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# Temperature Measurement of Glowing Embers with Color Pyrometry

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## Abstract and Figures

The surface temperature of glowing embers are important for determining the burning rate of embers as they fall and their ability to ignite a target fuel when they land. Temperature measurements of glowing embers using Color Pyrometry (CP) are presented. CP allows for temperature measurement of an incandescing gray or black body based on the intensity’s spectral distribution rather than absolute intensity, and it can be accomplished with a DSLR camera. The CP measurements are compared with measurements using an infrared (IR) camera; it was observed that the CP measurements were more accurate than IR measurements because IR measurements need knowledge of the surface’s emissivity. We show that the emissivity is very important because the transient accumulation and shedding of ash on the surface of the glowing ember changes the emissivity dynamically, consequentially changing the measured temperature. The insensitivity of CP to emissivity allows this method to provide a more robust temperature measurement. Experiments using CP were performed with embers of different sizes in different airflows. The results show that the temperature of the glowing embers increases with increasing air flow. The measured mean glowing temperatures ranged from $$750^\circ \mathrm{C}$$ at 1 m/s to $$950^\circ \mathrm{C}$$ at 4 m/s. A glowing combustion model explains the dependence of the temperature on the air speed.
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Noname manuscript No.
(will be inserted by the editor)
Temperature Measurement of Glowing Embers with
Color Pyrometry
James L Urban, Michela Vicariotto, Derek
Dunn-Rankin, & Carlos Fernandez-Pello
the date of receipt and acceptance should be inserted later
Abstract The surface temperature of glowing embers are important for deter-
mining the burning rate of embers as they fall and their ability to ignite a target
fuel when they land. Temperature measurements of glowing embers using Color
Pyrometry (CP) are presented. CP allows for temperature measurement of an in-
candescing gray or black body based on the intensity’s spectral distribution rather
than absolute intensity, and it can be accomplished with a DSLR camera. The CP
measurements are compared with measurements using an infrared (IR) camera;
it was observed that the CP measurements were more accurate than IR measure-
ments because IR measurements need knowledge of the surface’s emissivity. We
show that the emissivity is very important because the transient accumulation and
shedding of ash on the surface of the glowing ember changes the emissivity dy-
namically, consequentially changing the measured temperature. The insensitivity
of CP to emissivity allows this method to provide a more robust temperature mea-
surement. Experiments using CP were performed with embers of diﬀerent sizes in
diﬀerent airﬂows. The results show that the temperature of the glowing embers in-
creases with increasing air ﬂow. The measured mean glowing temperatures ranged
from 750oCat 1 m/s to 950oCat 4 m/s. A glowing combustion model explains
the dependence of the temperature on the air speed.
Keywords Embers ·glowing combustion ·wildland ﬁres
1 Introduction
Each year as wildland and Wildland Urban Interface (WUI) ﬁres wreak havoc on
communities around the world, and the intensity and damage of these types of
ﬁres continue to rise. Recently, the October Fire Siege of 2017 has wrought heavy
damage to the northern California wine country (Napa and Sonoma counties) [1].
One of the ﬁres in the siege, the Tubbs ﬁre, alone has broken the record as the most
destructive wildﬁre in California history [2] and three others in the siege are also
on the list of Cal Fires list of the Top 20 Most Destructive California Wildﬁres [1].
Address(es) of author(s) should be given
This is a pre-print of a published article:
Urban, J. L., Vicariotto, M., Dunn-Rankin, D., & Fernandez-Pello, C. (2019). Temperature measurement of glowing
firebrands with color pyrometry. Fire Technology. https://doi.org/10.1007/s10694-018-0810-3
2 James L Urban, Michela Vicariotto, Derek Dunn-Rankin, & Carlos Fernandez-Pello
The ﬁre siege consisted of a peak number of 21 major wildﬁres that together killed
44 people [3], burned 245,000 acres, forced the evacuation of 100,000 people [1], and
destroyed an estimated 8,900 structures [2]. Dash cam and body cam videos from
law enforcement personnel show signiﬁcant sprays of glowing embers/ﬁrebrands
[4] and there are reports of ember spotting igniting spot ﬁres well outside the ﬁre
perimeter [5].
There have also been many notable wildland and WUI ﬁres in the past third
century, examples include: the 1991 Oakland Hills Fire [6], the 2003, 2007, and
2014 San Diego Firestorms and the 2012 Waldo Canyon Fire,[7], the Texas Bastrop
county ﬁres [8] and the Valley Fire [9]. There have also been many signiﬁcant WUI
ﬁres which occurred outside of the United States [7], in the Mediterranean Area
[10], South America [11], Australia [12], and New Zealand [13]. There does not
appear to be a consensus on the fraction of structure ignitions caused by embers
[7], as it is often diﬃcult to determine the ignition source after the ﬁre. However,
recent studies attribute signiﬁcant [14], if not the majority of structure ignitions
to ember spotting [15].
