AN INVESTIGATION OF EXTINGUISHMENT BY THERMAL AGENTS USING 

DETAILED CHEMICAL MODELING OF OPPOSED-FLOW DIFFUSION FLAMES 

William M. Pitts and Linda G. Blevins 

National Institute of Standards and Technology 

Gaithersburg, MD 2OX9Y-X653 USA 

INTRODUCTION 

The manufacture of the halons widely used in fire extinguishing systems was banned in 1994 due 

10 their deleterious effect on stratospheric ozone. Since the late 1980s there have been ongoing 

research efforts to identify replacement agents having comparable properties. This search has 

proven difficult and continues today with a large directed effort known as the Next Generation 

Fire Suppression Technology Program (NGP). As part of the NGP. the National Institute of 

Standards and Technology is investigating whether highly effective thermal agents are feasible. 

Thermal agents are defined as those that obtain their effectiveness solely by heal extraction and 

dilution. Excluded from investigation are species that directly or indirectly disrupt the combuslion chemistry such as halons, which derive much of their effectiveness by the release of bromine 

atoms that catalytically remove hydrogen atoms in the flame zone. 

A great deal is known about the effects of thermal agents on tlames. The paper by Sheinson el 

iil. provides a good introduction [ 11. A number of endothermic physical processes can extract 

heat from a gaseous flame zone, thus lowering the temperature and ultimately leading to tlame 

extinguishment. These include simple heating (it., heat capacity) of an agent, phase changes 

such as vaporization of a liquid or sublimation of a solid, endothermic molecular decomposition 

(which is classified as a physical proce. s long iis the initial agent and its products do not participate in the combustion chemistry), and simple dilution, which can modify flame temperatures 

by spreading the heat release over larger volumes and by affecting three-body flame reactions. 

The flame temperature is also expected to be a function of the thermal diffusivity of an agent. 

During the past two decades the understanding of the chemical kinetics involved in combustion 

has reached the point where realistic detailed mechanisms involving large number of reactants 

and reactions can be written for simple combustion systems, and mathematical techniques have 

been developed for simultaneously solving the large number of differential equations that result. 

While still involvins significant approximations. such modeling has advanced to the point where 

it can be used to gain useful insights into the behavior of praclically relevant tlames. 

This paper describes the results of a detailed chemical kinetic modeling investigation of laminar 

opposed-flow mcthane/air diffusion flame designed to provide an improvcd understanding of the 

extinguishment of fires by thermal agents. A particular focus was to test the hypothesis that the 

effectiveness of a thermal agent depends on the location of heat absorption relative to the flame 

zone. An internal report has been prepared, which summarizes the kinetic modeling in detail and 

also includes the results of an extensive database search of potential thermal agents and modeling 

results for the effectiveness of thermal agents in cooling liquid surfaces [2]. 
DETAILED CHEMICAL KINETIC MODELING 

A number of different types of combustion systems have been modeled using detailed chemical 

kinetic approaches including plug flow reactors, perfectly stirred reactors, premixed laminar 

flames, and opposed flow laminar diffusion flames. For studies of fire extinguishment, we have 

chosen an opposed flow laminar diffusion flame model because for the majority offires the fuel 

and air are initially separated and therefore burn as diffusion flames. Most fires are large enough 

to be turbulent, so a laminar flame model is not strictly correct. However, the most widely used 

model for turbulent combustion incorporating detailed chemistry is the laminar flamelet concept, 

which treats the combustion as laminar flame sheets subject to the local strain rate field associated with the fire-induced motions of the fluid [3,4]. Thus, the opposed flow diffusion flame is a 

particularly appropriate detailed chemical kinetic model for describing fire behavior. 

Three configurations of opposed flow diffusion flames have been described in the literature: 

opposed jet, flow over a porous cylinder, and flow over a porous sphere. For the porous bodies, 

fuel typically flows from the surface, and the surrounding flow is the oxidizer. The three configurations are related in that a diffusion flame is stabilized in the region near the stagnation point in 

the flow. Numerous papers in the literature discuss these types of flames. Two excellent reviews 

have been provided by Tsuji [5] and Dixon-Lewis 161. The review by Tsuji includes a discussion 

of the use of such burners to characterize flame inhibition experimentally. Recently, an opposed 

jet experiment has been used to investigate potential replacement agents for halons [7]. The most 

common of the three configurations used in the recent past is the opposed jet laminar diffusion 

flame, which was chosen for the current investigation. 

