David R. Weise

Fire spread in chaparral—'go or no-go?'

Current fire models are designed to model the spread of a linear fire front in dead, small-diameter fuels. Fires in predominantly living vegetation account for a large proportion of annual burned area in the United States. Prescribed burning is used to manage living fuels; however, prescribed burning is currently conducted under conditions that result in marginal burning. We do not understand quantitatively the relative importance of the fuel and environmental variables that determine spread in live vegetation. To address these weaknesses, laboratory fires have been burned to determine the effects of wind, slope, moisture content and fuel characteristics on fire spread in fuel beds of common chaparral species. Four species (Adenostoma fasciculatum, Ceanothus crassifolius, Quercus berberidifolia, Arctostaphylos parryana), two wind velocities (0 and 2 m s −1 ) and two fuel bed depths (20 and 40 cm) were used. Oven-dry moisture content of fine fuels (<0.63 cm diameter) ranged from 0.09 to 1.06. Seventy of 125 fires successfully propagated the length (2.0 m) of the elevated fuel bed. A logistic model to predict the probability of successful fire spread was developed using stepwise logistic regression. The variables selected to predict propagation were wind velocity, slope percent, moisture content, fuel loading, species and air temperature. Air temperature and species terms were removed from the model for parsimony. The final model correctly classified 94% of the observations. Comparison of results with an empirical decision matrix for prescribed burning in chaparral suggested some agreement between the laboratory data and the empirical tool.

Firebrands and spotting ignition in large-scale fires

Spotting ignition by lofted firebrands is a significant mechanism of fire spread, as observed in many large-scale fires. The role of firebrands in fire propagation and the important parameters involved in spot fire development are studied. Historical large-scale fires, including wind-driven urban and wildland conflagrations and post-earthquake fires are given as examples. In addition, research on firebrand behaviour is reviewed. The phenomenon of spotting fires comprises three sequential mechanisms: generation, transport and ignition of recipient fuel. In order to understand these mechanisms, many experiments have been performed, such as measuring drag on firebrands, analysing the flow fields of flame and plume structures, collecting firebrands from burning materials, houses and wildfires, and observing firebrand burning characteristics in wind tunnels under the terminal velocity condition and ignition characteristics of fuel beds. The knowledge obtained from the experiments was used to develop firebrand models. Since Tarifa developed a firebrand model based on the terminal velocity approximation, many firebrand transport models have been developed to predict maximum spot fire distance. Combustion models of a firebrand were developed empirically and the maximum spot fire distance was found at the burnout limit. Recommendations for future research and development are provided.

Use of the cone calorimeter to detect seasonal differences in selected combustion characteristics of ornamental vegetation

The flammability of living vegetation is influenced by a variety of factors, including moisture content, physical structure and chemical composition. The relative flammability of ornamental vegetation is of interest to homeowners seeking to make their homes ‘fire safe’. The relative importance of the factors influencing fire behaviour characteristics, such as flammability, is unknown. In the present study, oxygen consumption calorimetry was used to obtain selected combustion characteristics of ornamental vegetation. Peak heat release rate, mass loss rate, time to ignition and effective heat of combustion of 100 × 100-mm samples of foliage and small branches were measured using a bench-scale cone calorimeter. Green and oven-dry samples of 10 species were collected and tested seasonally for a period of 1 year. Similar measurements were made on whole shrubs in an intermediate-scale calorimeter. The range of cone calorimeter peak heat release rates for green and oven-dry samples was 1–176 and 49–331 kW m−2, respectively. Moisture content significantly reduced heat release rates and increased time to ignition. Peak heat release rates for Olea europea and Adenostoma fasciculatum were consistently highest over the year of testing; Aloe sp. consistently had the lowest heat release rate. The correlation of peak heat release rates measured by the cone calorimeter and an intermediate-scale calorimeter was statistically significant yet low (0.51). The use of the cone calorimeter as a tool to establish the relative flammability rating for landscape vegetation requires additional investigation.

Effects of Moisture on Ignition Behavior of Moist California Chaparral and Utah Leaves

Abstract Individual cuttings from eight plant species native to California chaparral or Utah were burned in a well-controlled, well-instrumented facility. Gas temperatures above a flat-flame burner were controlled at 987 ± 12°C and 10 ± 0.5 mol% O2, resulting in a heat flux at the leaf surface varying from 80–140 kW/m2. High moisture leaves were observed to burst due to the rapid escape of vapor from the leaf interior. Bubbles in or on the leaf surface were observed for leaves with moderate moisture contents. A large number of leaf temperature measurements were made, along with measurements of the ignition time and temperature, flame height, and flame duration. Average ignition temperatures were species dependent, ranging from 227°C to 453°C, with a large degree of scatter from leaf to leaf. Correlations of time to ignition and ignition temperature were made, but showed only a weak dependence on leaf thickness and almost no dependence on mass of moisture in the leaf. Leaf samples with similar mass showed that Utah juniper took longer to burn than the other species, and that the Utah broadleaf species burned more rapidly than all the other species.

IGNITION BEHAVIOR OF LIVE CALIFORNIA CHAPARRAL LEAVES

Current forest fire models are largely empirical correlations based on data from beds of dead vegetation. Improvement in model capabilities is sought by developing models of the combustion of live fuels. A facility was developed to determine the combustion behavior of small samples of live fuels, consisting of a flat-flame burner on a moveable platform. Qualitative and quantitative combustion data are presented for representative samples of California chaparral: manzanita (Arctostaphylos parryana); oak (Quercus berberidifolia); ceanothus (Ceanothus crassifolius), and chamise (Adenostoma fasciculatum). Times to ignition were significantly influenced by shape effects, whereas ignition temperature was more dependent on chemical composition.