Ember spotting is the process by which burning pieces of biomass or other
ﬂammable materials are moved signiﬁcant distances by the wind and/or gas move-
ment induced by the ﬁre. Typically, there are three aspects of embers spotting: the
production of the embers, their transport, and ﬁnally the spot ﬁre ignition. The
transport of the embers is the better-understood part of the process [16–19]. How-
ever, topics such as ember exposure (number, mass, or energy ﬂuxes of embers)
[20], and ember ignition capability including the structure ignition vulnerabilities
have only recently been studied [21].
To help prevent embers in WUI ﬁres from igniting structures, an ember ex-
posure scale such as the one proposed by NIST [14] could be used by developers
of performance-based building codes which would provide guidelines for defend-
ing structures against ember attack. A crucial step towards developing an ember
exposure scale is the quantiﬁcation of relevant properties of ember exposure such
as the ember ﬂux (mass or number of embers per unit area) the ember temper-
atures, burning rates and Heat Release Rates (HRR). Diagnostic capabilities for
the remote measurement of temperatures of burning or glowing embers, could be
combined with existing particle tracking algorithms to provide much of this data.
Traditional thermography or Thermal Imaging (TI), which estimates the tem-
perature of an object by measuring the intensity of infrared light emitted by the
object can be used to measure the temperature of embers [22, 23], but it is of lim-
ited value in this application because of both fundamental and technical challenges.
First, thermography requires knowledge of the emissivity of the material under
study to accurately invert the Stefan-Boltzmann equation to recover the tempera-
ture. Second, thermal imaging sensors are currently limited in both their resolution
and their response time, making them ill-suited to the observation of small, quickly
moving embers. Color pyrometry addresses these concerns admirably, in that it
is less sensitive to material emissivity, object size, and speed. Further, it can be
performed with conventional visible-light cameras, which are small, provide excep-
tional spatial resolution, and are readily available [24]. Color pyrometry using dig-
ital cameras has been applied eﬀectively to measure temperatures of hot objects in
ﬂames such as silicon carbide ﬁbers (thin ﬁlament pyrometry) [25–27]. Point mea-
surements of temperatures in coal particle combustion have been performed using
Color Pyrometry since as early as the 1970’s [28] and recently temperature mea-
Temperature Measurement of Glowing Embers with Color Pyrometry 3
surements of moving coal particles have been measured [29]. Surface temperature
of burning wood particles in a high temperature reactor have also been imaged
using CP [30].
In this study we measure the temperature of static glowing embers with CP
under controlled airﬂow velocities and we identify behaviors such as ash accumula-
tion on embers, which could eﬀect temperature measurements with other methods.
This is investigated through experiments using various sized embers exposed to
various air speeds using CP and, in some of the conditions, IR photography which
demonstrates that diﬀerent levels of ash accumulation or air speeds could eﬀect
the temperature of the ember. The results provide data under controlled condi-
tions which could be used to inform future ember combustion models and could
be compared to future temperature measurements of moving embers. The results
could be used to make more physically grounded ember transport, and similar
experiments could be performed to get measurements for temperatures of fallen
embers or ember piles.
2 Color Pyrometry (CP)
Traditional CP allows for a temperature measurement of a gray body by measuring
the intensities of at minimum two discernible wavelengths of light and calculating
the surface temperature which would produce those intensities. CP can be accom-
plished using a suitable camera with a Bayer ﬁlter array. The Bayer ﬁlter separates
light into three channels in the visible spectrum, (i.e. Red, Green, and Blue) [31].
The Bayer ﬁlter allows the camera’s detector to measure a spectral intensity over
diﬀerent overlapping wavelength bands. Instead of two discrete spectral intensity
ratios, ratios of the integrated spectral intensities observed by the color channels
are used to the same end. These color ratios, Ci,j between the ith and jth color
can be predicted with Eq. 1.
Ci,j =R(λ)IP lank(λ, T )τ(λ)ηi(λ)
R(λ)IP lank(λ, T )τ(λ)ηj(λ)(1)
where IP lank(λ, T ) is the spectral intensity distribution of light emitted with
respect to wavelength, λ, according to the Planck blackbody radiation law, and T
is the temperature of the glowing body, τ(λ) is the transmissivity of the optics, and
ηi(λ) is the spectral sensitivity of the sensor. The only optical equipment used for
these experiments is a BG-40 ﬁlter. It is used to balance the intensities of the color
channels - improving the measurement ratio-signal and avoiding saturation of the
red color channel. In this work we assume that the emissivity was constant with
respect to wavelength, and thus would cancel out in Eq. 1. The integrals in Eq. 1
are integrated over wavelengths from 400 to 720 nm (the camera’s response spec-
trum) with resolution of 1 nm for temperatures from 570-1200oC. This produces a
predicted color ratio for every temperature in the range considered. Then the color
ratios can be measured using the data from the image produced by the camera.
