|Temperatures in flames and fires
|by Dr. Vytenis Babrauskas, Fire Science and Technology Inc.
|It is unfortunately not too rare to find that fire investigators
estimate flame temperatures by looking up a handbook value, which turns out to
the adiabatic flame temperature. Statements are then made about whether
some materials could have melted, softened, lost strength, etc., based on
comparing such a flame temperature against the material's melting point, etc.
The purpose of this short paper is to point out the fallacies of doing this, and
to present some more appropriate information for a more realistic
First, we must point out that measuring of flame temperatures to
a high degree of precision is quite difficult, and many combustion research
scientists have devoted decades to studying the task. The difficulties come from
two sources: (1) intrusiveness of instrumentation; and (2) interpretation
difficulties due to the time-varying nature of the measurement. Non-intrusive
(e.g., optical laser techniques) methods are available, but these are difficult
and expensive to make and are generally not applied to the study of building
fires. In most cases, thermocouples are used for temperature measurement. These
have a multitude of potential errors, including surface reactions, radiation,
stem loss, etc. A whole textbook is available on the subject of instrumentation
for studying flames . As we see
below, the flames of most interest for unwanted fires are turbulent. This time
fluctuation presents tremendous difficulties in making measurements and in
interpreting them meaningfully. Such flames move about in little "packets."
Thus, a measurement at a single location returns a complicated average value of
reacting and unreacting packets flowing by. Some of these issues are elucidated
Even careful laboratory reconstructions of fires cannot bring in
the kind of painstaking temperature measuring technologies which are used by
combustion scientists doing fundamental research studies. Thus, it must be kept
in mind that fire temperatures, when applied to the context of measurement of
building fires, may be quite imprecise, and their errors not well characterized.
|Before we discuss details of flame temperatures, it is important
to distinguish between some of the major flame types. Flames can be divided into
- laminar, premixed
- laminar, diffusion
- turbulent, premixed
- turbulent, diffusion
An example of a laminar premixed flame is a Bunsen burner flame.
Laminar means that the flow streamlines are smooth and do not bounce around
significantly. Two photos taken a few seconds apart will show nearly identical
images. Premixed means that the fuel and the oxidizer are mixed before the
combustion zone occurs.
A laminar diffusion flame is a candle. The fuel comes from the
wax vapor, while the oxidizer is air; they do not mix before being introduced
(by diffusion) into the flame zone. A peak temperature of around 1400°C
is found in a candle flame .
Most turbulent premixed flames are from engineered combustion
systems: boilers, furnaces, etc. In such systems, the air and the fuel are
premixed in some burner device. Since the flames are turbulent, two sequential
photos would show a greatly different flame shape and location.
Most unwanted fires fall into the category of turbulent
diffusion flames. Since no burner or other mechanical device exists for mixing
fuel and air, the flames are diffusion type.
|Adiabatic flame temperature
|When one consults combustion textbooks for the topic of 'flame
temperature,' what one normally finds are tabulations of the adiabatic flame
temperature. 'Adiabatic' means without losing heat. Thus, these temperatures
would be achieved in a (fictional) combustion system where there were no losses.
Even though real-world combustion systems are not adiabatic, the reason why such
tabulations are convenient is because these temperatures can be computed from
fundamental thermochemical considerations: a fire experiment is not necessary.
For methane burning in air, the adiabatic flame temperature is 1949°C, while for
propane it is 1977°C, for example. The value for wood is nearly identical to
that for propane. The adiabatic flame temperatures for most common organic
substances burned in air are, in fact, nearly indistinguishable. These
temperatures are vastly higher than what any thermocouple inserted into a
building fire will register!
|Flames temperatures of open flames
|For convenience, we can subdivide the turbulent diffusion flames
from unwanted fires into two types: flames in the open, and room fires. First we
will consider open flames.
The starting point for discussing this topic can be the work of
the late Dr. McCaffrey, who made extensive measurements  of temperatures in
turbulent diffusion flames. He used gas burners in a "pool fire" mode (i.e.,
non-premixed) and studied various characteristics of such fire plumes. He
described three different regimes in such a fire plume:
- Slightly above the base of the fire begins the continuous
flame region. Here the temperatures are constant and are slightly below
- Above the solid flame region is the intermittent flame
region. Here the temperatures are continuously dropping as one moves up the
plume. The visible flame tips correspond to a temperature of about
- Finally, beyond the flame tips is the thermal plume region,
where no more flames are visible and temperature continually drop with height.
French researchers at the University of Poitiers recently made
the same types of measurements and reported numerical values 
indistinguishable from McCaffrey's. Cox and Chitty  measured similar plumes
and obtained very similar results: a temperature of 900°C in the continuous
flame region, and a temperature of around 340°C at the flame tips. The latter
value does not appear to be a universal constant. Cox and Chitty later measured
slightly higher heat release rate fires, and found a flame tip temperature of
around 550°C. In a later paper , researchers from the same laboratory
examined turbulent diffusion flames under slightly different conditions, and
found peak values of 1150-1250°C for natural gas flames, which is rather higher
than 900°C. The above results were from fires of circular or square fuel shape.
