Samuel L. Manzello

The wildland-urban interface fire problem - current approaches and research needs

Wildfires that spread into wildland–urban interface (WUI) communities present significant challenges on several fronts. In the United States, the WUI accounts for a significant portion of wildland fire suppression and wildland fuel treatment costs. Methods to reduce structure losses are focussed on fuel treatments in either wildland fuels or residential fuels. There is a need for a well-characterised, systematic testing of these approaches across a range of community and structure types and fire conditions. Laboratory experiments, field measurements and fire behaviour models can be used to better determine the exposure conditions faced by communities and structures. The outcome of such an effort would be proven fuel treatment techniques for wildland and residential fuels, risk assessment strategies, economic cost analysis models, and test methods with representative exposure conditions for fire-resistant building designs and materials.

Ignition of mulch and grasses by firebrands in wildland–urban interface fires*

Firebrands or embers are produced as trees and structures burn in wildland–urban interface (WUI) fires. It is believed that firebrand showers created in WUI fires may ignite vegetation and mulch located near homes and structures. This, in turn, may lead to ignition of homes and structures due to burning vegetation and mulch. Understanding the ignition events that are due to firebrands is important to mitigate fire spread in communities. To assess the ignition propensity of such materials, simulated firebrands of uniform geometry, but in two different sizes, were allowed to impinge on fuel beds of shredded hardwood mulch, pine straw mulch, and cut grass. The moisture content of these materials was varied. Firebrands were suspended and ignited within the test cell of the Fire Emulator/Detector Evaluator (FE/DE) apparatus. The FE/DE was used to investigate the influence of an air flow on the ignition propensity of a fuel bed. Ignition regime maps were generated for each material tested as a function of impacting firebrand size, number of deposited firebrands, air flow, and material moisture content.

Firebrand generation from burning vegetation

A series of real-scale fire experiments were performed to determine the size and mass distribution of firebrands generated from Douglas-fir (Pseudotsuga menziesii) trees. The experiments were performed in the Large Fire Laboratory at the National Institute of Standards and Technology. The Douglas-fir trees used for the experiments ranged in total height from 2.6 to 5.2 m and the tree moisture content was varied. An array of pans filled with water was used to collect the firebrands that were generated from the burning trees. This ensured that firebrands would be quenched as soon as they made contact with the pans. The firebrands were subsequently dried and the sizes were measured using callipers and the dry mass was determined using a precision balance. For all experiments performed, the firebrands were cylindrical in shape. The average firebrand size measured from the 2.6-m Douglas-fir trees was 3 mm in diameter, 40 mm in length. The average firebrand size measured for the 5.2-m Douglas-fir trees was 4 mm in diameter with a length of 53 mm. The mass distribution of firebrands produced from two different tree sizes under similar tree moisture levels was similar. The only noticeable difference occurred in the largest mass class. Firebrands with masses up to 3.5 g to 3.7 g were observed for the larger tree height used (5.2 m). The surface area of the firebrands scaled with firebrand weight.

On the collision dynamics of a water droplet containing an additive on a heated solid surface

An experimental study is presented for water–droplet impingement containing an additive upon a heated stainless–steel surface. A solution of 30% (mass fraction) sodium acetate trihydrate, CH3COONa3H2O, was used for the experiments. The impaction process was recorded using a high–speed digital camera at 1000 frames per second. The initial droplet diameter was fixed at 2.7 ± 0.1 mm and all experiments were performed in atmospheric air. Three different impact Weber numbers were considered, namely 15, 80 and 181. The droplet–evaporation lifetime was measured as a function of temperature for 30% (mass fraction) sodium acetate trihydrate. Collision dynamics were investigated for each impact Weber number, with the temperature of the stainless–steel surface varied from film evaporation to film boiling. The temporal variation of the liquid–film diameter was measured as a function of temperature for each impact Weber number. Experiments were also performed using water for direct comparison of the collision dynamics with the additive–containing droplets. The collision dynamics were observed to be different for water droplets containing an additive at low impact Weber number, but became increasingly similar as the impact Weber number was increased.

An experimental study of high Weber number impact of methoxy-nonafluorobutane C4F9OCH3 (HFE-7100) and n-heptane droplets on a heated solid surface ☆

Abstract An experimental study is presented for methoxy-nonafluorobutane (C4F9OCH3, HFE-7100) droplet impingement on a heated stainless steel surface. The impaction process was recorded using a high-speed digital camera at 1000 frames per second. The initial droplet diameter was fixed at 1.7±0.1 mm , and all experiments were performed in atmospheric air. The impact velocity was fixed at 2.0 m/s thus defining an impact Weber number of 750. The temperature of the stainless steel surface was varied from 20 to 300 °C, above the Leidenfrost temperature of HFE-7100. Experiments were also performed using n-heptane to investigate whether the collision dynamics were similar if the impact Weber number was matched to HFE-7100 and collision considered within the same boiling regimes as HFE-7100. While the collision dynamics were qualitatively similar, the evolution of liquid film diameter with time was different. Existing models used to describe the evolution of liquid film diameter with time were found to be inadequate to describe HFE-7100 and n-heptane impact.

