Experimental investigation of structure vulnerabilities to firebrand showers

Attempting to experimentally quantify the vulnerabilities of structures to ignition from firebrand showers has remained elusive. The coupling of two facilities has begun to unravel this difficult problem. The NIST Firebrand Generator (NIST Dragon) is an experimental device that can generate a firebrand shower in a safe and repeatable fashion. Since wind plays a critical role in the spread of WUI fires in the USA and urban fires in Japan, NIST has established collaboration with the Building Research Institute (BRI) in Japan. BRI maintains one of the only full scale wind tunnel facilities in the world designed specifically for fire experimentation; the Fire Research Wind Tunnel Facility (FRWTF). The present investigation is aimed at extensively quantifying firebrand penetration through building vents using full scale tests. A structure was placed inside the FRWTF and firebrand showers were directed at the structure using the NIST Dragon. The structure was fitted with a generic building vent, consisting of only a frame fitted with a metal mesh. Six different mesh sizes openings were used for testing. Behind the mesh, four different materials were placed to ascertain whether the firebrands that were able to penetrate the building mesh assembly could ignite these materials. Reduced scale test methods afford the capability to test new vent technologies and may serve as the basis for new standard testing methodologies. As a result, a new experimental facility developed at NIST is presented and is known as the NIST Dragon's LAIR (Lofting and Ignition Research). The NIST Dragon's LAIR has been developed to simulate a wind driven firebrand attack at reduced scale. The facility consists of a reduced scale Firebrand Generator (Baby Dragon) coupled to a bench scale wind tunnel. Finally, a series of full scale experiments were conducted to visualize the flow of firebrands around obstacles placed downstream of the NIST Dragon. Firebrands were observed to accumulate in front of these obstacles at a stagnation plane, as was observed when the structure was used for firebrand penetration through building vent experiments, due to flow recirculation. The accumulation of firebrands at a stagnation plane presents a severe threat to ignitable materials placed near structures.     

Experiments to provide the scientific-basis for laboratory standard test methods for firebrand exposure

Firebrand ignition of structures is a major factor in large outdoor fire spread. Standard laboratory test methods are required to evaluate and compare the performance of different building elements and/or vegetative fuels ability to resist firebrand ignition. It is important to determine full-scale assembly performance when exposed to firebrand showers since weak points in a given assembly can be investigated. Such studies will lead to determining the necessary configuration of building component mock-ups that can be used in standard laboratory test methods. The basis of this paper is to present a comparison of results from full-scale roofing assembly experiments, to mockups using the recently developed experimental capability at National Research Institute of Fire and Disaster (NRIFD). The results demonstrated that similar firebrand penetration behavior/trends were observed for mock-ups of full-scale roofing assemblies, as compared to experiments where full-scale roofing assemblies were used, all under similar wind speeds. The experimental findings presented in this paper represent an important step to develop reduced-scale test methods for firebrand exposure.     

Quantifying the vulnerabilities of ceramic tile roofing assemblies to ignition during a firebrand attack

An experimental campaign was conducted to investigate the vulnerabilities of ceramic tile roofing assemblies to ignition under a controlled firebrand attack using the NIST firebrand generator. The results of a parametric study on the ignition propensity of ceramic tile roofing assemblies under a firebrand attack using the firebrand generator installed inside the Fire Research Wind Tunnel Facility (FRWTF) at the Building Research Institute in Tsukuba, Japan is presented. Over the range of parameters considered, ceramic tile roofing assemblies were found to be vulnerable to ignition during a firebrand attack.     

Experimental investigation of firebrands: Generation and ignition of fuel beds

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 results of the real scale fire experiments were used to determine firebrand sizes to perform reduced scale ignition studies of fuel beds in contact with burning firebrands. The firebrand ignition apparatus allowed for the ignition and deposition of both single and multiple firebrands onto the target fuel bed. The moisture content of the fuel beds used was varied and the fuels considered were pine needle beds, shredded paper beds, and shredded hardwood mulch. Firebrands were constructed by machining wood (Douglas Fir) into small cylinders of uniform geometry and the size of the cylinders was varied. The firebrand ignition apparatus was installed into the Fire Emulator/Detector Evaluator (FE/DE) to investigate the influence of an air flow on the ignition propensity of fuel beds. Results of this study are presented and compared to relevant studies in the literature.     

