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Burning Rates of Wood Cribs with Implications for Wildland Fires

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Abstract

Wood cribs are often used as ignition sources for room fire tests and the well characterized burning rates may also have applications to wildland fires. The burning rate of wildland fuel structures, whether the needle layer on the ground or trees and shrubs themselves, is not addressed in any operational fire model and no simple model exists. Several relations exist in the literature for the burning rate of wood cribs, but the cribs used to generate them were built with fairly limited geometries. This work explores the burning rate of cribs with a wide variety of geometries and aspect ratios in the loosely-packed regime to evaluate the rigor of several correlations from the literature. Specifically, stick thicknesses ranged from 0.16 cm to 1.27 cm and lengths from 6.4 cm to 61.0 cm resulting in aspect ratios (stick length/thickness) from 10 cm to 160. As wildland fuel beds occur both directly on the ground and suspended in the air, the effect of the vertical gap between the ground and crib base was also examined. The critical vertical gap was shown to be larger than previously thought (7.6 cm for all cribs) and a function of the aspect ratio. It was quite apparent that as the aspect ratio increases, a significant portion of the required oxidizer comes from the bottom of the crib. A relation is then found to adjust the predicted values for the reduction in burning rate due to insufficient vertical gap.

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Abbreviations

av :

Area of single vertical shaft (s2) (cm2)

as :

Surface area of single vertical shaft (4 sh) (cm2)

Av :

Total area of vertical shafts (cm2)

As :

Total stick surface area (cm2)

b:

Stick thickness (cm)

B:

Enthalpy ratio (Eq. 13) (dimensionless)

cp :

Specific heat (kJ/kg K)

C:

Fuel property constant in Block’s Theory (Eq. 3) (g/s*cm1.5)

d:

Height above ground of crib bottom (cm)

f:

Friction factor (dimensionless)

F:

Ratio of the thermal diffusivity of Douglas-fir to the wood tested (dimensionless)

g:

Acceleration due to gravity (m/s2)

G:

Modified Froude number defined in Eq. 6 (dimensionless)

h:

Crib height (cm)

Hc :

Heat of combustion of pyrolyzates (kJ/kg)

Hp :

Heat required for pyrolysis (kJ/kg)

l:

Crib/stick length (cm)

n:

Number of sticks per layer (dimensionless)

N:

Number of layers (dimensionless)

P:

Perimeter of vertical shaft (4s) (cm)

R:

Burning rate (g/s)

Rmax :

Maximum burning rate measured for a given crib design (g/s)

s:

Spacing between sticks (cm)

Ts :

Temperature in shaft (°C)

T0 :

Ambient temperature (°C)

γ:

Fuel-to-air mass ratio for pyrolyzate-air reaction (dimensionless)

λ:

Ratio of gas mass flux leaving to air entering (Eq. 16) (dimensionless)

φGross :

Crib porosity as defined by Gross (Eq. 1) (cm1.1)

φHeskestad :

Crib porosity as defined by Heskestad (Eq. 7) (cm)

ν∞ :

Kinematic viscosity of ambient air (m2/s)

ρ:

Density of air in the vertical shafts (kg/m3)

ρ0 :

Density of ambient air (kg/m3)

Ψ:

Drag coefficient defined in Eq. 5 (dimensionless)

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Acknowledgments

The authors wish to thank James McGuire, Jennifer Kennedy, and Sophia Vernholm for their tireless and careful construction of the cribs and Cyle Wold for setting up the data acquisition system. Funding for this work was provided by the National Fire Decision Support Center.

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Authors and Affiliations

  1. USDA Forest Service, RMRS Missoula Fire Sciences Lab, 5775 W US Highway 10, Missoula, MT, 59808, USA

    Sara McAllister & Mark Finney

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  1. Sara McAllister
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Correspondence to Sara McAllister.

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McAllister, S., Finney, M. Burning Rates of Wood Cribs with Implications for Wildland Fires. Fire Technol 52, 1755–1777 (2016). https://doi.org/10.1007/s10694-015-0543-5

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