Laminar opposed flow diffusion flames are usually modeled as one-dimensional flow systems by 

using a similarity transform to reduce the two-dimensional equations. Fuel and oxidizer velocity 

profiles at the burner exits are most often assumed to be those appropriate either for potential 

flow, which has a constant strain rate (a,) (velocity gradient), or for exit plug flow, which has a 

local strain rate (a) equal to 0 s-'. Since the latter boundary condition corresponds more closely 

to most experiments, it is used for the calculations reported here. As the opposed flow velocities 

are increased, the flame is subjected to higher and higher strain rates. It is well known that, as 

the strain rate becomes larger, the flame gradually becomes weaker and weaker until it abruptly 

undergoes extinction [8,9]. A number of different parameters are used to quantify the effect of 

strain rate on a laminar flame including the constant strain rate (al,); a global strain rate (u,~); the 

maximum strain rate outside of the thermal boundary layer on the oxidizer side (a,,); and the 

stoichiometric scalar dissipation (n.J. 

For this investigation, a series of laminar opposed flow diffusion flames of methane and oxidizer 

has been calculated as a function of their opposed flow velocities (assumed to have equal magnitudes) and the concentration of various thermal agents added to the air. For each concentration 

of added agent, an extinction condition is identified that corresponds to a given velocity magnitude and corresponding measures of strain rate and stoichiometric scalar dissipation. A focus 

of this work is to identify the minimum concentration of an agent required to extinguish buoyancy dominated fires. The cup burner is an example of one experimental approach for determining this concentration. The extinguishing concentration is therefore expected to correspond to a 

particular extinction condition. 

146 Halon Options Technical Working Conlerence 27.2') April 1999 
CALCULATIONAL APPROACH 

The goal is to calculate the effects of thermal agents on laminar methane/air opposed flow 

diffusion flames. There are 21 number of codes described in the literature for making such 

calculations. The code Oppdif [IO] developed by Sandia National Laboratories was chosen. 

This code is now available commercially from Rcaction Design” of San Diego, CA. Oppdif is 

built on a number ofgcncral purpose subroutines, collectively known as Chemkin-Ill [I I]. which 

handle many of the tasks associated with the calculation. Data describing the reaction mechanism and thermodynamic and transport properties are either incorporated iii Oppdif or accessed 

as databases. 

Oppdif solvcs the psuedo-one-dimensional equations describing a laminar axisymmetric opposed 

flow diffusion f1:une. The approach is that originally used by Kee et al. 1121 for premixed flames 

to solve problems in which the radial velocity gradient is constant at the boundaries and later 

extended to diffusion flames by Chelliah et al. 1131 This condition includes plug tlows with a = 

0 s-’ at the burner exits, which were used for all of the calculations here. 

A detailed chemical kinetic mechanism is required to model the flame. After reviewing the 

literature, we chose the widcly used mcthane/air mechanism developed with the support of the 

Gas Research Institute. The version used was GRI-Mech 1.2 [ 141 which consists of 32 chemical 

species undergoing 177 reactions. One- and two-carbon reactions are included. Thermodynamic 

and transport dara are provided as separate files. The mechanism was created by the GRIsponsored researchers by starting with appropriate estimates for the rate constants and then 

optimizing the mechanism to provide the best agcemcnt with experimental measurements in 

such systems as premixed tlames, shock tube studies, and flow reactor investigations [ 141. It 

should be noted that the mechanism was not 

optimized using diffusion flame results. MethaneiAir Diffusion Flame 

2050 , I 

To determine the extinction behavior for a flame as 

functions of flow conditions and agent concentration, 

ii burning solution was first obtained for a particular 

combination of methane and oxidizer having 

rclativcly low fuel and oxidizer exit velocity 

magnitudes. The velocity magnitudes were then 

increased until a condition was reached where the 

flame went out or a solution was not obtained. By 

approaching the extinction velocity in small 

incrcmcnts, it was possible to obtain the extinction 

point to within a step size of 0.01 cm/sec. 

Figure I shows a plot of maximum temperature, 7,,(,.,, 

1850 .- 

x 

m 

z 

. 0 

0 

0. 

0 SO 100 150 200 250 300 

Flow Velocities (crnls) 

versus flow velocity (equal magnitudes for the fuel 

and oxidizer opposed velocities) calculated for ii 

laminar opposed flow methane/air diffusion flame. 

Figure I. Maximum flame temperature 

versus exit velocity magnitudes. 

“Certain commercial equipment, instruments, or material are identified herein to specify the experimcnta1 procedure adequately. Such identification does not imply recommendation or endorsement by NIST, 

nor does it imply that the materials or equipment are necessarily the best available for the purpose. 

H;ilon Oplion\ Technical Working (~’anfcrsnoc 27-2‘) April IWl) 147 
The fuel is 100% methane and air is assumed to be composed of 88.1 % N?, 21 .O% 02, and 0.9% 

Ar by volume. As expected, T,,,,, decreases with increasing velocity. Extinction is calculated to 

occur for a velocity of 320.12 cm/sec with a maximum flame temperature of 1785 K [8,9]. 