The temperature can be determined by ﬁnding the temperature which produces
the same predicted color ratio [26, 27]. While the analysis here may seem quite
complex, it only has to be done once to calibrate the system (camera and optical
system). Alternatively, the camera and optical system can be calibrated in a more
4 James L Urban, Michela Vicariotto, Derek Dunn-Rankin, & Carlos Fernandez-Pello
Fig. 1 View of experimental apparatus from the top. The camera heights are such that the
center of their lenses are in-plane with the center of the ember sample. The camera lenses
were oﬀset horizontally and vertically such that they were as close as possible to prevent any
diﬀerences caused by viewing angle.
crude manner by taking images of a blackbody at a known temperature and curve
ﬁtting the temperature of the blackbody to the observed color ratio.
3 Experiments
A schematic of the experimental set-up is shown in Figure 1. The ember sample
was mounted on a metal spike and placed in the approximate centerline of a
bench-scale wind tunnel. The wind tunnel test section is 550 mm in length with
a 130x80 mm2cross section. The ember was placed 225 mm from the entrance of
the opening of the test section. Laboratory air is ﬂown through the wind tunnel
at speeds between 1 and 4 m/s. An opening was made on the side of the tunnel to
allow for optical access without the use of glass which would aﬀect the spectrum
of light transmitted to the camera.
For the CP measurements, a Nikon D90 camera was used, with an aperture
setting of f/2 to f/4 and ISO sensitivity set to 200 and the focal length was ap-
proximately 0.5m. Care must be taken to turn oﬀ all image enhancements options
on the camera and to save the images as RAW images. The RAW images are con-
verted to a linear 16-bit depth TIFF ﬁles using the open source software DCraw,
with all color correction and white balance oﬀ. The RGB intensities from the TIFF
ﬁle allows for calculation of the color ratios at each pixel, then the surface temper-
ature at each pixel location using data obtained through calibration [26, 27]. An
infrared camera FLIR SC-620 was also used for comparison of the two temperature
measurement methods, and qualitative veriﬁcation of the CP approach.
Experiments were performed on simulated embers formed from birch dowel
rods, 6.5, 9.5, 11, and 15.9 mm in diameter and cut to an aspect ratio of 1. The
dowels were heated and ignited with a ﬂame from a butane torch. The heating
times were 15, 20, 22, and 25 seconds for the 6.5, 9.5, 11, and 15.9 mm diameter
embers, respectively.
During tests images were collected at a rate of roughly 0.3 Hz. For the tests
comparing the CP and IR temperature measurements,pictures were taken simul-
Temperature Measurement of Glowing Embers with Color Pyrometry 5
taneously with the D90 and FLIR cameras. The CP measurements of ember tem-
peratures were compared with those measured with a ﬁne bead thermocouple with
a diameter of 0.005in (0.1 mm). However, the point of contact of the thermocou-
ple bead on the ember surface cooled (ceased glowing/turned black) immediately
making thermocouple measurements of dubious accuracy. The precision of the
CP measurement was determined by evaluating the temperature distribution of a
glowing steel ball bearing which had been heated in a furnace. The CP tempera-
ture measurement showed an approximately Gaussian distribution with a standard
deviation of 9.7oC.
4 Results
During the experiments the embers exhibited glowing combustion and a wake ﬂame
trailing the ember for a short period after ignition. The wake ﬂame was often lifted
from the ember and the surface beneath it exhibited less glowing than the front.
After this short period the ember sustained glowing combustion at its surface and
its mass decreased with time. In some cases, ash will be formed over the ember
surface and was intermittently removed from the surface by the wind. The visual
intensity of the glowing appeared to increase with increasing wind speeds.
4.1 Comparison of CP and IR measurements
To compare the temperature measuring capabilities of CP and TI, the results of
the ember temperature measurements using CP were compared with TI using an
IR camera. Both the CP and IR cameras were pointed at the same ember and a
small set of tests were completed to compare them. Characteristic results of these
comparison tests are shown in Figure 2, where (a) has results taken right after
the ember was ignited and (b) has results taken 6 seconds later. In (a) and (b)
the top-left sub-ﬁgure contains temperature images from the IR camera, the top-
right has temperatures from the CP, and the bottom sub-ﬁgure has distributions
of the pixel temperatures. The temperature distribution from the IR camera is
displayed assuming three diﬀerent emissivity values: of 0.95, 0.4, and 0.68. These
values correspond to carbon black [32], ash [33], and the mean of the two values.
The missing/dark section at the bottom in the second image is the sample holder
which has been exposed by the receding ember surface.
From the temperature distributions functions in (a), the IR camera results
best agree if the emissivity used by the IR camera was selected between 0.95
and 0.68 while for (b) the CP and IR results appear to better ﬁt if the mean
value, 0.68 is used. From the results of Figure 2b, it seems that the accumulated
ash was reducing the eﬀective emissivity of the ember as the ember burns, as
initially noted by Manzello et al. [22] in a previous study. The changing emissivity
makes accurate measurement with an IR camera or pyrometer diﬃcult because
it requires temporal and spatial characterization of the emissivity for the entire
exposed surface. While using an ”eﬀective emissivity” could be suﬃcient for some
applications, the value would change as ash accumulates and is shed, making
accurate temperature measurement through intensity based pyrometry inaccurate.
