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EcoSmart Fire as Structure Ignition Model in Wildland Urban Interface: Predictions and Validations

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Abstract

EcoSmartFire is a Windows program that models heat damage and piloted ignition of structures from radiant exposure to discrete landscaped tree fires. It calculates the radiant heat transfer from cylindrical shaped fires to the walls and roof of the structure while accounting for radiation shadowing, attenuation, and ground reflections. Tests of litter burn, a 0.6 m diameter fire up to 250 kW heat release under a Heat Release Rate (HRR) hood, with Schmidt-Boelter heat flux sensors in the mockup wall receiving up to 5 kW/m2 radiant flux, in conjunction with Fire Dynamic Simulator (FDS) modeling verified a 30% radiant fraction, but indicated the need for a new empirical model of flame extinction coefficient and radiation temperature as function of fire diameter and heat release rate for use in ecoSmartFire. The radiant fluxes predicted with both ecoSmartFire and FDS agreed with SB heat flux sensors to within a few percent errors during litter fire growth. Further experimental work done with propane flame heating (also with 30% radiant fraction) on vertical redwood boards instrumented with embedded thermocouples validated the predicted temperature response to within 20% error for both models. The final empirical correlation for flame extinction coefficient and temperature is valid for fire diameters between 0.2 and 7.9 m, with heat release rates up to 1000 kW. From the corrected radiant flux the program calculates surface temperatures for a given burn time (typically 30 s) and weather conditions (typically dry, windy, and warm for website application) for field applications of many trees and many structural surfaces. An example was provided for a simple house exposed to 4 burning trees selected on a Google enhanced mapping that showed ignition of a building redwood siding. These temperatures were compared to damage or ignition temperatures with output of the percentage of each cladding surface that is damaged or ignited, which a homeowner or a landscaper can use to optimize vegetation landscaping in conjunction with house exterior cladding selections. The need for such physics-based fire modeling of tree spacing was indicated in NFPA 1144 for home ignitability in wildland urban interface, whereas no other model is known to provide such capability.

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Abbreviations

A :

Surface area (m2)

Bi :

Biot number

D :

Tree flame diameter (ft (m))

c :

Heat capacity (J kg−1 K−1)

δ m :

Cladding thickness (mm)

ε m :

Tree flame emissivity (–)

E b :

Blackbody total emissive flux (kWm−2)

F i :

Cylindrical flame view factor in “I” direction (–)

Fo :

Fourier number (−)

Fr :

Froude number based on heat release rate (–)

h :

Heat transfer coefficient (W m−2 K−1)

H :

Tree flame height (ft (m))

HRR :

Heat release rate of tree (kW)

k :

Thermal conductivity (Wm−1 K−1)

κ :

Smoke extinction coefficient (m−1)

L :

Distance from surface element to flame center (m)

m :

Tree wet mass (kg)

MC :

Dry basis moisture content of tree foliage (%)

q j :

Line of sight irradiance from tree fire j to surface element (kWm−2)

ρ :

Cladding density (kg m−3)

r :

Flame radius (m)

R j :

Ground reflection coefficient around tree j (–)

S j :

Mean beam length of tree fire j (m)

σ:

Stefan-Boltzmann constant, (5.6696 × 10−8 × Wm−2 K−4)

τ j :

Tree flame transmittance (–)

θ 0 :

Upward rotation angle to flame top (radian)

T :

Temperature (K)

u :

Unit vector in x direction of element normal vector

v :

Unit vector in y direction of element normal vector

w :

Unit vector in z direction of element normal vector

χ r :

Radiant fraction of heat release rate

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Acknowledgements

Initial inspiration to undertake this rewrite of EcoSmart Fire was provided by Greg McPherson and Joe Purohit who worked on the larger ecoSmart landscapes web project with funding from CalFire. Part of the core EcoSmart Fire programming work was supported under their CalFire contract. Greg and Joe helped shape the initial scope of the project. Further validation work and continuing software refinement was supported by the Forest Products Laboratory. We thank FPL project leaders Carol Clausen and Sam Zelinka for their ongoing encouragement.

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Authors and Affiliations

  1. Forest Products Laboratory, USDA, FS, Madison, WI, 53726, USA

    Mark A. Dietenberger & Charles R. Boardman

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  1. Mark A. Dietenberger
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  2. Charles R. Boardman
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Correspondence to Mark A. Dietenberger.

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Dietenberger, M.A., Boardman, C.R. EcoSmart Fire as Structure Ignition Model in Wildland Urban Interface: Predictions and Validations. Fire Technol 53, 577–607 (2017). https://doi.org/10.1007/s10694-016-0632-0

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