Jason Forthofer

Predicting the ignition of crown fuels above a spreading surface fire. Part I: model idealization

A model was developed to predict the ignition of forest crown fuels above a surface fire based on heat transfer theory. The crown fuel ignition model (hereafter referred to as CFIM) is based on first principles, integrating: (i) the characteristics of the energy source as defined by surface fire flame front properties; (ii) buoyant plume dynamics; (iii) heat sink as described by the crown fuel particle characteristics; and (iv) energy transfer (gain and losses) to the crown fuels. Fuel particle temperature increase is determined through an energy balance relating heat absorption to fuel particle temperature. The final model output is the temperature of the crown fuel particles, which upon reaching ignition temperature are assumed to ignite. CFIM predicts the ignition of crown fuels but does not determine the onset of crown fire spread per se. The coupling of the CFIM with models determining the rate of propagation of crown fires allows for the prediction of the potential for sustained crowning. CFIM has the potential to be implemented in fire management decision support systems.

Role of buoyant flame dynamics in wildfire spread

Significance Wildfires burn millions of hectares per year on every inhabited continent, but the physical mechanism governing spread is not known. Models of wildfire spread are widely used for prediction, firefighter training, and ecological research but have assumed various formulations of known heat transfer processes (radiation and convection) absent a definitive theory of their organization. New experimental evidence reported here reveals how buoyancy generated by the fire induces vorticity and instabilities in the flame zone that control the convective heating needed to ignite fuel particles and produce spread. Large wildfires of increasing frequency and severity threaten local populations and natural resources and contribute carbon emissions into the earth-climate system. Although wildfires have been researched and modeled for decades, no verifiable physical theory of spread is available to form the basis for the precise predictions needed to manage fires more effectively and reduce their environmental, economic, ecological, and climate impacts. Here, we report new experiments conducted at multiple scales that appear to reveal how wildfire spread derives from the tight coupling between flame dynamics induced by buoyancy and fine-particle response to convection. Convective cooling of the fine-sized fuel particles in wildland vegetation is observed to efficiently offset heating by thermal radiation until convective heating by contact with flames and hot gasses occurs. The structure and intermittency of flames that ignite fuel particles were found to correlate with instabilities induced by the strong buoyancy of the flame zone itself. Discovery that ignition in wildfires is critically dependent on nonsteady flame convection governed by buoyant and inertial interaction advances both theory and the physical basis for practical modeling.

Measurements of convective and radiative heating in wildland fires

Time-resolved irradiance and convective heating and cooling of fast-response thermopile sensors were measured in 13 natural and prescribed wildland fires under a variety of fuel and ambient conditions. It was shown that a sensor exposed to the fire environment was subject to rapid fluctuations of convective transfer whereas irradiance measured by a windowed sensor was much less variable intime, increasing nearly monotonically with the approach of the flamefrontandlargelydecliningwithitspassage.Irradiancebeneathtwocrownfirespeakedat200and300kWm � 2 ,peak irradiance associated with fires in surface fuels reached 100kWm � 2 and the peak for three instances of burning in shrub fuels was 132kWm � 2 . The fire radiative energy accounted for 79% of the variance in fuel consumption. Convective heatingatthesensorsurfacevariedfrom15%tovaluesexceedingtheradiativeflux.Detailedmeasurementsofconvective and radiative heating rates in wildland fires are presented. Results indicate that the relative contribution of each to total energy release is dependent on fuel and environment.

A comparison of three approaches for simulating fine-scale surface winds in support of wildland fire management. Part I. Model formulation and comparison against measurements

For this study three types of wind models have been defined for simulating surface wind flow in support of wildland fire management: (1) a uniform wind field (typically acquired from coarse-resolution (~4km) weather service forecast models); (2) a newly developed mass-conserving model and (3) a newly developed mass and momentum-conserving model (referred to as the momentum-conserving model). The technical foundation for the two new modelling approaches is described, simulated surface wind fields are compared to field measurements, and the sensitivity of the new model types to mesh resolution and aspect ratio (second type only) is discussed. Both of the newly developed models assume neutral stability and are designed to be run by casual users on standard personal computers. Simulation times vary from a few seconds for the mass-conserving model to ~1h for the momentum-conserving model using consumer-grade computers. Applications for this technology include use in real-time fire spread prediction models to support fire management activities, mapping local wind fields to identify areas of concern for firefighter safety and exploring best-case weather scenarios to achieve prescribed fire objectives. Both models performed best on the upwind side and top of terrain features and had reduced accuracy on the lee side. The momentum-conserving model performed better than the mass-conserving model on the lee side.

An examination of flame shape related to convection heat transfer in deep-fuel beds.

Fire spread through a fuel bed produces an observable curved combustion interface. This shape has been schematically represented largely without consideration for fire spread processes. The shape and dynamics of the flame profile within the fuel bed likely reflect the mechanisms of heat transfer necessary for the pre-heating and ignition of the fuel during fire spread. We developed a simple laminar flame model for examining convection heat transfer as a potentially significant fire spread process. The flame model produced a flame profile qualitatively comparable to experimental flames and similar to the combustion interface of spreading fires. The model comparison to flame experiments revealed that at increasing fuel depths (>0.7 m), lateral flame extension was increased through transition and turbulent flame behaviour. Given previous research indicating that radiation is not sufficient for fire spread, this research suggests that flame turbulence can produce the convection heat transfer (i.e. flame contact) necessary for fire spread particularly in vertically arranged, discontinuous fuels such as shrub and tree canopies.

A comparison of three approaches for simulating fine-scale surface winds in support of wildland fire management. Part II. An exploratory study of the effect of simulated winds on fire growth simulations

The effect of fine-resolution wind simulations on fire growth simulations is explored. The wind models are (1) a wind field consisting of constant speed and direction applied everywhere over the area of interest; (2) a tool based on the solution of the conservation of mass only (termed mass-conserving model) and (3) a tool based on a solution of conservation of mass and momentum (termed momentum-conserving model). Fire simulations use the FARSITE fire simulation system to simulate fire growth for one hypothetical fire and two actual wildfires. The momentum-conserving model produced fire perimeters that most closely matched the observed fire spread, followed by the mass-conserving model and then the uniform winds. The results suggest that momentum-conserving and mass-conserving models can reduce the sensitivity of fire growth simulations to input wind direction, which is advantageous to fire growth modellers. The mass-conserving and momentum-conserving wind models may be useful for operational use as decision support tools in wildland fire management, prescribed fire planning, smoke dispersion modelling, and firefighter and public safety.

An Evaluation of NDFD Weather Forecasts for Wildland Fire Behavior Prediction

AbstractWildland fire managers in the United States currently utilize the gridded forecasts from the National Digital Forecast Database (NDFD) to make fire behavior predictions across complex lands...

Section B Fire and Explosion - A Study of Flame Spread in Engineered Cardboard Fuel Beds Part I: Correlations and Observations of Flame Spread

Wind-aided laboratory fires spreading through laser-cut cardboard fuel beds were instrumented and analyzed for physical processes associated with spread. Flames in the spanwise direction appeared as a regular series of peaks and troughs that scaled directly with flame length. Flame structure in the stream-wise direction fluctuated with the forward advection of coherent parcels that originated near the rear edge of the flame zone. Thermocouples arranged longitudinally in the fuel beds revealed the frequency of temperature fluctuations decreased with flame length but increased with wind speed. The downstream extent of these fluctuations from the leading flame edge scaled with Froude number and flame zone depth. The behaviors are remarkably similar to those of boundary layers, suggesting a dominant role for buoyancy in determining wildland fire spread.