The maximum strain rate on the oxidizer side for the extinction condition is *a,,* = SO9 sec-'. 

This value is roughly 25% higher than measured experimentally [7,13.15]. Tanoff et al. have 

shown that calculated values of a,, are highly dependent on the detailed mechanism used [ 161. In 

their work, GRI-Mech also overpredicts the extinction strain rate. 

METHANE FLAMES BURNING IN AIR DILUTED WITH THERMAL AGENTS 

Despite the large number of studies previously reported for methane combustion, we were only 

able to identify two experimental measurements of extinguishing concentration for methane 

flames burning in air diluted with an added thermal agent. Both studied the effect of added 

nitrogen. Simmons and Wolfhard [I71 and Ishizuka and Tsuji [ 181 reported extinguishing concentrations of 33.8% and 3 1.9%. respectively. These two measurements are in good agreement. 

Simple averaging yields an estimate of 33% added nitrogen. Simmons and Wolfhard calculated ~ -. 

the adiabatic flame temperature for a 

stoichiometric premixed flame corresponding 

to their extinction condition as 1820 K. 

Ishizuka and Tsuji used thermocouples to 

measure the maximum flame temperature in 

their diffusion flame as 1483 K for the extinguishing condition. A series of calculations 

was performed for methane opposed-flow 

diffusion flames reacting with air containing 

various percentages of added nitrogen. 

Figure 2 shows the results. Several points 

are immediately obvious. As the percentage 

of added nitrogen increases, the fuel and 

oxidizer exit velocity magnitudes sufficient 

to cause flame extinction decrease. The 

maximum flame temperature at extinction 

also decreases as the nitrogen concentration 

increases. The plots of T,,$,, versus velocity 

magnitude become more steep as the 

concentration of nitrogen increases. 

Nitrogen Dilution of CHJAir Diffusion Flames 

- 

. 

. 

i 

0 % 

w 5% - A 10% 

v 15 Yo 

20% 

25% 

0 30% 

33% 

1500 

0 50 100 150 2W 250 300 

Figure 2. Maximum flame temperature versus 

flow velocity magnitudes for methane 

burning in nitrogen-diluted air. 

The experimental concentration of added nitrogen that extinguishes the flame is 33%. The calculated value of T,,, at extinction for this concentration is 1545 K. This is very close to the experimental value of 1483 K [18]. In fact, the agreement is probably better than indicated since the 

thermocouple measurements of Ishizuka and Tsuji were not corrected for radiative heat losses, 

which would be expected to decrease the measured temperature. The calculated temperature is 

expected to be a slight overestimate due to neglect of the radiation heat losses. 

These estimates for the maximum flame temperature at extinguishment are consistent with others 

available in the literature. Sheinson et ai. indicate that hydrocarbon combustion is not sustainable when the maximum flame temperature drops below roughly 1600 K [I]. This estimate is 

148 Halon Options Technical Working Conference 27-29 April 199Y 
based upon work by Roberts and Quince [ 191. Very recently, Babb et al. have rcported radiationcorrected thermocouple tcmpcrature measurements in heptane flames ncar their extinction points 

using air diluted by N? and CO? [20]. For both thermal agents. the maximum tlame temperatures 

at extinguishment were on the order of 1500 K. Similar measurements using propane as fuel 

gave maximum flainc tcmpcratures of I600 ti and 1700 K for added Nz and CO2, respectively. 

For the conditions corresponding to cxtinguishment with added nitrogen, the velocities of the 

fuel and air at extinction arc calculated to be 2 I .42 cm/scc. Corresponding values for the various 

mcasures used to characterize the flow condition at extinction are ux = 37.5 scc~', *u,,:k = 

27.0 sec-', and n, = 0.80 sec~'. 

An important question concerning the extinction of diffusion flames is-What strain rate is 

appropriate to use when determining the minimum value of an added thermal agent rcquired to 

extinguish diffusion flames at normal gravity'? The only discussion of this point of which we are 

aware was presented by Hamins et al. [2 I]. These authors compared cup-burner measurements 

(heptane fuel) of extinguishing concentrations for a variety of agents with corresponding 

measurements madc in it counterflow tlamc. The counterflow measurements wcrc made over a 

range of tlow velocities, which were characterized in terms of a global strain rate appropriate for 

an air flow impinging on a liquid surface. In agreement with thc current calculations, the required strain rate to cause extinction of the counterflow tlamc decreased with the concentration of 

added agent. When the concentrations of added agents for the opposed flame were comparable 

to those observed in thc cup-burncr test, the global strain rate was on the order of 50 sec~'. Due 

rent boundary conditions and fuels between the current invcstigation and those for 

Hamins ct al. [?I], as wcll as slightly different definitions for the global strain rates, absolute 

quantitative comparisons are not appropriate. However, it is clear thal the sti-ain riitcs have 

comparable magnitudes in each case. It is important to note that the strain rate appropriate for 

Characterizing the extinguishing condition is much lower than typically used for experimental 

and modeling investigations of opposed-llow laminar diffusion flames. 