6 James L Urban, Michela Vicariotto, Derek Dunn-Rankin, & Carlos Fernandez-Pello
Fig. 2 Results of IR camera and CP directly after the ember (diameter = 9.5 mm, airspeed
of 3.6 m/s) is ignited (a), and again after the ember has burned for approximately six seconds
and accumulated signiﬁcant ash (b). Results for (a) and (b) include IR and CP images as well
as temperatures distribution of glowing ember from the CP (blue) and the IR camera in green
at various assumed eﬀective emissivities, eff , ranging from carbon black, 0.95, to ash, 0.4.
4.2 Color Pyrometry Measurements
Temperatures of glowing embers at diﬀerent air ﬂow speeds were measured using
CP as indicated above. It was observed that the local temperature of the ember
ﬂuctuates as it burns. Based on visual observations and the CP images, the accu-
mulation of ash around the ember both limits light emission from the ember, but
also oxygen delivery to the glowing surface. The ash is periodically removed by the
wind causing the observed temperature ﬂuctuations. Figure 3 shows the history of
an ember. The successive columns (i-v) correspond to diﬀerent times during the
experiment (separated by 6s) from ignition of the ember to burnout. The ﬁrst row
contains images taken by the camera and post-processed with the default camera
settings, the second row contains the CP temperature images colored to show the
temperature using the colorbar at the right of the ﬁgure, and the ﬁnal row are the
pixel temperature distribution of the ember surface from the image in the second
row. With exception of the temperature right after ignition and near burn-out,
the mean temperature of the ember surface during all tests was observed to be
in the range of 750 to 950oCand maximum ember surface temperatures of up to
1100oC.
From these results we can observe several things: ﬁrst the surface tempera-
tures of an ember can be considerably higher than reported temperatures in the
literature. Anthenien et al. [17] uses an ember temperature of 720oC, reported
by [34]. The temperatures reported by Ohlemiller [34] are for smoldering combus-
tion in a U channel of wood which is a signiﬁcantly diﬀerent scenario to that of
a glowing ember in an air ﬂow. Moreover, measurements by Ohlemiller [34] were
performed with a thermocouple which could cause localized cooling of the ember
and leading to a lower temperature measurement. Recently, the temperature of
embers in a laboratory environment has been measured with an infrared imager
Temperature Measurement of Glowing Embers with Color Pyrometry 7
Fig. 3 (row a) Images of 15.9 mm diameter ember burning in an airﬂow of 3.85 m/s at various
times (approx. 6 seconds apart), (row b) CP images of ember surface (ﬂame is cropped out) and
(row c) pixel histograms and distribution of pixel temperatures. The reader is recommended
to view the full-color version online. It should be noted that the color in the images in row a
is diﬀerent than what might be observed with the naked eye because they were taken through
the BG-40 ﬁlter, which preferentially transmits light in the blue and green wavelengths more
than in the red wavelengths, and then processed using default settings for DCraw.
[35] and temperatures between 600 to 900oCare reported, although it is unclear
what emissivity value was used.
The local variations in surface temperature shown in Fig. 3 are believed to
primarily be based on diﬀerences in the local heat transfer with the surrounding
air ﬂow and the oxygen transport to the glowing combustion reaction zone. Other
eﬀects such as ash accumulation and surface cracking would then aﬀect these pro-
cesses at diﬀerent locations of the ember. To better examine the role of the airﬂow
on the ember combustion, we can compare temperature of embers in diﬀerent air
ﬂows.
The results of the average glowing temperatures are shown in Figure 4. This
average temperature was calculated by taking the average of the pixel temperatures
from the CP images, and then taking the average for all images during the test
until the ember completely burned or so signiﬁcantly that it fell oﬀ the sample
holder, which took longer for larger embers. The results show a trend of increased
ember glowing temperature with wind speed. Speciﬁcally, a roughly 150 - 200oC
increase in temperature was measured when the wind speed was increased from 1
to 4 m/s. A clear trend with ember diameter however was not observed, although
smaller sizes exhibited more variation in average temperature between diﬀerent
embers. The observed increase in ember surface temperature with ﬂow velocity
is due to the fact that the increased air speed leads to more oxygen delivery to
the surface reaction, increasing the heat release rate. This, in turn, increases the
ember’s glowing surface temperature resulting in brighter radiation.
8 James L Urban, Michela Vicariotto, Derek Dunn-Rankin, & Carlos Fernandez-Pello
Fig. 4 Average glowing temperatures measured from ember experiments (scatter points) as
a function of air ﬂow velocity and Reynolds number.