Yuan and Cox  measured line-source type fires. They found a temperature of
898°C in the continuous flame region, and a flame tip temperature of around
340°C. This suggests that such results are not dependent on the shape of the
In studying fires in a warehouse storage rack geometry, Ingason
 found an average solid-flame temperature of 870°C. At the visible flame
tips, the average temperature was 450°C, but the range was large, covering
300~600°C. In a related study, Ingason and de Ris  found typical flame tip
temperatures of 400°C for burner flames of propane, propylene, and carbon
Sullivan et al.  cite Australian studies on wildfire flames, finding that flame tip temperature corresponds to 300°C, while peak values around 927°C can be expected.
Heskestad  adopts a criterion of
500°C rise as defining the flame tip temperature, i.e. an actual
temperature of about 520°C.
Taking all of the above information in account, it appears that
flame tip temperatures for turbulent diffusion flames should be estimated as
being around 320~400°C. For small flames (less than about 1 m base diameter),
continuous flame region temperatures of around 900°C should be expected. For
large pools, the latter value can rise to 1100~1200°C.
|Flame temperatures in room fires
|There is fairly broad agreement in the fire science community
that flashover is reached when the average upper gas temperature in the room
exceeds about 600°C. Prior to that point, no generalizations should be made:
There will be zones of 900°C flame temperatures, but wide spatial variations
will be seen. Of interest, however, is the peak fire temperature normally
associated with room fires. The peak value is governed by ventilation and fuel
supply characteristics  and so such values will form a wide frequency
distribution. Of interest is the maximum value which is fairly regularly found.
This value turns out to be around 1200°C, although a typical post-flashover room
fire will more commonly be 900~1000°C. The time-temperature curve for the
standard fire endurance test, ASTM E 119  goes up to 1260°C, but this is
reached only in 8 hr. In actual fact, no jurisdiction demands fire endurance
periods for over 4 hr, at which point the curve only reaches 1093°C.
The peak expected temperatures in room fires, then, are slightly
greater than those found in free-burning fire plumes. This is to be expected.
The amount that the fire plume's temperature drops below the adiabatic flame
temperature is determined by the heat losses from the flame. When a flame is far
away from any walls and does not heat up the enclosure, it radiates to
surroundings which are essentially at 20°C. If the flame is big enough (or the
room small enough) for the room walls to heat up substantially, then the flame
exchanges radiation with a body that is several hundred °C; the consequence is
smaller heat losses, and, therefore, a higher flame temperature.
|Temperatures of objects
|It is common to find that investigators assume that an object
next to a flame of a certain temperature will also be of that same temperature.
This is, of course, untrue. If a flame is exchanging heat with a object which
was initially at room temperature, it will take a finite amount of time for that
object to rise to a temperature which is 'close' to that of the flame. Exactly
how long it will take for it to rise to a certain value is the subject for the
study of heat transfer. Heat transfer is usually presented to engineering
students over several semesters of university classes, so it should be clear
that simple rules-of-thumb would not be expected. Here, we will merely point out
that the rate at which target objects heat up is largely governed by their
thermal conductivity, density, and size. Small, low-density, low-conductivity
objects will heat up much faster than massive, heavy-weight ones.
 Fristrom, R. M., Flame Structure and Process, Oxford
University Press, New York (1995).
 Cox, G., and Chitty, R., Some Stochastic Properties of Fire
Plumes, Fire and Materials 6, 127-134 (1982).
 Gaydon, A. G., and Wolfhard, H. G., Flames: Their
Structure, Radiation and Temperature, 3rd ed., Chapman and Hall,
 McCaffrey, B. J., Purely Buoyant Diffusion Flames: Some
Experimental Results (NBSIR 791910). [U.S.] Natl. Bur. Stand.,
Gaithersburg, MD (1979).
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Average Centerline Temperatures of a Buoyant Pool Fire Obtained by Image
Processing of Video Recordings, Fire Safety J. 24, 107-130
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Properties of Unbounded Fire Plumes, Combustion and Flame 39,
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Turbulent Diffusion Flames, Combustion and Flame 91, 226-238
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Fires, Fire Safety J. 27, 123-139 (1997).
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1209-1220 in Fire Safety Science-Proc. Fourth Intl. Symp., Intl. Assn.
for Fire Safety Science, (1994).
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Geometries, Fire Safety J. (1997).
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in Depth, pp. 427-438 in Fire Safety Science--Proc. Fifth Intl. Symp.,
Intl. Assn. for Fire Safety Science (1997).
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Compartment Fires, Fire and Materials 2, 39-53
(1978); and 3, 17 (1979).
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Construction and Materials (ASTM E 119). American Society for Testing and
 Sullivan, A. L., Ellis, P. F., and Knight, I. K., A Review of Radiant Heat Flux Models Used in Bushfire Applications, Intl. J. Wildland Fire 12, 101-110 (2003).
Written 28 April 1997; revised 25 February 2006. Copyright © 1997, 2006 by Fire
Science and Technology Inc.