It is interesting to speculate about why lower and lower strain rates can not be sustained for 

tlames at normal gravity. The most likely reason is that buoyancy effects result in ii lower limit 

for the minimum strain rate perpendicular to ii tlame surface. Buoyancy always accelerates hot 

combustion gases relative to the cold oxidizer with the result that the flame surface is subject to ii 

nonzero strain rate. The results of Hamins et al. [21] and the current findings suggest this minimum strain rate is on the order of a few tens of inverse seconds. 

As iilrcady pointed out. we have identified no additional measurements of extinguishing concentrations for thcrmal agents added to methane/air diffusion flames. However. lshizuka and Tsuji 

did make measurements for methane burning in an artificial "air" consisting of 2 1 % oxygen and 

79%) argon [ 181. This "air" was diluted with argon until extinguishment occurred. Thc result 

corresponded to an added argon concentration of 54.3%. The measured maximum tliime temperature at extinguishment was 1443 K, or roughly 40 ti less than for standard air diluted with 

nitrogen. To test the ability to predict extinguishing concentrations of added thermal agents, a 

series of calculations for argon "air" diluted with argon were made. 
Figure 3 shows a plot of the maximum Argon Dilution of CHjAr “Air” Diffusion Flame 

2300 

calculated flame temperature as a function of 

the percentage of argon added to the argon “air” 

and the fuel and oxidizer velocity magnitudes. 

Comparison with Figure shows that replacing 

nitrogen with argon has a dramatic effect on the 

flame behavior. First, flame temperatures are 

considerably higher for a given diluent 

concentration and fuel and oxidizer velocity 

magnitudes for the argon “air.” This is due to 

0% 

10% 

. 20% 

v 30% 

40% 

50% 

0 54% 

1800 

E 

the lower heat capacity of argon as compared to 

nitrogen. As a result of the increase in temperature, much higher flow velocities are required 0 200 400 600 800 1000 1200 

1500 

to generate the strain rate necessary to cause Flow Velocities (crnis) - 

extinction. For the same reason, higher concentrations of argon must be added to achieve 

extinguishment than in the nitrogen case. 

Figure 3. Maximum flame temperature 

versus exit velocity magnitude for 

methane flame in Ar “air.” 

The flame formed by an oxidizer having 50% argon is calculated to undergo extinction with a 

maximum temperature of 1610 K and methane and oxidizer velocity magnitudes of 37.8 cm/sec. 

The corresponding results for 54% argon, corresponding to the experimental extinguishment 

value, are 1473 K and 15.9 cm/sec. These values are both slightly smaller than found for the 

methane/air flame diluted with nitrogen, 154.5 K and 21.4 cm/sec, respectively. However, they 

are remarkably close when one recalls that the use of argon instead of nitrogen should result in a 

significantly different flame structure due to differences in heat capacity and thermal diffusivity 

between nitrogen and argon. In fact, if one simply assumes that extinguishment occurs for the 

same maximum flame temperature. Le., roughly 1550 K. as for the nitrogen-diluted air flame, it 

is possible to estimate the required argon concentration as 52% (Figure 3). This is only 4% less 

than the experimental value. Thus, an assumption that flame extinguishment occurs for the 

concentration necessary to reduce the maximum calculated flame temperature at extinction to 

1550 K should provide an excellent estimate for the percentage of an arbitrary thermal agent 

required to extinguish a fire. This approach is used for estimating extinguishing concentrations 

in the present work. 

Opposed tlow diffusion flame calculations have been used to estimate the required extinguishing 

concentrations, i.e.. the concentration necessary to lower the maximum flame temperature at 

extinction to 1550 K, for methane burning in air diluted with Ar, He, COz, and H?O. Each of 

these gases is expected to act primarily as a thermal agent. The resulting plots (not shown) of 

maximum flame temperature versus the magnitudes of the fuel and oxidizer velocities exhibit 

trends similar to those for nitrogen and argon shown in Figures 2 and 3, respectively. The 

estimates for extinguishing concentration are tabulated below (Table 1). 

As already discussed, the only experimental values for opposed flow methane diffusion flames 

we have identified are for nitrogen dilution. Cup-burner determinations of extinguishing concentrations using heptane as fuel have been reported for some of these agents by Sheinson et al. [l], 

Babb et al. [20], Hamins et al. [22], and Moore et al. [23]. These values are included in Table 1. 