Fig. 5 Diagram of ember glowing combustion model
5 Discussion
5.1 Glowing Combustion Model
To better understand the experimental results shown in Fig. 4 it is useful to develop
a simpliﬁed model of the smoldering combustion of an ember in a convective air
ﬂow. For this purpose, an analytical a model of the thermo-chemical mechanisms
taking place during the glowing combustion of an ember in a convective oxidizer
ﬂow is developed in this section. The objective of the model is to predict the
glowing surface temperatures of a glowing ember in the vicinity of its stagnation
point show in Fig. 5.
For the analysis, we assume the ember is eﬀectively a sphere because although
it starts as a cylinder, the aspect ratio is unity and the corners become rounded
as it burns, approaching a spherical geometry. As a glowing ember is exposed to
Temperature Measurement of Glowing Embers with Color Pyrometry 9
the air ﬂow, heterogeneous surface oxidation reactions take place, which heat the
ember, and produce CO and CO2, and leave behind ash. The heated surface also
is cooled by convective heat transfer to the surrounding ﬂow and radiation heat
transfer to the surroundings. There are also conductive heat losses into the inside
of the ember, which are assumed to be small enough that they can be neglected due
to the low thermal conductivity of the wood ember. The heterogeneous reaction is
assumed to only be signiﬁcant in a thin outer layer of the ember that is presumed
fully charred.
Visual observation of glowing embers in the experimental portion of this work
reveals that the glowing is signiﬁcantly stronger in the leading face of the ember.
This is because the local Nusselt and Sherwood numbers change with the polar
angle from the stagnation point of the sphere. With these assumptions the temper-
ature of the leading face of the ember can be calculated by balancing the energy
equation on the surface of the ember, as
QrxnhM T ρairYO2=hH T (TT) + σ(T4T4
) (2)
where the left-hand term is the heat released by the reaction, and the terms on
the right-hand side are the convective and radiative heat losses, respectively. The
temperatures Tand Tare that of the glowing combustion zone and the ambient
surroundings, respectively. ef f and σare the eﬀective emissivity and the Stefan-
Boltzmann constant respectively. The properties of air are evaluated at the ﬁlm
temperature (mean of ambient and glowing combustion temperatures), with the
exception of the density of air, ρair, which is evaluated at the ambient temperature.
YO2is the mass fraction of oxygen in air. The eﬀective emissivity was allowed to
vary between values discussed above (0.4 to 0.95) and represents the uncertainty
in the model from the emissivity.
Qrxn is the energy released from the char oxidation reaction that is absorbed
by the ember, per unit mass of oxygen, 2.59MJ/kg (oxygen basis). This assumes
that only a portion, 0.285, of the energy released by the reaction is absorbed by
the ember similar to [36]. As with previous glowing ember burning rate models
[16, 17, 36] we assume that the rate is dominated by convective mass transfer
of oxygen to the surface. The mass transfer and heat transfer coeﬃcients are
calculated using a Sherwood number and Nusselt number correlations for a sphere:
hMT =S h ·DO2/Air/demb (3)
hHT =N u ·kair/demb (4)
where Nu is the Nusselt number, Sh is the Sherwood number, DO2/Air is the dif-
fusion coeﬃcient of oxygen in air and kair is the thermal conductivity of air, both
evaluated at the ﬁlm temperature, and demb is the ember diameter. If the velocity
of the air is increased, the Reynolds number will increase, increasing both convec-
tion mass and heat transfer. The increase in the mass transfer of oxygen causes
the reaction to progress faster and the glowing reaction should reach a higher tem-
perature under steady conditions. The local Nusselt and Sherwood numbers are
given by the equations:
10 James L Urban, Michela Vicariotto, Derek Dunn-Rankin, & Carlos Fernandez-Pello
Nu = 2 + F s(Re, φ)·Re1/2P r1/3(5)
Sh = 2 + F s(Re, φ)·Re1/2Sc1/3(6)
where Re is the Reynolds number, P r is the Prandtl number, Sc is the Schmidt
number, and F s is the local Fr¨ossling number as a function of the angle from the
stagnation point, φ, [37]. There is no change in F s with φfor φ < π/2 (90o), and
depending on the ﬂow and surface roughness, the ﬂow may separate creating a
wake. For Reynolds number below 7500 [37] (the maximum value in the experi-
ments was 3852), the Fossling number in the wake is lower valued [37], indicating
less eﬀective convective heat transfer. This is reasonable, as the ﬂow near the
stagnation point has the freshest air and consequently, the air is also the coldest
(closest to ambient temperature) while in the wake the air would be hotter and
have lower amounts of O2.
The dependence of F s on Re captures enhanced convection by grid-level turbu-
lence. For the experiment here we assumed that grid level turbulence was present
but not strong and we used the lowest, non-zero value in the correlation in Ref.
[37]. With these assumptions, the local Fr¨ossling number at the stagnation point is
given in Eq. 7, where νis the kinematic viscosity of air at ambient temperature
and νsurf is the kinematic viscosity the temperature of the surface, for this the
average glowing temperature was used.