Experimental measurements during combustion of moist individual foliage samples

Individual samples of high moisture fuels from the western and southern United States and humidified aspen excelsior were burned over a flat-flame burner at 987° ± 12°C and 10 ± 0.5 mol% O2. Time-dependent mass and temperature profiles of these samples were obtained and analysed. It was observed that significant amounts of moisture remained in the individual samples after ignition occurred. Temperature histories showed a plateau at 200°–300°C at the leaf perimeter rather than at 100°C, with a plateau of 140°C for the leaf interior. Implications are that classical combustion models should be altered to reflect the behaviour of moisture in high moisture (live) samples. Mass release rates were determined at ignition and maximum flame height; these appeared to vary due to surface area and perimeter, but no significant correlation was found for all species.

A Simple Physical Model for Forest Fire Spread Rate

Based on energy conservation and detailed heat transfer mechanisms, a simple physical model for fire spread is presented for the limit of one-dimensional steady-state contiguous spread of a line fire in a thermally-thin uniform porous fuel bed. The solution for the fire spread rate is found as an eigenvalue from this model with appropriate boundary conditions through a fourth order Runge-Kutta method. Three experiments on fire spread are compared to the model simulations and good agreement is demonstrated. The comparisons with wind tunnel experiments on white birch fuel beds show that the physics in this model successfully evaluates wind and slope effects on the fire spread rate. The grassland fuel experiments with various fuel characteristics also compare well to the simulations. Limited comparison with data on fire spread in shrubs, obtained in China, also shows good agreement. These comparisons suggest that this model may serve as the basis for an improved operational model.

An Investigation of Crown Fuel Bulk Density Effects on the Dynamics of Crown Fire Initiation in Shrublands 1

Crown fire initiation is studied by using a simple experimental and detailed physical modeling based on Large Eddy Simulation (LES). Experiments conducted thus far reveal that crown fuel ignition via surface fire occurs when the crown base is within the continuous flame region and does not occur when the crown base is located in the hot plume gas region of the surface fire. Accordingly, the focus in this article is on crown fuel ignition when the crown base is situated within the intermittent flame region. In this region, the flame shape and height changes with time over the course of pulsation. This causes the flame to impinge on the crown fuel base and the hot gas is forced through the crown fuel matrix. Under certain conditions, it is observed that the crown fuel bulk density affects the impingement of flame and the ignition of crown fire. The crown fuel properties used were estimated for live chamise (Adenostoma fasciculatum) with a fuel moisture content of 44% (dry basis). As the crown fuel bulk density is increased from 0.75 kg·m−3 to 1.75 kg·m−3, it is observed that the average hot gas velocity inside the crown matrix decreases from 0.70 m·s−1 to 0.52 m·s−1, thus, resulting in less entrained air passing through the crown fuel and more energy accumulation inside the crown fuel matrix. Higher bulk density also influences the surface fire. As the hot gas flows into the crown fuel matrix is retarded, the average hot gas temperature at the crown fuel base increases from 768 K to 1,205 K. This is because the mixing rate of air and combustible gas around the base of crown fuel increases. Although higher fuel bulk density means more fuel must be heated, the increase in accumulated energy per unit volume within the crown fuel matrix is higher than the additional heat needed by the fuel. Thus, the average crown fuel temperature increases and ignition occurs at higher bulk density.

A numerical study of slope and fuel structure effects on coupled wildfire behaviour.

Slope and fuel structure are commonly accepted as major factors affecting the way wildfires behave. However, it is possible that slope affects fire differently depending on the fuel bed. Six FIRETEC simulations using three different fuel beds on flat and upslope topography were used to examine this possibility. Fuel beds resembling grass, chaparral, and ponderosa pine forests were created in such a way that there were two specific locations with identical local fuel beds located around them. These fuel beds were each used for a flat-terrain simulation and an idealised-hill simulation in order to isolate the impacts of the topography without the complications of having different local fuels. In these simulations, fuel bed characteristics have a significant effect on the spread rate and perimeter shape of the fires on both flat ground and on the idealised smooth hill topography. The analysis showed that these simulated fires evolved as they travelled between the locations even on flat ground, and the accelerations and decelerations that affect the fire occurred at different times and at different rates depending on the fuel bed. The results of these simulations and analyses indicate that though some general principles are true for all fuel beds, there are differences in the way that fires react to non-homogeneous topographies depending on the fuel bed.

Entrainment regimes and flame characteristics of wildland fires

This paper reports results from a study of the flame characteristics of 22 wind-aided pine litter fires in a laboratory wind tunnel and 32 field fires in southern rough and litter-grass fuels. Flame characteristic and fire behaviour data from these fires, simple theoretical flame models and regression techniques are used to determine whether the data supportthederivedmodels.Whenthedatadonotsupportthemodels,alternativemodelsaredeveloped.Theexperimental fires are used to evaluate entrainment constants and air/fuel mass ratios in the model equations. Both the models and the experimental data are consistent with recently reported computational fluid dynamics simulations that suggest the existence of buoyancy- and convection-controlled regimes of fire behaviour. The results also suggest these regimes are delimited by a critical value of Byrams convection number. Flame heights and air/fuel ratios behave similarly in the laboratory and field, but flame tilt angle relationships differ. Additional keywords: air/fuel mass ratio, combustion regimes, entrainment constant, flame height, flame tilt angle.