scholarly journals Experimental investigation of the physical mechanisms governing the spread of wildfires

2010 ◽  
Vol 19 (5) ◽  
pp. 570 ◽  
Author(s):  
Frédéric Morandini ◽  
Xavier Silvani
Keyword(s):  
Heat Transfer ◽  
Heat Flux ◽  
Flame Front ◽  
Fire Regime ◽  
Radiant Heat ◽  
Fire Spread ◽  
Fire Plume ◽  
Physical Mechanisms ◽  
Fire Front ◽  
Observed Behaviour

One of the objectives of the present study is to gain a deeper understanding of the heat transfer mechanisms that control the spread of wildfires. Five experimental fires were conducted in the field across plots of living vegetation. This study focussed on characterising heat transfer ahead of the flame front. The temperature and heat flux were measured at the top of the vegetation as the fire spread. The results showed the existence of two different fire spread regimes that were either dominated by radiation or governed by mixed radiant–convective heat transfer. For plume‐dominated fires, the flow strongly responds to the great buoyancy forces generated by the fire; this guides the fire plume upward. For wind‐driven fires, the flow is governed by inertial forces due to the wind, and the fire plume is greatly tilted towards unburned vegetation. The correlations of the temperature (ahead of the flame front) and wind velocity fluctuations change according to the fire regime. The longitudinal distributions of the radiant heat flux ahead of the fire front are also discussed. The data showed that neither the convective Froude number nor the Nelson convection number – used in the literature to predict fire spread regimes – reflect the observed behaviour of wind‐driven fires.

Convective heat transfer in fire spread through fine fuel beds

2010 ◽  
Vol 19 (3) ◽  
pp. 284 ◽  
Author(s):  
W. R. Anderson ◽  
E. A. Catchpole ◽  
B. W. Butler
Keyword(s):  
Heat Transfer ◽  
Wind Speed ◽  
Moisture Content ◽  
Flame Front ◽  
Free Stream ◽  
Surface Wind ◽  
Surface Wind Speed ◽  
Fuel Moisture ◽  
Fire Front ◽  
Fuel Moisture Content

An extensive set of wind-tunnel fires was burned to investigate convective heat transfer ahead of a steadily progressing fire front moving across a porous fuel bed. The effects of fuel and environmental variables on the gas temperature profile and the ‘surface wind speed’ (gas velocity at the fuel bed surface) are reported. In non-zero winds, the temperature of the air near the fuel bed surface decays exponentially with distance from the fire front. In zero winds, the temperature decreases rapidly within a very short distance of the flame front, then decays slowly thereafter. The maximum air temperature decreases as the free stream wind speed, packing ratio and fuel moisture content increase. The characteristic distance of the exponential decay increases strongly with the free stream wind speed and decreases with the packing ratio and surface area-to-volume ratio of the fuel. The surface wind speed depends strongly on the free stream wind speed, and to a lesser extent on packing ratio, fuel bed depth and fuel moisture content. There are three general regimes for the surface flow: (1) a constant velocity flow of approximately half the free stream flow, far from the flame front; (2) an intermediate zone of minimum flow characterised by low or reversed flow; and (3) a region near the flame front where the velocity rises rapidly almost to the free stream velocity. The boundaries between the three regions move further from the flame front with increasing wind speed, in a way which is only slightly affected by fuel geometry.

Experimental Measurement of Heat Transfer to a Cylinder Immersed in a Large Aviation-Fuel Fire

1973 ◽  
Vol 95 (3) ◽  
pp. 397-404 ◽  
Author(s):  
L. H. Russell ◽  
J. A. Canfield
Keyword(s):  
Heat Transfer ◽  
Heat Flux ◽  
Radiant Heat ◽  
Aviation Fuel ◽  
Temperature History ◽  
Cylinder Surface ◽  
Measured Temperature ◽  
Convection Coefficient ◽  
Transfer Parameters ◽  
Experimental Effort

Presented are the results of an experimental effort to quantify some of the heat transfer parameters pertaining to the luminous flame that results from the uncontrolled combustion of an 8-ft × 16-ft pool of JP-5 aviation fuel. The temperature and effective total radiant heat flux, both as temporal mean quantities, were measured as functions of position within the quasi-steady burning flame as it existed in a quiescent atmosphere. A grid of infrared radiometers and radiation-shielded thermocouples served as the primary sensing equipment. A determination was made of the perimeter-mean convection coefficient applicable to a horizontally oriented, smooth, 8.530-in-dia circular cylinder immersed at a particular location within the JP-5 flame. The value of this coefficient was the result of a solution to a nonlinear, inverse conduction problem in which the convective heat flux at the cylinder surface was estimated by utilizing a measured temperature history inside the cylinder. An expression relating this coefficient to more general flame/cylinder systems was developed.