F s(Re) = ν
νsurf .16
(1.03 + .002 ·Re) (7)
Fig. 6 Average glowing temperatures measured from ember experiments (scatter points) as
a function of air ﬂow velocity and Reynolds number. Superimposed are curves of the simple
ember glowing combustion model using an emissivity value of 0.68 (mean of ash and car-
bon emissivities) and shaded regions marking the temperatures predicted if the emissivity is
changed by ±0.05.
Temperature Measurement of Glowing Embers with Color Pyrometry 11
The glowing surface temperature of the ember can be predicted by solving Eq.
2 together with Eqs. 3 to 7 numerically for the ember sizes and ﬂow speeds used
in the experiments. The results of the model are included in Figure 6 with the
results from the average pixel temperatures measured with CP in the experiments
previously shown in Fig. 4. The model results include the predicted temperature
assuming the emissivity is the mean of ash and carbon emissivities, 0.68 shown
by the lines and a shaded region showing how the temperature would vary if
the emissivity were changed by ±0.05, which demonstrates the strong eﬀect on
the emissivity. This helps explain the scatter in the experiments, in fact if the
emissivity was varied from 0.4 to 0.95 the variation in the model results would
encompass the majority of the average temperatures.
It is seen that the model predicts the experimental results qualitatively, with
the glowing temperature monotonically increased with ﬂow speed. This is due to
the increase in the heat release rate of the glowing combustion with the wind
speed, which overcome the increased convective heat losses from the ember to the
surrounding ambient. The predicted ember temperatures monotonically increase
with a sublinear trend with velocity while the experimental data appears to follow a
linear trend, albeit scattered. Another diﬀerence is that the model predicts a larger
diﬀerence in temperature than that observed in the experiments. One possible
explanation is that the model does not account for heat and mass transfer to
the ember through the ash. The model is also observed to under-predict the
temperature of large particles, a possible explanation is that a larger fraction of
the heat generated by the oxidation reaction is retained by the larger embers, and
in the model presented here we assume that fraction is constant. The remaining
variation in the experiments is likely caused by uncontrollable variable factors
including the inhomogeneity of wood, complex cracking and pyrolyzate venting
phenomena.
6 Conclusion
This work shows that color pyrometry can be applied to measure the surface
temperature of glowing embers. The method provides a robust, high resolution,
method to measure the temperature of embers and other incandescing solids ob-
jects. Unlike IR cameras, the method does not require knowledge of the emissivity
of the glowing surface. It was also observed that ash produced by the char ox-
idation of the ember causes signiﬁcant variation in its surface temperature and
also locally occludes the surface of the ember. The temperature variations are due
to periodic removal of the ash by the air ﬂowing past the ember. The ash makes
intensity-based methods for measuring the surface temperature (e.g. IR cameras)
more diﬃcult to use as the ash will have a lower emissivity than the charring wood.
This diﬃculty can be overcome by using the color pyrometry technique to measure
the ember temperature. In the present experiments we have measured the glowing
on the ember directly with color pyrometry and found the average temperature to
be roughly 750 to 950oC. These temperatures are roughly 20 to 220oChigher than
surface temperatures attributed to embers before, with better agreement seen at
lower ﬂow conditions.
To examine the variation of the glowing ember temperature with air ﬂow ve-
locity, a model was developed to predict the dependence of the ember temperature
12 James L Urban, Michela Vicariotto, Derek Dunn-Rankin, & Carlos Fernandez-Pello
on the air ﬂow velocity. The model predicts qualitatively well the experimental ob-
servations with the glowing temperature monotonically increasing with ﬂow speed,
and helps explain the nature of the results.
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... However, most of these data are qualitative, and the studies largely report on ignition probabilities of the fuel studied. Very recently, the research groups at the University of Edinburgh [23], University of Maryland [29][30][31][32], University of California Berkeley [33], and others [34] performed advanced experiments to study thermal characteristics of embers (e.g., ember temperature and heat transfer as well as factors and conditions that affect each one) and identify conditions under which the wooden substrate would ignite. These experiments provide insights into measurement techniques and associated uncertainties. ...
... Addressing the limitations of infrared thermography associated with its dependency on surface emissivity, Urban et al. [33] recently used color pyrometry to measure temperature of an ember placed in a wind tunnel. They compared color pyrometry with infrared thermography measurements taken simultaneously and concluded that the color pyrometry was more accurate and appropriate for ember temperature measurements than infrared thermography. ...
