|
Introduction
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 assessment.
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 [1].
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 in [2].
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.
Flame types
Before we
discuss details of flame temperatures, it is important to distinguish
between some of the major flame types. Flames can be divided into 4 categories:
- 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 [3].
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 [4] 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 900°C.
- 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 320°C.
- 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 [5] indistinguishable from McCaffrey's.
Cox and Chitty [6] 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 [7], 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 [8] 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 fuel source.
In studying
fires in a warehouse storage rack geometry, Ingason [9] 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 [10] found typical flame tip temperatures
of 400°C for burner flames of propane, propylene, and carbon monoxide
fuels.
In the SFPE
Handbook, Heskestad [11] recommends using a value of 650°C for the temperature
rise at the flame tip, i.e., an actual temperature of about 670°C.
This seems notably high compared to the experimental data cited above,
and Heskestad does not provide any explanation where his value comes from.
Also in the Handbook, Mudan and Croce [12] summarize some continuous-flame
region measurements for various liquid pools. With the exception of a
few data points, most values lie between 827°C to 1127°C. The variations
appear to be more attributable to experimental technique than to type
of liquid being burned. Most of the values are for quite large (many meters
in diameter) pools. Fundamental radiation considerations would suggest
that smaller pools might show somewhat lower temperatures, but data to
demonstrate this point seem sparse. Curiously, in a later study [13],
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 [14] 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 [15] 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.
References
[1] Fristrom,
R. M., Flame Structure and Process, Oxford University Press, New
York (1995).
[2] Cox,
G., and Chitty, R., Some Stochastic Properties of Fire Plumes, Fire
and Materials 6, 127-134 (1982).
[3] Gaydon,
A. G., and Wolfhard, H. G., Flames: Their Structure, Radiation and
Temperature, 3rd ed., Chapman and Hall, London (1970).
[4] McCaffrey,
B. J., Purely Buoyant Diffusion Flames: Some Experimental Results (NBSIR
791910). [U.S.] Natl. Bur. Stand., Gaithersburg, MD (1979).
[5] Audoin,
L., Kolb., G., Torero, J. L., and Most., J. M., Average Centerline Temperatures
of a Buoyant Pool Fire Obtained by Image Processing of Video Recordings,
Fire Safety J. 24, 107-130 (1995).
[6] Cox,
G., and Chitty, R., A Study of the Deterministic Properties of Unbounded
Fire Plumes, Combustion and Flame 39, 191-209 (1980).
[7] Smith,
D. A., and Cox, G., Major Chemical Species in Turbulent Diffusion Flames,
Combustion and Flame 91, 226-238 (1992).
[8] Yuan,
L.-M., and Cox, G., An Experimental Study of Some Line Fires, Fire
Safety J. 27, 123-139 (1997).
[9] Ingason,
H., Two Dimensional Rack Storage Fires, pp. 1209-1220 in Fire Safety
Science-Proc. Fourth Intl. Symp., Intl. Assn. for Fire Safety Science,
(1994).
[10] Ingason,
H., and de Ris, J., Flame Heat Transfer in Storage Geometries, Fire
Safety J. (1997).
[11] Heskestad,
G., Fire Plumes, p. 2-13 in SFPE Handbook of Fire Protection Engineering,
NFPA/SPFE (1995).
[12] Mudan,
K. S., and Croce, P. A., Fire Hazard Calculations for Large Open Hydrocarbon
Fires, p. 3-203, op. cit.
[13] Heskestad,
G., Flame Heights of Fuel Arrays with Combustion in Depth, pp. 427-438
in Fire Safety Science--Proc. Fifth Intl. Symp., Intl. Assn. for
Fire Safety Science (1997).
[14] Babrauskas,
V., and Williamson, R. B., Post-Flashover Compartment Fires, Fire
and Materials 2, 39-53 (1978); and 3, 17
(1979).
[15] Standard
Test Methods for Fire Tests of Building Construction and Materials (ASTM
E 119). American Society for Testing and Materials, Philadelphia.
|