Firebrands generated from a full-scale structure burning under well-controlled laboratory conditions

Firebrand production from a real-scale structure under well-controlled laboratory conditions was investigated. The structure was fabricated using wood studs and oriented strand board (OSB). The entire structure was placed inside the Building Research Institute's (BRI) Fire Research Wind Tunnel Facility (FRWTF) in Japan to apply a wind field of 6 m/s onto the structure. As the structure burned, firebrands were collected using an array of water pans. The size and mass distributions of firebrands collected in this study were compared with sparsely available firebrand generation data from actual full-scale structure burns, individual building component tests, and historical structure fire firebrand generation studies. In this experiment, more than 90% of firebrands were less than 1 g and 56% were less than 0.1 g. It was found that size and mass of firebrands collected in this study were similar to the literature studies, yet differences existed as well. Different experimental conditions, as well as varied firebrand collection strategies, were believed to be responsible for the differences in firebrand size and mass measured in the present work, and those in the literature. The present study has provided much needed data on firebrand generation from structures.     

Experimental investigation of firebrand accumulation zones in front of obstacles

It is well accepted that as structures are exposed to wind, stagnation planes are produced around structures. Past work by the authors demonstrated for the first-time that wind-driven firebrand showers may accumulate in these stagnation planes. While those experiments demonstrated this important phenomenon, due to the limited duration of firebrand showers of the original NIST Batch-Feed Firebrand Generator, it was not possible to perform a more systematic study. To this end, a series of detailed experiments were performed using the recently developed NIST Continuous-Feed Firebrand Generator capable of firebrand showers of unlimited duration. Full-scale walls of varying size were placed downstream of the device and the wind speed was varied in increments up to 10 m/s. The experiments were conducted in the Building Research Institute's Fire Research Wind Tunnel Facility (FRWTF). For a given wall size exposed to specific firebrand size/mass distribution, it was observed that wind speed influences not only the spatial location and extent of the accumulated firebrands in the stagnation plane in front of the wall, but also the nature of the smoldering combustion intensity of the accumulated firebrands. The experiments demonstrated that higher wind speeds (10 m/s) did not promote accumulation of firebrands in stagnation planes in front of walls. The data may be used to provide guidance to appropriate separation distances that combustibles should be placed near structures and is also of great use to develop and validate numerical models of firebrand accumulation.     

Investigation on the ability of glowing firebrands deposited within crevices to ignite common building materials

A series of experiments was conducted to determine the range of conditions that glowing firebrands may ignite common building materials. The surface temperature of glowing firebrands burning under different applied airflow was quantified using an infrared camera. As the applied airflow was increased, the surface temperature of glowing firebrands was observed to increase. A crevice was constructed using plywood and oriented strand board (OSB) and the angle was varied to investigate the influence that this parameter has on promoting ignition after contact with glowing firebrands. The number of firebrands deposited within the constructed crevices was varied. Single firebrands were unable to ignite the materials used in this study over a range of applied airflows. For the tightest fuel bed angle of 60°, the glowing firebrands deposited on the fuel bed always resulted in smoldering ignition. For plywood, contact with glowing firebrands produced smoldering ignition followed by a transition to flaming ignition. At the fuel bed angle of 90°, no definitive ignition behavior was observed for either material; different ignition criteria (either no ignition or smoldering ignition) were observed under identical experimental conditions. As the fuel bed angle was increased up to 135°, ignition never occurred for both test fuel beds. For a given airflow and fuel bed material, the ignition delay time was observed to increase as the fuel bed angle was increased. A large difference was observed in the ignition delay time for plywood and OSB at a fuel bed angle of 90°. Based on these ignition results, the critical angle for ignition exists between 90° and 135° at a given airflow. These results clearly demonstrate that firebrands are capable of igniting common building materials.     

Summary of workshop for fire structure interaction and urban and wildland-urban interface (WUI) Fires–operation Tomodachi–fire research

A workshop, known as “Operation Tomodachi—Fire Research” was held in Tokyo, Japan from July 1 to July 4, 2012. Tomodachi means friendship in Japanese. This workshop, under the direction of Dr. Samuel L. Manzello of EL-NIST and Dr. Tokiyoshi Yamada of the University of Tokyo, was conducted in partnership with the Japan Association of Fire Science and Engineering (JAFSE). The objective was to: (1) develop scientific knowledge and translate it to building codes and standards that will be of use to both countries to reduce the devastation caused by unwanted fires, (2) provide a forum for next generation researchers to present their work in order to develop new research collaborations, (3) and allow USA participants a chance to visit excellent large-scale research facilities available in Japan that are of use to the research topics of this workshop. This is a formal continuation of the kickoff meeting held at NIST's Engineering Laboratory (EL-NIST) in June 2011. USA presentations were delivered from: NIST, Purdue University, University of Texas-Austin, Michigan State University, University of Michigan, Insurance Institute for Business and Home Safety (IBHS), Worcester Polytechnic Institute (WPI), University of California-Berkeley, California Polytechnic University (CALPOLY), Underwriters Laboratories (UL), and the University of Delaware (organizations are listed based on the order of oral presentation). Japanese presentations were delivered from: The University of Tokyo, Building Research Institute (BRI), Takenaka Corporation, Center for Better Living, Shimizu Corporation, Tokyo University of Science (TUS), National Institute for Land and Infrastructure Management (NILIM), Kyoto University, National Research Institute of Fire and Disaster (NRIFD), Yamagata University, and Kobe University (organizations are listed based on the order of oral presentation). All of the presentations are documented in a recent NIST Special Publication (NIST SP 1137). The present paper provides a detailed summary for the need of this workshop as well as the findings obtained from the event. It is desired that this activity will motivate the next generation of researchers to explore and develop research collaborations related to emerging areas of fire safety science. The authors are hopeful that new and exciting activities specific to other countries may come out of this type of event.     

