An official website of the United States government
Firebrand (ember) attack has been shown to be one of the key mechanisms of wildfire spread into wildland–urban interface communities. After the firebrands land on a substrate material, the ignition propensity of the material depends on not only the attributes (e.g. shape, size, and numbers) but also the distribution of the firebrands. To help characterize this process, this study aims to investigate the effects of gap spacing on the burning behaviors of a group of wooden samples. Experiments are conducted using nine wooden cubes, 19 mm on each side. These samples are arranged in a 3 × 3 square pattern on suspension wires and are ignited by hot coils from the bottom surface. The gap spacing (s) between the samples varies in each test (ranging from 0 to 30 mm). After ignition, the samples are left to burn to completion. The burning process is recorded using video cameras. Sample mass loss and temperatures are monitored during the flaming and smoldering processes. The results show that the flame height and the sample mass loss rate have non-monotonic dependencies on the gap spacing. When the gap spacing reduces, the flame height and the mass loss rate first increase due to enhanced heat input from the adjacent flames to each sample. When s ≤ 10 mm, flames from individual samples are observed to merge into a single large fire. As s further decreases, the air entrainment at the flame bottom decreases and the flame lift-off distance at the flame center increases, resulting in an increased flame height, decreased flame heat feedback to the solid samples, and a decreased mass loss rate. The decreased mass loss rate eventually leads to a decrease in the flame height as well. The gaseous flame height is correlated to the solid burning rate. The correlation generally follows previous empirical equations for continuous fire sources. For the smoldering combustion, compared to a single burning sample, the smoldering temperature and duration significantly increase due to the thermal interactions between adjacent burning samples. To help interpret the results of the burning experiments, thermogravimetric analysis is also performed in air and nitrogen, resulting in heating rates ranging from 10 to 100 K/min.
more »
« less
Ignition propensity of structural materials exposed to multiple firebrands in wildland-urban interface (WUI) fires: effects of firebrand distribution and ambient wind
Firebrands are known to be able to ignite not only vegetation but also various structures found in wildland-urban interface (WUI) area. Especially, firebrands located close to each other on a combustible substrate increase the likelihood of ignition and the subsequent fire. To elucidate the ignition mechanism of firebrands, experiments are performed using a 3 by 3 square array of flaming firebrands deposited on a 6.35 mm thick birch plywood. The spacing of the firebrand is varied in each experiment, ranging from 10 to 30 mm. The deposited mass of firebrands lies between 13 and 15 g. Ambient wind is imposed parallel to the plywood surface to investigate its effect on the ignition and the subsequent flame spread over the fuel. Three different wind speeds 0, 0.5, and 0.75 m/s are tested. During the experiments, mass loss of the plywood and the deposited firebrands is recorded. Video cameras are used to monitor the burning process. An infrared camera is also used to monitor the temperature of the firebrands and the plywood. The experiment results indicate that the firebrands with the spacing greater than 20 mm are able to burn only the surface of the plywood until the firebrands burn out. When the spacing between firebrands is smaller than 20 mm, the plywood is ignited and continues to burn even after the firebrands are fully consumed. It is also observed that the flame is able to spread downstream at 10 mm spacing under ambient wind speed of 0.5 m/s. Results from this study demonstrate the significant influence of spacing between the firebrands on the ignition and the burning behavior of the substrate materials.
more »
« less
- Award ID(s):
- 1836428
- PAR ID:
- 10390457
- Date Published:
- Journal Name:
- 2022 Spring Technical Meeting of The Central States Section of the Combustion Institute
- Format(s):
- Medium: X
- Sponsoring Org:
- National Science Foundation
More Like this
-
-
Firebrand attack has been shown to be one of the key mechanisms of wildfire spread into Wildland-Urban Interface (WUI) communities. The ignition propensity of materials caused by firebrands depends on not only the attributes (e.g., shape, size, numbers) but also the distribution of firebrands after landing on the substrate materials. To help characterize this process, this study aims to first investigate the effects of gap spacing on the burning behaviors of a group of wooden samples. Experiments were conducted using 9 wooden cubes, 19mm-long on each side. These samples were arranged in a 3 by 3 square pattern on suspension wires. The gap spacing (s) between the cube samples varies from 0 to 30 mm. Burning process was recorded using video cameras. Sample mass loss and temperatures were monitored during the flaming and smoldering processes. The results show that when s ≤ 10 mm, flames from individual samples merged. When the gap spacing reduces, the mass loss rate first increases but starts decreasing at s = 10 mm where flame merging occurs. The flame height has a similar non-monotonic dependency on the gap spacing and the maximum flame height occurs at s = 5 mm. Compared to the case with s = 10 mm, cases with a smaller gap spacing (s = 2.5 and 5 mm) have a larger flame height but a smaller sample mass loss rate. This indicates that a reduced air entrainment leads to an increase in the flame height despite of a decreased flame heat feedback to the solid samples. The heating rates of each sample were also calculated to investigate the local burning behaviors. The analysis showed a weaker flame heat feedback to the sample at the center for cases with under-ventilated combustion. Last, gaseous flame height was corelated to the solid burning rate. The correlation was also compared with previous empirical equations concerning liquid pool fires of different heat release rates.more » « less