Measurement of visible and near-IR optical properties of soot produced from laminar flames

This study describes the measurements of the dimensionless extinction constant, K e , of soot in the visible and IR spectrum using the National Institute of Standards and Technology Large Agglomerate Optics Facility. Soot was produced using a 11 mm i.d. laminar diffusion flame burner fueled with acetylene and ethene. Light extinction measurements were performed using light sources at 543.5, 632.8, 856, 1314, and 1565 nm. The mean values of present measurements of K e range from 7.95 to 10.0. These unique experiments provide accurate values of K e to be used for measurements of soot concentration and temperature in the IR spectrum. These measurements represent the first fuel-specific data available in the near-IR spectrum. The measured K e values for all wavelengths are significantly larger than values calculated using reported values of the refractive index and the Rayleigh theory. Transmission electron microscopy and optical microscopy analyses were used to analyze soot morphology and aerosol properties to estimate the influences of beam shielding and light scattering on the observed variations of K e .

Measurement of the Optical Extinction Coefficients of Post-Flame Soot in the Infrared

Abstract The optical extinction coefficients of post-flame soot have been measured in the wavelength range 2.8 to 4.1 μm. A laminar diffusion burner was combined with an infrared spectrograph and gravimetric measurements to determine the mass specific extinction coefficient, σs, and the dimensionless extinction coefficient, Ke. Using ethene gas as the fuel, the burner was operated at four global equivalence ratios (φ = 0.8, 1.0, 2.0, and 3.0) to examine the effect of the fuel-air ratio on the extinction coefficient. The extinction coefficient was found to decrease with increasing values of the global equivalence ratio for φ = 1.0, 2.0, and 3.0. The results for φ = 0.8 and φ = 1.0 were in agreement to within the uncertainty of the measurements. Measurements were obtained using propane gas as the fuel (φ = 1.0) and resulted in extinction coefficients equivalent to those of ethene. Transmission electron microscopy (TEM) images revealed differences in the morphology of the particles, consistent with the quantitative differences observed in the extinction data. The data indicate that the equivalence ratio has a strong effect on the optical properties of post-flame soot agglomerates.

On the interaction of a liquid droplet with a pool of hot cooking oil

An experimental study is presented for distilled water droplets impacting on a heated pool of cooking oil. The impaction process was recorded using a high-speed digital camera at 1000 frames per second. The initial droplet diameter was fixed at 3.170.1 mm and all experiments were performed at room temperature (20 � C). The impact Weber (We) number of the water droplets was fixed at 200. As the water droplet impacted the hot peanut oil pool, it fragmented, and ultimately produced a vapor explosion. Experiments were also performed applying methoxy-nonafluorobutane C4F9OCH3 (HFE-7100) to hot peanut oil with similar impact We number. Dramatic differences were observed when HFE-7100 droplets were used. At peanut oil temperatures above E180 � C, HFE-7100 droplets did not result in a vapor explosion. Published by Elsevier Science Ltd.

Firebrand generation data obtained from a full-scale structure burn

A full-scale, proof-of-concept experiment was conducted to investigate firebrand production from a burning structure. In this experiment, researchers from National Institute of Standards and Technology (NIST) were invited to set up instrumentation and collect firebrands using an array of water pans during a structure burn-down. The size and mass distribution of firebrands collected from the burning structure was compared with those measured from vegetation as well as historical firebrand investigations and found to be larger and broader than those of prior studies from historical firebrand investigations. Language: en

Investigating the Vulnerabilities of Structures to Ignition From a Firebrand Attack

A unique experimental apparatus, known as the Firebrand Generator, was used to generate a controlled and repeatable size and mass distribution of glowing firebrands. The size and mass distribution of firebrands produced from the generator was selected to be representative of firebrands produced from burning vegetation. The vulnerability of roofing materials to firebrand attack was ascertained using fluxes of firebrands produced using this device. The experiments were performed at the Fire Research Wind Tunnel Facility (FRWTF) at the Building Research Institute (BRI) in Tsukuba, Japan. A custom mounting assembly was constructed to support full scale sections of common roofing materials inside the FRWTF. The sections constructed for testing included full roofing assemblies (base layer, tar paper, and shingles) as well as only the base layer material, such as oriented strand board (OSB). The custom mounting assembly allowed for the construction of flat roofs as well as the construction of angled roofs (valleys). Results of this study are presented and discussed.