexities of the fuel with the surrounding
environment, which controls the burning dynamics. In this work, a method
is presented to accurately understand the processes which control the burning
behaviour of a wildland fuel layer using numerical simulations coupled with
laboratory experiments. Simulations are undertaken with ForestFireFOAM,
a modification of FireFOAM that uses a Large Eddy Simulation solver to
represent porous fuel by implementing a multiphase formulation to conservation
equations (mass, momentum, and energy). This approach allows the fire-induced
behaviour of a porous, reactive and radiative medium to be simulated.
Conservation equations are solved in an averaged control volume at a scale
sufficient to contain both coexisting gas and solid phases, considering strong
coupling between the phases. Processes such as drying, pyrolysis, and char
combustion are described through temperature-dependent interaction between
the solid and gas phases. Di↵erent sub-models for heat transfer, pyrolysis,
gas combustion, and smouldering have been implemented and tested to allow
better representation of these combustion processes. Numerical simulations are
compared with experiments undertaken in a controlled environment using the
FM Global Fire Propagation Apparatus. Pine needle beds of varying densities
and surface to volume ratios were subject to radiative heat fluxes and flows to
interrogate the ignition and combustion behaviour. After including modified
descriptions of the heat transfer, degradation, and combustion models, it is
shown that key flammability parameters of mass loss rates, heat release rates,
gas emissions and temperature fields agree well with experimental observations.
Using this approach, we are able to provide the appropriate modifications to
represent the burning behaviour of complex wildland fuels in a range of conditions
representative of real fires. It is anticipated that this framework will support
larger-scale model development and optimisation of fire simulations of wildland
fuels.
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