The maximum difference between values calculated for methane and the experimental values for 

150 Halon Options Technical Working Conference 27-29 April 1999 
TABLE I. EXTINGUISHING CONCENTRATIONS (MOLE FRACTION) OF 

THERMAL AGENTS. 

Thermal Agent Current Cup Burner Cup Burner Cup Burner Cup Burner 

Work 111 [201 [221 1231 

Nitrogen 0.33 0.30 0.33 0.32 0.30 

Argon 0.43 0.4 ~ I 0.4 1 0.38 

Helium 0.34 0.32 0.3 I ~ 

Carbon Dioxide 0.22 0.2 I 0.20 0.23 0.20 

Water 0.28 

heptane is 12%), with the vast ma.jority being less than 10%. With the exception of carbon diox 

ide. the cup-burner measurements are somewhat lower than for the counterflow flame. These 

differences could be due to the use of different fuels or to the effects of burner configuration. 

The heptane fires are burning just above a liquid fuel in a cotlow of oxidizer. and it may be easier 

to blow out this type of flame than the opposed flow diffusion flame. which is stabilized away 

from surfaces. The close tracking of the calculated results and the experimental findings suggest 

that detailed chemical kinetic modeling can accurately predict the amount of a thermal agent 

required to extinguish opposed-flow diffusion and cup-burner flames. 

The differences in the extinguishing concentrations of helium and argon are interesting since 

these agents are both monatomic gases and have the same heat capacities. The difference is clear 

in both the calculations and the experimental values. The fact that helium is a more efficient 

extinguishing agent means that at least one other parameter. in addition to heat capacity, is 

important in determining extinguishing efficiency. A related observation was reported by 

Coward and Hariwell for the inerting of premixed flames and was attributed to the much higher 

~ 

~ ~ ~ ~ 

thermal conductivity of helium, which distributes 

the heat of combustion over a larger region of spacc 

and therefore weakens the flame [24]. The same 

explanation is most likely valid for diffusion flames. 

Sheinson et al. reached the same conclusion [I]. 

Comparison of the dependence of calculated maximum flame temperature on the fuel and oxidizer exit 

velocity magnitudes for the different thermal agents 

shows that. for given velocities and extinction temperatures, they have vety similar shapes. This is true 

despite the fact that the amount of added agent and. 

therefore, the oxygen concentration and stoichiometric ratio vary dramatically with agent. This is 

confirmed in Figure 4, which shows the maximum 

flame temperature at extinction as a function of fuel 

and oxidizer exit velocity magnitudes for the five 

thermal agents added to air as well as the results for 

argon added to argon "air." The data fall within a 

narrow hand on either side of a well-defined curve. 

Inert-Diluted Methane Flames 

1800 ---~,-~ 

A L ~.~-! ~~2 ~ 

0 50 100 150 200 250 300 

Extinction Flow Velocity (crnls) 

Figure 4. Maximum flame temperature 

versus exit tlow velocity 

magnitude for thermal agents. 
A survey of thermodynamic data bases has identified potential halon replacements based on their 

ability to absorb heat [2]. One example is methoxy-nonafluorobutane (CdFqOCH3; HFE7100), 

which is available commercially [Z]. The required extinguishing concentration was estimated 

as 5.5% assuming an extinction temperature of 1550 K. Two experimental measurements of the 

concentration of HFE7 100 required to extinguish diffusion flames have been identified. Unpublished measurements from the New Mexico Engineering Research Institute using a standard cup 

burner with heptane fuel yielded a value of 6.1 vel.%.* In a patent disclosure, Flynn and Scott 

also reported that 6.1 % was sufficient to extinguish a butane flame in a “micro-cup burner” [26]. 

These values are 11% higher than estimates from detailed chemical kinetic modeling. 

. . 

0. 

0% - 20% 

40% 

60% 

70% 

80% 

SURROGATE AGENT STUDIES OF EXTINCTION AND EXTINGUISHMENT 

An important advantage of modeling investigations is the ability to perform calculations for 

conditions that are not physically possible in order to learn details concerning the role of various 

parameters. One question, which has been the subject of speculation, is the relative importance 

of heat extraction and dilution on the effectiveness of a thermal agent. Here dilution refers to any 

effects of a thermal agent not simply due its ability to extract heat. To obtain insights into this 

behavior, an artificial agent was created by starting with argon and setting its heat capacity to 

zero. Direct comparison with the results for added argon allows the relative roles of heat extraction and dilution to he characterized. Sheinson et al. have discussed the effects of dilution on 

extinguishment [I]. They concluded that they are relatively small compared to direct heat removal due to heat capacity for the thermal agents CFq and SFs. 