Technical Report
Full-text available
The ignitability of structural components due to ember attack is a common cause of the structural fires in wildland-urban interface (WUI) communities. To fire-harden structures in WUI communities, it is important to be able to quantitatively predict the ignitability of wooden substrates in response to ember exposure. To commence this effort, in this manuscript, past studies have been compiled and analyzed to identify the knowledge gaps. Key topics are reviewed, including ignition of structures in WUI fires, measurement of thermal response of solid wood products used in residential structures, controlling mechanisms of ignition and sustained smoldering of wood, measurement of ember properties, real-scale and bench-scale experiments assessing ember ignitability of structural components, and surrogate ignition sources for assessing smoldering propensity of the wooden substrates. Existing standard test methods have also been reviewed in the light of their ability to represent common exposures observed in the WUI environment. The experimental data from past studies have provided guidance in developing building codes and standards for reducing the susceptibility of structures to WUI fires. Relationships have been developed between ember size, fuel moisture content, and ignition frequency of common structural components. However, most of these data are qualitative, and the studies largely report on ignition probabilities of the fuels studied under specific test conditions. Advanced experiments have been designed to quantify the thermal characteristics of embers (i.e., ember temperature and heat transfer as well as factors and conditions that affect each one) thus providing information on net heat flux, peak heat flux, and heat flux and temperature distributions of ember piles. These studies also assess wind conditions and heating times required for initiation of smoldering and subsequent transitioning to flaming combustion, and provide an initial understanding of selected factors affecting ember ignitability of wood-based materials. Despite these efforts, available empirical data on the ember ignition of solid wood substrates are limited and is not sufficient to predict the ignitability and sustained smoldering of wood products exposed to an arbitrary set of conditions they may face in the WUI. Without the ability to quantitatively predict the ember ignitability of wooden substrates outside of specific, pre-defined experimental conditions, it is difficult to develop a system for risk assessment and tools to reduce structural losses from wildland fires. A comprehensive review of alternative ignition sources as surrogates for an ember or pile of embers has indicated that electrical resistance heaters are capable of initiating smoldering ignition in solid wooden substrates. The power output of such heaters can be carefully controlled to generate well-defined heat transfer conditions comparable to those measured for piles of real embers, which may develop in WUI fires. It is anticipated that the use of surrogate ignition sources will provide a robust, accurate, and efficient approach to validate the performance of computational models designed to quantitatively predict the smoldering behavior of a combustible solid based on simulation of the chemical and physical mechanisms controlling this behavior and knowledge of the material's thermophysical properties.
... In the present study, the uncertainty due to potential cooling from the WC-HFG is estimated to be on the order of ∼10% at most. The potential cooling was estimated for a firebrand pile at a temperature of 800 • C, an average based on Urbas et al. (2004), Caton et al. (2016), and Urban et al. (2019), with a gauge held at the water temperature of 20 • C, assuming radiative heat transfer between the firebrands and the WC-HFG. Given the difference in size between the pile and the gauge, cooling by the WC-HFG is ∼10% for the worst case scenario (i.e., the lowest longtime heat fluxes measured). ...
... While averaged temperatures for the inner TSCs for the large pile reach about 700 • C, the temperatures of individual TSCs reached well over 900 • C instantaneously at higher wind speeds. These temperatures correspond well with previously measured temperatures by Urban et al. (2019) using color pyrometry. ...
Article
Full-text available
This study investigated the thermal conditions preceding ignition of three dense woody fuels often found on structures by firebrands, a major cause of home ignition during wildland-urban interface (WUI) fires. Piles of smoldering cylindrical firebrands, fabricated from wooden dowels, were deposited either on a flat inert surface instrumented with temperature and heat flux sensors or on a target fuel (marine-grade plywood, oriented-strand board, or cedar shingles) to investigate critical conditions at ignition. The former provided thermal data to characterize the time before and at ignition, while the latter provided smoldering and flaming ignition times. Tests were conducted in a small-scale wind tunnel. Larger firebrand piles produced higher temperatures at the center of the pile, thought to be due to re-radiation within the pile. Ignition was found to be dependent on target fuel density; flaming ignition was additionally found to be dependent on wind speed. Higher wind speeds increased the rate of oxidation and led to higher temperatures and heat fluxes measured on the test surface. The heat flux at ignition was determined by combining results of inert and ignition tests, showing that ignition occurred while transient heating from the firebrand pile was increasing. Ultimately, critical ignition conditions from firebrand pile exposure are needed to design appropriate fire safety standards and WUI fire modeling.
... The temperatures were measured using IF cameras, and these demonstrated an internal temperature can reach values above 800ᵒC. Recently the transient accumulation and shedding of ash on the surface of the glowing embers were investigated [Urban et al. 2019], in it was concluded that emissivity is very important and may change the measured temperature. They measured temperatures in the embers surface from 750ᵒC to 900ᵒC, but no time duration or temperature surface transmission was presented. ...
... The primary optimisation to reduce cost and scale would be to substitute the infrared camera with a modified consumer grade camera. Since it has been demonstrated that accurate measurement of a falling, rotating particle's temperature is complicated by the variability in orientation and actual temperature and the relatively low time in the field of view, a bespoke calibration is likely to be required so high resolution optical pyrometry is not required and alternative techniques may be appropriate [23]. Reducing the thermal information to this level would allow for significant cost reduction while only introducing the need for a laboratory calibration to determine the appropriate thresholds to distinguish between hot and cold firebrands. ...