Radiation Energy Density and Radiation Heat Flux in Small Rectangular Cavities

1972 ◽  
Vol 94 (3) ◽  
pp. 289-294 ◽  
Author(s):  
R. P. Caren
Keyword(s):  
Heat Transfer ◽  
Heat Flux ◽  
Energy Density ◽  
Classical Theory ◽  
Radiant Energy ◽  
Radiation Energy ◽  
Radiant Heat ◽  
Radiant Heat Transfer ◽  
Radiation Heat ◽  
Radiation Energy Density

The present paper investigates the impact of one or more small cavity dimensions on the radiation energy density and radiation heat flux in rectangular metallic cavities. The emphasis of the present analysis is the exact treatment of the modal structure of the electromagnetic field in a small cavity in determining the properties of the thermal radiation field in the cavity. The excitation spectrum of the modes is assumed to be given by the Planck distribution function. The Poynting theorem is invoked in order to determine the radiative heat flux absorbed by the walls from the radiation in the cavity. Variation of the dimensions of the rectangular cavity allows the effects of cavity size and shape on the radiant energy density and radiant heat transfer to be assessed, particularly in several interesting limiting cases. It is found that significant deviations from the classical theory occur whenever any of the cavity dimensions satisfy the inequality lT ≤ 1 cm-deg K. It is further found that, when two or more of the cavity dimensions satisfy the above inequality, the radiant energy density and radiant heat transfer are significantly reduced in comparison to the results of classical theory. However, when only one dimension is limited, as in the case of a closely spaced parallel-surface geometry, the radiant energy density and radiant heat transfer are significantly increased compared to the classical theory.

scholarly journals Effect Of External Radiant Heat Flux On Upward Fire Spread: Measurements On Plywood And Numerical Predictions

1994 ◽  
Vol 4 ◽  
pp. 421-432 ◽  
Author(s):  
M. Delichatsios ◽  
Peter Wu ◽  
M. Delichatsios ◽  
G. Lougheed ◽  
G. Crampton ◽  
...  
Keyword(s):  
Heat Flux ◽  
Radiant Heat ◽  
Fire Spread ◽  
Radiant Heat Flux ◽  
Numerical Predictions

scholarly journals Experimental study on the radiant heat flux of wall-attached fire plume generated by rectangular sources

2021 ◽  
Vol 159 ◽  
pp. 106605 ◽  
Author(s):  
Xianjia Huang ◽  
Yuhong Wang ◽  
He Zhu ◽  
Le He ◽  
Fei Tang ◽  
...  
Keyword(s):  
Experimental Study ◽  
Heat Flux ◽  
Radiant Heat ◽  
Radiant Heat Flux ◽  
Fire Plume

Incorporating vegetation attenuation in radiant heat flux modelling

2015 ◽  
Vol 24 (5) ◽  
pp. 640 ◽  
Author(s):  
Glenn Newnham ◽  
Raphaele Blanchi ◽  
Kimberley Opie ◽  
Justin Leonard ◽  
Anders Siggins
Keyword(s):  
Heat Flux ◽  
Complex Terrain ◽  
Spatial Information ◽  
South Australia ◽  
Radiant Heat ◽  
Separation Distance ◽  
Radiant Heat Flux ◽  
Fire Event ◽  
Fire Front ◽  
Spatially Varying

Models of radiant heat flux (RHF) are critical for understanding wildfire behaviour and the effect a fire may have on homes and people. Various models have been presented in the literature for wildfire RHF, many being based on the Stephan–Boltzmann equation for radiative heat transfer. Most models simplify the fire and receiver interaction by considering a single fuel type at a given separation distance from a receiving point (e.g. on a building requiring protection). However, wildfire is an inherently spatial phenomenon, in that a fire may progress across the landscape towards a building across complex terrain and through spatially varying fuel types. This spatial variation influences the fire behaviour as well as the level of RHF incident on the building. In this study, we present methods for incorporating spatially varying topography and fuels into existing RHF modelling equations. In this way, we achieve a time-dependent profile of the RHF incident on homes, while accounting for attenuation due to fuels and topography that lie between the building and the fire front. The model is applied to the prediction of damage in a fire that occurred in South Australia in 2005. Although only coarse spatial information was available for determining the spatial distribution of fuels, modelled RHF was a significant indicator of house damage. Attenuation due to vegetation between homes and the fire was shown to reduce the modelled RHF exposure of homes. However, this was not shown to increase the significance of predicted house damage in the case of this fire event.