-
The FireFlux II experiment was conducted in a tall grass prairie located in south-east Texas on 30 January 2013 under a regional burn ban and high fire danger conditions. The goal of the experiment was to better understand micrometeorological aspects of fire spread. The experimental design was guided by the use of a coupled fire–atmosphere model that predicted the fire spread in advance. Preliminary results show that after ignition, a surface pressure perturbation formed and strengthened as the fire front and plume developed, causing an increase in wind velocity at the fire front. The fire-induced winds advected hot combustion gases forward and downwind of the fire front that resulted in acceleration of air through the flame front. Overall, the experiment collected a large set of micrometeorological, air chemistry and fire behaviour data that may provide a comprehensive dataset for evaluating and testing coupled fire–atmosphere model systems.more » « less
-
Abstract High frequency (30 Hz) two-dimensional particle image velocimetry data recorded during a field experiment exploring fire spread from point ignition in hand-spread pine needles under calm ambient wind conditions are analysed in this study. In the initial stages, as the flame spreads approximately radially away from the ignition point in the absence of a preferred wind-forcing direction, it entrains cooler ambient air into the warmer fire core, thereby experiencing a dynamic pressure resistance. The fire-front, comprising a flame that is tilted inward, is surrounded by a region of downdraft. Coherent structures describe the initial shape of the fire-front and its response to local wind shifts while also revealing possible fire-spread mechanisms. Vortex tubes originating outside the fire spiral inward and get stretched thinner at the fire-front leading to higher vorticity there. These tubes comprise circulation structures that induce a radially outward velocity close to the fuel bed, which pushes hot gases outward, thereby causing the fire to spread. Moreover, these circulation structures confirm the presence of counter-rotating vortex pairs that are known to be a key mechanism for fire spread. The axis of the vortex tubes changes its orientation alternately towards and away from the surface of the fuel bed, causing the vortex tubes to be kinked. The strong updraft observed at the location of the fire-front could potentially advect and tilt the kinked vortex tube vertically upward leading to fire-whirl formation. As the fire evolves, its perimeter disintegrates in response to flow instabilities to form smaller fire “pockets”. These pockets are confined to certain points in the flow field that remain relatively fixed for a while and resemble the behavior of a chaotic system in the vicinity of an attractor. Increased magnitudes of the turbulent fluxes of horizontal momentum, computed at certain such fixed points along the fire-front, are symptomatic of irregular fire bursts and help contextualize the fire spread. Most importantly, the time-varying transport terms of the turbulent kinetic energy budget equation computed at adjacent fixed points indicate that local fires along the fire-front primarily interact via the horizontal turbulent transport term.more » « less
-
We demonstrate the use of nanosecond pulse transient plasma (NPTP) to improve the control (and acceleration) of the combustion of solid rocket propellants. Here, we fabricate end-burning propellant samples (i.e., grains) with a co-axial center wire electrode using hydroxyl terminated polybutadiene (HTPB), isodecyl pelargonate (IDP), modified diphenyl diisocyanate (MDI), and ammonium perchlorate (AP) as the fuel, plasticizer, curative, and oxidizer, respectively. High voltage (20 kV) nanosecond pulses (20 nsec) produce a streamer discharge that provides electronic throttling of the solid rocket propellant. These studies are carried out over a wide range of oxidizer mass fractions, including those considered insensitive munitions (IM). In addition, real time imaging is performed characterizing the plasma-formation, evolution of the ignition process, and plasma enhanced flamefuel coupling. We believe the plasma-based mechanisms of enhancement are 3-fold: 1.) The plasma provides highly energetic electrons that drive new chemical reaction pathways via highly reactive atomic species such as H, O, and Cl, 2.) The plasma sputters chunks of the solid fuel material up into the flame where it is combusted, producing an agitated flame profile. 3.) The plasma provides increased turbulence and multi-scale mixing due to hydrodynamic effects (i.e., ionic winds), which further improves the combustion process. Having electronic control of the burn rate introduces the ability to “throttle” solid rocket motors and introduce new flight profile options beyond a pre-selected profile such as the typical boost-sustain profile. While we are unable to quantify the burn rate or thrust from these relatively simple observations, we observe clear evidence of the effect of the plasma on the combustion of these solid rocket fuels even at high oxidizer content.more » « less
- Free Publicly Accessible Full Text
-
Accepted Manuscript1.0
- Conference Paper:
- The DOI is not currently available.