Development of rapidly deployable instrumentation packages for data acquisition in wildland–urban interface (WUI) fires

In an effort to quantify structure ignition mechanisms during wildland–urban interface (WUI) fires, rapidly deployable instrumentation packages were developed. For a structure under a WUI fire attack, the packages are designed to: (1) provide temporally resolved images of structure ignition mechanisms and (2) provide quantitative data on total heat flux, wind speed, wind direction, ambient temperature, and relative humidity near a structure. The unique design of the packages allowed for wireless transmission of all data signals collected to a hardened location. Prior to attempting to use these instrumentation packages in real WUI fires, a proof-of-concept test was conducted under a prescribed fire. In these tests, a shed was used as a surrogate for a typical structure that would be found in the WUI. The proof-of-concept test was successful and has demonstrated that relatively inexpensive instrumentation can be used to image structure ignition in the path of an approaching crown fire and that directional flame thermometers (DFT) were acceptable instrumentation to measure total heat flux in place of cumbersome water cooled total heat flux gages.     

Smoke concentrations inside and outside of a corridor-like enclosure

This work presents smoke measurements and correlations inside and outside of a corridor-like enclosure fires in order to determine the effects of burning on smoke concentrations inside and outside the enclosure. Thirty eight experiments were performed in a three metre long corridor-like enclosure having a cross section 0.5 m×0.5 m, door like openings in the front panel and a gaseous burner located near the closed end. Smoke concentrations were measured at two locations inside the enclosure and also in the exhaust duct of a hood collecting the fire gases from the enclosure. It was found that smoke concentration in the exhaust duct decreased whereas smoke concentration inside the enclosure increased after the flames started moving towards the opening and external burning occurred. This increased smoke concentration inside the enclosure was caused by reversion of the flow pattern inside the enclosure after the flames moved past a point towards the opening. Namely, the flow pattern changed direction behind the flame front in the sense that hot gases in the upper layer were travelling backwards towards the closed end of the corridor thus contributing to smoke increase inside the enclosure. This change of flow pattern was confirmed in all experiments by bidirectional probe velocity measurements in the upper and lower layer as well as by oxygen concentrations and temperature measurements inside the enclosure. These results are useful for CFD validation and specifically applicable for assessing smoke hazards in corridor fires in buildings where smoke concentrations can be much larger than anticipated owing to leakage to adjacent rooms behind a moving flame front.     

Experimental validation of a numerical model for the transport of firebrands

The paper deals with the experimental validation of a numerical model for the transport and combustion of cylindrical and disk-shaped firebrands. The model solves the conservation equations of brand mass, kinetic and angular momentum, and volume. Validation consists in predicting the mass and spatial distributions of glowing firebrands that were produced from the experimental generator developed by Manzello and coworkers [S.L. Manzello, J.R. Shields, T.G. Cleary, A. Maranghides, W.E. Mell, J.C. Yang, Y. Hayashi, D. Nii, T. Kurita, On the development and characterization of a firebrand generator, Fire Saf. J. 43 (2008) 258–268]. Ten thousand firebrands per run are released with initial conditions that are randomly generated according to probability distribution functions deduced from experimental mass and spatial distributions under no-wind conditions. Whatever the wind conditions considered, numerical results are found to be in good agreement with experimental data.     

Stochastic modeling of firebrand shower scenarios

Firebrand shower and its subsequent spot fires are responsible for more than half of the ignitions reported during wildfires, in particular at wildland urban interface (WUI) areas. The firebrand transport is a highly stochastic and nonlinear problem which directly influences the spotting distribution. Hence, a coupled stochastic model of firebrand showers, that is thoroughly and systematically validated against large scale wind tunnel experiments of lofting and downwind transport of model firebrands, is presented. It is shown that the developed model predicts the first and second order statistics of the flight accurately in relation to the experimental data. The sensitivity of the model to the initial conditions of the flight as well as the velocity field characteristics are examined.     

