Ignition and flame-growth modelling on realistic building and landscape objects in changing environments
Mark A. DietenbergerUSDA Forest Service, Forest Products Laboratory, 1 Gifford Pinchot Drive, Madison, WI 53726, USA. Email: mdietenberger@fs.fed.us
International Journal of Wildland Fire 19(2) 228-237 https://doi.org/10.1071/WF07133
Submitted: 2 August 2007 Accepted: 22 May 2009 Published: 31 March 2010
Abstract
Effective mitigation of external fires on structures can be achieved flexibly, economically, and aesthetically by (1) preventing large-area ignition on structures by avoiding close proximity of burning vegetation; and (2) stopping flame travel from firebrands landing on combustible building objects. Using bench-scale and mid-scale fire tests to obtain flammability properties of common building constructions and landscaping plants, a model is being developed to use fast predictive methods suitable for changing environments imposed on a parcel lot consisting of structures and ornamental plants. Eventually, the property owners and associated professionals will be able to view various fire scenarios with the ability to select building materials and shapes as well as select ornamental plant species and their placement for achieving the desired fire mitigation. The mathematical formulation presented at the 2006 BCC Research Symposium is partially shown here and some results are compared with (1) specialised testing of Class B burning brands (ASTM E108) in the cone calorimeter (ASTM E1354); (2) our refurbished and modified Lateral Ignition and Flame Travel Test (ASTM E1321 and E1317); (3) room-corner tests with oriented-strand board (ISO 9705); and (4) cone calorimeter tests of fire-resistive materials such as fire retardant-treated plywood and single-layer stucco-coated oriented-strand board.
Additional keywords: calorimetry, fire mitigation, flammability modelling.
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Nomenclature
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αs = ϵs, surface absorptivity equal to surface emissivity for most building materials
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δf, exponential decay with characteristic length for flame extension (m)
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ϵf, flame emissivity
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ωm, decay coefficient for material burn-off (1 s–1)
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ρ, dry body density (kg m–3)
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α = Kq/ρCq, thermal diffusivity (m2 s–1)
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σ, Stefan–Boltzmann constant (kW K–4 m–2)
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τm, material time constant (s)
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Af, flame area on combustible object (m2)
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Ap, combustible object pyrolysis area (m2)
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Cq, heat capacity (kJ kg–1 K–1)
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H(ti – t1), Heaviside function
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hcf, flaming convective coefficient (kW m–1 K–1)
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Qt, HRR, total heat release rate (kW)
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Q″m,ig, material peak HRR flux (kW m–2)
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Kq, thermal conductivity coefficient (kW m–1 K–1)
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ℓ, material thickness (m)
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(ℓ, t) time stepping changes in surface heat fluxes (kW m–2)
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(0, t) time stepping changes in back-side heat fluxes (kW m–2)
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, irradiance (kW m–2)
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, imposed heat flux from ignition burner or the firebrand flame and glow (kW m–2)
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si, growth acceleration coefficients (Eqn 15)
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, series expansion solution (Eqn 2)
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t, current time (s)
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, temperature change (K)
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Tf, averaged measured flame temperature (K)
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Tig, ignition temperature (K)
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vp, quasi-steady speed of surface flame spread (m s–1)
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w, flame width (m)
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, dimensional depth (m)
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y, surface distance (m)