Figure 5 is a plot of calculated maximum flame temperature versus fuel and oxidizer velocity 

magnitudes for the zero-heat-camcity argon added to air. It is obvious that this species does 

m 

5 

E 1700 

.: - 

3 1600 3 

- 

decrease the strength of the flame, hut its effect is 

much smaller than for the argon. Based on an 

extinguishment temperature of 1550 K, the extinguishing concentration can he estimated as 73% or 

roughly 1.7 times of the amount required for 

argon. This corresponds to an oxygen concentration of 5.7%. Interestingly, the dependence of the 

maximum flame temperature at extinction on the 

fuel and oxidizer velocities is very similar for both 

sets of calculations. Assuming that the effects of 

heating an inert and dilution are additive and 

linear in concentration, the effectiveness of Ar as 

a thermal agent is estimated to he 59% due to 

dilution and 41% due to heat extraction. Comparison of the detailed flame structures for methane 

flames in air and in air diluted with zero-heatcapacity argon shows that the primary reason for 

the weakening of the flame burning in the diluted 

. A 

. 

. - e . . . e’ A 

e . 

e a ~ 

Dilution by Argon Having No Heal Capacity 

.’ , 

1500 0 u 50 100 150 200 250 300 

Flow Velocities (cmis) 

Figure 5. Maximum flame temperature 

versus exit velocity magnitude 

agent with no heat capacity. 

for 

* Tapscott, R. E., Personal Communication, 26 February 1999. 

152 Halon Options Technical Working Conference 27-29 April IYYY 
air is the passage of more oxygen through the flame to the fuel side. The unreacted oxygen that 

leaks through the flame front acts as a thermal agent. 

One of the goals of the current work was to test the hypothesis that the effectiveness of a thermal 

agent depends on the location, relative to the high temperature flame zone, where heat extraction 

occurs. A surrogate thermal agent, X, was used for these calculations. The molecular weight, 

thermodynamic properties, and transport properties of X are identical to those of argon, but it can 

undergo a simple reaction with ambient gases to generate a new species, Y, that is, 

X+M+Y+M (1) 

Y is also very similar to argon, the only difference being that its heat of formation is assigned an 

arbitrary positive value instead of being zero. As a result, when Reaction (1) takes place it 

extracts heat and cools the local surroundings by an amount equal to the heat of reaction, AHx-y. 

Since X and Y do not react with any other species, the reaction is simply a heat sink and, therefore, meets the definition of a thermal agent. 

The rate constant for Reaction (1) is expressed as 

where A is the pre-exponential factor, p is the temperature exponent, E,, is the energy of activation, R is the gas constant, and Tis temperature. By varying the parameters A, p, and E,,, it is 

possible to change the rate and temperature range over which the reaction occurs and hence the 

location relative to the flame zone for heat extraction. For the following calculations, initial 

values were chosen for A and p, and only the value of E, was changed in order to vary kXAY. 

Figure 6 compares calculated flame temperature 

versus distance from the fuel exit for two flames 

having fuel and oxidizer exit velocity magnitudes of 

25 cdsec and with 5% X added to the air. For each, 

A = 1~10'~ cm3/(mole-s), p = 0, and AHx+= 96.1 

kJ/mole. The only difference between the two calculations is the value of E,, which equals 25. l kJ/ 

mole, for one and 50.2 kJ/mole for the other. For the 

lower E,, X begins to react immediately upon 

leaving the oxidizer exit, which results in the 

temperature drop evident on the oxidizer side for 

positions well removed from the flame zone. When 

the E, is increased to 50.2 kJ/mole, the conversion of 

X to Y is very slow at room temperature, and there is 

no significant drop in temperature in the ambient 

region of the flow. However, as the temperature 

increases, X begins to convert to Y, and heat is 

absorbed in higher temperature flame regions. 

Interestingly, maximum flame temperatures are 

MethanelAir Diluted With 5% X 

Velocity Magnitudes = 25 cws 

2wo 

g l400 

% lOW / I 

5 

? 2 1m 

c 

/ 

/ 600 / 

/ 4000 / 

+ BOD 

-_~~ 00 05 10 15 20 

Distance From Fuel Exit (cm) 

Figure 6. Temperature versus distance 

from fuel exit for methanelair 

diluted with 5% X flame. 

identical within the uncertainty of the calculations. Since flame extinguishment depends 

primarily on the maximum flame temperature at extinction, this suggests the effectiveness of a 

Halon Options Technical Working Conference 27-29 April 1999 153 
thermal agent is independent of the spatial location where the heat extraction occurs, indicating 

that the original hypothesis concerning the effect of heat extraction position was incorrect. 