Verification of a Lagrangian particle model for short-range firebrand transport

Firebrands are a harbinger of damage to infrastructure; their effects cause a particularly important threat to people living within the wildland-urban-interface. Short-range firebrands travel with the wind with little or no lofting, and cause spotfires. In this work, the design of a novel firebrand generator prototype is discussed to achieve a uniform shower of firebrands. The transport of short-range firebrand is studied to verify the existing Lagrangian particle model of Fire Dynamics Simulator. Uniform, non-combusting cubiform and cylindrical firebrands are projected using the firebrand generator. The experimentally observed distribution of particles on the ground is compared with a simulated distribution using the fire dynamic simulator. The results show that the existing Lagrangian model gives a good agreement with the experimental data.     

公共安全视阈下的森林火灾应急产业发展对策研究

森林火灾作为最为严重的自然灾害和公共危机事件之一,是国家公共应急体系建设的重要内容。推进森林火灾应急产业体系构建意义重大,不仅为有预防和应对森林火灾提供应急物质保障、技术支持和专业服务,也有利于森林火灾相关行业的产业结构优化和调整,促进相关产业自主创新和技术研发的进步。从政策、技术、服务、教育、市场等方面出发,分析了现阶段森林火灾应急产业体系存在的问题,并从产业扶持机制、市场供给体系、核心技术培优机制、产业服务体系、专门人才培养机制等方面提出了加快推动森林火灾应急产业体系发展的对策建议。     

Time-dependent smoke yield and mass loss of pool fires in a reduced-scale mechanically ventilated compartment

Technical and pure grades of the combustibles heptane and dodecane were used in a series of small-scale fire tests conducted in a 1 m³ compartment that was mechanically ventilated at 5 and 8 air changes per hour (ACH). Combustible mass loss rates, soot mass concentrations, soot size distributions, several gas species concentrations, and compartment temperatures were measured during the fire. Results for the two pure-grade hydrocarbons were compared with results obtained for their respective technical grades. Technical-grade dodecane produced the highest soot emissions; pure n-heptane produced the lowest. Soot size distributions of all four combustibles attained a steady profile whose modal diameter was about 200 nm. Underventilated fires showed higher carbon monoxide yields than soot yields. Both compartment ventilation rates produced similar results, although the fire self-extinguished earlier for 5 ACH.     

Smoke production,radiation heat transfer and fire growth in a liquid-fuelled compartment fire

A detailed investigation is described of the interaction between fire development, smoke production and radiative exchange in a half-scale ASTM compartment in which the source is a heptane pool fire. Measurements of heat flux, fuel mass loss rate, ventilation flow rates, temperature and soot volume fraction are reported for the compartment for varying door widths. Data from the compartment are compared with open pool fire measurements using the same equipment. The confined geometry is shown to exert a strong influence on pool fire development and suggests that considerable caution is needed in employing open pool fire data as boundary conditions for CFD simulation. Numerical simulations based on the direct calculation of radiative exchange between the liquid fuel surface, the smoke-laden environment and bounding walls do reproduce the behaviour observed when combustion, soot production and radiation are modelled in detail and finely resolved spatially.     

Determination of smoldering time and thermal characteristics of firebrands under laboratory conditions

The laboratory experiment was conducted to simulate the transfer of smouldering particles produced in forest wildfires by a heated gas flow. The pine bark pieces with the linear dimensions L=(15; 20; 30) mm and a thickness of h=(4−5) mm were selected as model particles. The rate and temperature of the incident flow varied in the range of 1–3 m/s and 80–85 °C, respectively. The temperature of the samples was recorded using a thermal imager. To determine the minimum smouldering temperature of pine bark, the thermal analysis was conducted. The minimum smouldering temperature of pine bark was found to be 190 °C. This temperature will cause thermal decomposition of bark only at the first stage (oxidation of resinous components). In the study the smouldering time, the temperature and the weight of samples were obtained and analyzed under various experimental conditions. The data analysis shows that the increase in the particle size leads to the decrease in their mass loss, and the rate change of the incident flow does not practically influence the mass change. For particles with the linear dimensions of 10 mm and 20 mm, the mass varies from 6% to 25%. The maximum mass loss is observed for the flows with a rate of 1 and 2 m/s. The results have shown that the increase in the particle size leads to the increase in the smouldering time. The position of the particle plays an important role, the effect of which increases with increasing the particle size. The calculations showed that the smouldering time of bark samples is long enough for the particles to serve as new sources of spot fires. The particles were found to be transported to a distance of 218 m from the fire line which can certainly influence the propagation of the fire front.     

Predicting the heat release rates of liquid pool fires in mechanically ventilated compartments

In this paper we perform predictive simulations of liquid pool fires in mechanically ventilated compartments. We show that steady state burning rates are accurately predicted using a detailed model for the liquid phase heat transfer. The effect of lowered oxygen vitiation on the burning rate of pool fires is correctly captured. Simulations were done using the Fire Dynamics Simulator and the experiments considered were conducted in the OECD PRISME project. The main difference between the present study and previous simulation studies is the use of a detailed liquid evaporation model and the direct calculation of the vitiation and thermal environment interactions through the CFD solver.