Article
Full-text available
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Article
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Assessing the risk of forest fires in wildland urban interfaces (WUIs) is crucial for wildfire prevention and land management. With the goal of developing an efficient management of fire risk in wildland urban interfaces in European Mediterranean countries, this chapter recommends methods and advices for the identification, characterization and mapping of WUIs as well as for the assessment of the fire hazard, the vulnerability and the damage potential of these areas. These tools are the result of scientific researches and fruit of past experiences analysis. More over this chapter insists on the interest of providing wildland urban interface maps in the purpose to know the real extent of theses areas and to manage their development. It briefly presents the steps of a method to identify, to characterize and to map WUI in European Mediterranean countries combining relevant criteria which are connected to the spatial organization of inhabitant dwellings and the structure of the vegetation. A WUI' typology is established. The method can be easily applied by land agencies or managers easily and is suitable both in large areas or landscape level (small scales) and in local conditions (large scale). Because of their high vulnerability, ignition probability and combustibility, it is important and efficient to focus risk assessment in the WUIs. The chapter brings a method to assess and to map fire hazard levels and vulnerability levels according to WUIs and their environment. Introducing the risk of fire and particularly the vulnerability of the territory with such maps is a way to make the inhabitants becoming aware of fire risk in WUI. This will globally decrease the risk of fire either by reducing fire propagation with biomass removal and or by reducing fire ignition probability together with less carelessness. Accomplishing this goal is strictly related to the designation of suitable prevention messages and preventive actions which can be different according to WUI types.
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Knowledge of the in situ temperature, size, velocity, and number density of a population of burning coal particles yields insight into the chemical and aerodynamic behavior of a pulverized coal flame (e.g., through means of combustion model validation). Sophisticated and reasonably accurate methods are available for the simultaneous measurement of particle velocity and temperature; however, these methods typically produce single particle measurements in small analyzed volumes and require extensive instrumentation. We present a simple, inexpensive method for the simultaneous, in situ, three-dimensional (3D) measurement of particle velocity, number density, size, and temperature. The proposed method uses a combination of stereo imaging, 3D reconstruction, multicolor pyrometry, and digital image processing techniques. The details of theoretical and algorithmic backgrounds are presented, along with examples and validation experiments. Rigorous uncertainty quantification was performed using numerical simulations to estimate the accuracy of the method and explore how different parameters affect measurement uncertainty. This paper, Part II of two parts that discuss this method [Appl. Opt. 54, 4049 (2015)], describes particle temperature and size measurement in overexposed emission images.
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Knowledge of the in situ temperature, size, velocity, and number density of a population of burning coal particles yields insight into the chemical and aerodynamic behavior of a pulverized-coal flame, e.g., through means of combustion model validation. Sophisticated and reasonably accurate methods are available for the simultaneous measurement of particle velocity and temperature; however, these methods typically produce single particle measurements in small analyzed volumes and require extensive instrumentation. We present a simple and inexpensive method for the simultaneous, in situ, three-dimensional (3D) measurement of particle velocity, number density, size, and temperature. The proposed method utilizes a combination of stereo imaging, 3D reconstruction, multicolor pyrometry, and digital image processing techniques. The details of theoretical and algorithmic backgrounds are presented, along with examples and validation experiments. By utilizing numerical simulations, rigorous uncertainty quantification is performed in order to estimate the accuracy of the method and explore how different parameters affect measurement uncertainty. The method is described in two parts. The first part, presented in this paper, describes particle velocity and population density mapping by stereo streaking velocimetry using overexposed emission images.
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The Great Valparaiso Fire started as a wildfire on the outskirts of the city. The fire spread through the wildland–urban interface towards the city. In 5 days, it claimed the lives of 15 people, injured more than 500 people, destroyed over 2,900 homes, burned over 1,000 ha, and displaced approximately 12,500 people. In this Letter to the Editor, several issues that facilitated the tragic events are discussed.
Article
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.
Article
An experimental approach has been developed to quantify the characteristics and flux of firebrands during a management-scale wildfire in a pine-dominated ecosystem. By characterizing the local fire behavior and measuring the temporal and spatial variation in firebrand collection, the flux of firebrands has been related to the fire behavior for the first time. This linkage is seen as the first step in risk mitigation at the wildland urban interface (WUI). Data analyses allowed the evaluation of firebrand flux with respect to observed fire intensities for this ecosystem. Typical firebrand fluxes of 0.82–1.36 pcs m⁻² s⁻¹ were observed for fire intensities ranging between 7.35±3.48 MW m⁻¹ to 12.59±5.87 MW m⁻¹. The experimental approach is shown to provide consistent experimental data, with small variations within the firebrand collection area. Particle size distributions show that small particles of area 0.75–5×10⁻⁵ m² are the most abundant (0.6–1 pcs m⁻² s⁻¹), with the total flux of particles >5×10⁻⁵ m² equal to 0.2–0.3 pcs m⁻² s⁻¹. The experimental method and the data gathered show substantial promise for future investigation and quantification of firebrand generation and consequently a better description of the firebrand risk at the WUI.