A complete set of calculations was carried out 

for X having AHx+ = 96.1 kl/mole and E, = 

41.8 kJ/mole. Based on an extinction 

AT and X (E,dl.a Wlmole, AH.96.1 LJlmole) Dilution 

temperature concentration The corresponding of for 1550 X value is K, estimated the for extinguishing argon tobe was 15.9%. 43%. ;.:p a F m im 

? 

m 

$ 

3 1Bw 

1Bw Thus, the heat extracted by the reaction of X to 

Y has reduced the amount of agent required by 

of maximum flame temperature as a function of 

added argon with the corresponding results for 

close together, but agreement is not complete. 

At the lower concentrations the results for X fall 

slightly below those for Ar, while the opposite 

is true for the highest concentrations. 

. 00% 

0 ?5%Ar 

rn 30%Ar 

A 45%Ar 

0 59-x 

0 lO%X 

a 159.x 

nearly 2/3. Figure 7 compares calculated values 

velocity magnitude for 15%, 30%, and 45% 

ii 17w 

z 

I 

f 

5%, lo%, and 15% X. The two sets of data fall 15W 0 50 1W 150 2W 250 300 

Flow Velocities (crnls) 

Figure 7, Maximum flame temperature is 

dotted versus exit velocitv maeniThe effect of doubling the heat absorbed by X 

was considered by running a series of calculations for AHw-" = 192.1 kJ/mole. From these 

- 

tude for methane flames in air 

diluted with Ar and X having 

E, = 4 1.8 kJ/mole. .. . 

results the extinguishment concentration for X with the higher heat absorption can be estimated 

as 9.7%. This value is roughly 60% of that found with AHx+ = 95.2 kJ/mole, or 20% higher 

than would be expected if flame extinguishment was due solely to the heat extracted. The most 

likely source for this differences, as well as that seen in Figure 7, is the effect of dilution discussed earlier. The concentration of added agent required for flame extinguishment decreases as the 

amount of heat it can absorb increases, but the decreased concentration reduces the effectiveness 

of dilution, with the result that the effectiveness of extracting additional heat is offset somewhat. 

The role of dilution has implications with regard to estimates of extinguishing efficiency for 

thermal agents, which are often obtained by taking ratios of heat capacities for various agents. If 

the agents have a large heat capacity difference, and the percentages required for extinguishment 

therefore differ substantially, a simple linear dependence on heat capacity should not be observed. In fact, the agent having the largest heat capacity should be less effective than expected, as 

observed in the current calculations. 

SUMMARY 

It has been shown that detailed chemical kinetic modeling can be used to make quantitative 

predictions of the amount of a thermal agent required to extinguish a fire. Results for four wellknown thermal agents are in good agreement with experimental values. The calculations suggest 

that strain rates in fires are on the order of a few tens of inverse seconds and that the maximum 

flame temperature at extinction for the extinguishing condition is approximately 1550 K. The 

use of surrogate agents has revealed that (1) dilution effects are due to an increase in the amount 

154 Halon Options Technical Working Conference 27-29 April 1999 
of02 blccding through the tlanmc front and (2) the role of dilution is relatively small as compared 

to heat absorption. The location of the heat absorption relative to the flame front does not affect 

the ability of a thermal agent to extinguish a flame as long as the agent is convccted to the flamc 

zone. While good qualitative estimates of flame extinguishing effectiveness can be obtained 

based solely on the ability of an agent to absorb heat, the estimatcs are not perfect due to dilution 

effects. 

ACKNOWLEDGMENT 

This research is pnri oC the Departincnt of Defense Next Gencration Program Firc Suppression 

Technolozy Program, funded in part by the DOD Strategic Environmental Research and 

Development Program under Contract W74RDV73243630. 

REFERENCES 

I. 

2. 

3. 

4. 

5. 

6. 

7. 

8. 

9. 

IO. 

1 I. 

12. 

Sheinson. R. S., Penner-Hahn, J. E.. and Indritz, D., “The Physical and Chemical Action of 

Firc Suppressants.” Fire Sufcty ./ournu/. IS, 437-450, 1989. 

Pitts. W. M.. Yang, J., Huber, M., and Blcvins, L. G., Chuructc,ristic..s und l~/~~nr;ficution of 

Suprr-cfliwiiv Thernwl Fii-i,-E,~-tiiigui.shiii,q Agents-First Ammu/ Report, National 

Institute of Standards and Technology Internal Report, Gaithersburg, MD, 1999. 

Peters, N., “Laminar Diffusion Flamelet Models in Non-Premixed Turbulent Combustion,” 

Pwgress in Enqp und Conihustion Srirncc, 10, 3 19-339. 1984. 