Fire line rotation as a mechanism for fire spread on a uniform slope

2002 ◽  
Vol 11 (1) ◽  
pp. 11 ◽  
Author(s):  
Domingos Xavier Viegas
Keyword(s):  
Natural Convection ◽  
Flame Front ◽  
Fire Spread ◽  
Laboratory Scale ◽  
Initial Orientation ◽  
Experimental Results ◽  
Slope Gradient ◽  
Gradient Direction ◽  
Fire Front

The evolution of a linear flame front in a homogeneous fuel bed in a slope, for arbitrary values of the initial orientation of the fire front is studied. It is shown that, with the exception of initially horizontal or down-slope propagating fire lines, the propagation is not stationary. In its movement the fire front rotates, tending to become parallel to the slope gradient direction. The concept of fire line rotation as a tool to interpret and describe the evolution of a fire front is presented. Experimental results developed at a laboratory scale in a 30˚ slope are presented to support it. Some insight about the role played by natural convection induced by the fire is provided. A model using the concept of fire line rotation is proposed to predict the evolution of a fire front. Its application to the case of a point ignition fire in a slope is presented.

A review of radiant heat flux models used in bushfire applications

2003 ◽  
Vol 12 (1) ◽  
pp. 101 ◽  
Author(s):  
A. L. Sullivan ◽  
P. F. Ellis ◽  
I. K. Knight
Keyword(s):  
Heat Transfer ◽  
Heat Flux ◽  
Radiative Heat Transfer ◽  
Radiative Heat ◽  
Wildland Fire ◽  
Radiant Heat ◽  
Complex Nature ◽  
Radiant Heat Flux ◽  
Common Assumption ◽  
Made In

The need to determine the radiant heat flux (RHF) from bushfires for fire behaviour prediction, firefighter safety, or building protection planning purposes has lead to the development and implementation of a number of RHF models, most of which are based on the Stefan-Boltzmann equation of radiative heat transfer. However, because of the complex nature of bushfire flames, a number of assumptions are made in order to make the implementation of the radiative heat transfer equation practical for wildland fire applications. The main assumptions are: bushfire flame characteristics (geometry, temperature), flame radiative qualities (emission type, emissivity), and the view of the flame at the receiving element. The common assumption of a uniform emissivity of unity and an isothermal rectangular emitting surface produces a generic RHF model described here as an 'opaque box'. Because of the broad assumptions inherent in the opaque box model, it predicts the RHF of bushfires poorly. A comparison is made between the generic opaque box RHF model and the measurements of radiant heat flux emitted by a stationary propane-fuelled artificial bushfire flame front. Knowledge about the geometry and an understanding of the flame characteristics of a bushfire front are needed before generic RHF models will adequately describe the RHF emitted from bushfire flames.

Radiative Heat Transfer in Fibrous Insulations—Part I: Analytical Study

1983 ◽  
Vol 105 (1) ◽  
pp. 70-75 ◽  
Author(s):  
T. W. Tong ◽  
C. L. Tien
Keyword(s):  
Heat Transfer ◽  
Heat Flux ◽  
Transport Process ◽  
Analytical Study ◽  
Radiative Heat ◽  
Radiant Heat ◽  
Anisotropic Scattering ◽  
Radiative Properties ◽  
Radiant Heat Flux ◽  
The Mean

The purpose of this work is to develop models for predicting the radiant heat flux in lightweight fibrous insulations (LWFI). The radiative transport process is modeled by the two-flux solution and the linear anisotropic scattering solution of the equation of transfer. The radiative properties of LWFI consistent with these solutions have been determined based on extinction of electromagnetic radiation by the fibers. Their dependence on the physical characteristics of fibrous insulations has been investigated. It has been found that the radiant heat flux can be minimized by making the mean radius of the fibers close to that which yields the maximum extinction coefficient. The results obtained in this study are useful to those concerned with the design and application of LWFI.

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