Peiers, N, “Laminar Flamelet Conccpts in Turbulent Combustion.” Twwiry-First Symposinm (l~~t~,r~?u/i~j/i~rl) on Comhirstion, The Combustion Institutc, Pittsburgh. PA, pp. I23 I - 

1250, 1986. 

Tsu,ji. H.. “Counterflow Diffusion Flames,” Progre.s.s in Encr~qy u/7d Comhirstion Science. 

8, 93-1 19, 1982. 

Dixon-Lewis, G.. “Structure of Laminar Flames.” Twent~~-Third Sw1posi~tn7 (lnternutionul) 

on Comhirstion. The Combustion Institute, Pittsburgh, PA, pp. 305-324, 1990. 

Papas. P.. Fleming, J. W., and Sheinson, R. S., “Extinction of Non-Premixed Methane- and 

Propane-Air Counterflow Flames Inhibited with CF4, CF3H and CF3Br.” Tw~cnty-Si.ytli 

Swiposirrrn (lntrr-noliorrul) on Combidon, The Combustion Institute, Pittsburgh, PA. 

pp. 1405-141 I, 1996. 

Fendell, F. E., “Ignition and Extinction in Combustion of Initially Unmixed Reactants,” 

.Io~rrnu/ ($’Fluid Merhunics, 2 1, 28 1-303, 1965. 

LiZin. A,, “The Asymptotic Structure of Countertlow Diffusion Flames for Large Activation Encrgies,“ Acto A.stronuuticu. I. 1007- 1039. 1974. 

LU~L A.E., Kcc. R. J.. Grcar, J. F., and Rupley, F. M.. OPPDlF: A Fortruri ProLp-ucmlfor 

Comprr/in,q Opposed-Flow, Diflusioii Flunirs, SAND96-8243, Snndia National 

Laboratories, Livermore, CA, 1996. 

Kee, R. J.. Rupley, F. M.. Meeks, E., and Miller, J. A.. CHEMKIN-Ill: A FORTRAN 

Chcmic.ul Kinetics PucXu~qc~.for thc Anulysis of Gos-Phu.w Chemicul crnd Plusmu Kincti(.,s, 

SAND96-82 16, Sandia National Laboratories, Livermore, CA, 1996. 

Kcc, R. J., Miller. J. A.. Evans, G. H., and Dixon-Lewis, G., “A Computational Model of 

the Structure and Extinction of Strained, Opposcd Flow, Premixed Methane-Air Flames,” 

Tw,er?ty-Sec.ond Swzpo.sium (lnrernutior?crl) on Comhustion, The Combustion Institute, 

Pittsburgh, PA, pp. 1479.1494, 1988. 

Hdon Options Tcchnic;il Workin. Conkrcncc 27-79 April I WI 15.5 
13. 

14. 

15. 

16. 

17. 

18. 

19. 

20. 

21. 

22. 

23. 

24. 

25. 

26. 

Chelliah, H. K., Law, C. K., Ueda, T., Smooke, M. D., and Williams, F. A., “An 

Experimental and Theoretical Investigation of the Dilution, Pressure, and Flow-Field 

Effects on the Extinction Condition of Methane-Air-Nitrogen Diffusion Flames,” TwentyThird Symposium (International) on Combustion, The Combustion Institute, Pittsburgh, 

PA, pp. 503-5 I 1, 1990. 

GRI-MECH 1.2, Frenklach, M., Wang, H. Yu, C.-L., Goldenberg, M., Bowman, C. T., 

Hanson, R. K., Davidson, D. F., Chang, E. J., Smith, G. P., Golden, D. M., Gardiner, W. 

C., and Lissianski, V., http://www.me.berkeley.edu/gri_mech/. 

Du, J. and Axelbaum, R. L., “The Effects of Flame Structure on Extinction of CH4-02-N? 

Diffusion Flames,” TweurySixth Symposium (International) Combustion, The Combustion 

Institute, Pittsburgh, PA, pp. 1137-1 142, 1996. 

Tanoff, M. A., Dobbins, R. R., Smooke, M. D., Burgess, Jr., D. R., Zachariah, M. R., and 

Tsang, W., “C 1 and C2 Fluorinated Hydrocarbon Effects on the Extinction Characteristics 

of Methane Vs. Air Counterflow Diffusion Flames,” Proceedings, Halon Options Technical 

Working Conference. Albuquerque, NM, pp. 1 16-126, 1997. 

Simmons, R. F. and Wolfhard, H. G., “Some Limiting Oxygen Concentrations for 

Diffusion Flames in Air Diluted with Nitrogen,” Combustion Flame, 1, 155- 161, 19.57. 

Ishizuka, S. and Tsuji, H., “An Experimental Study of Effect of Inert Gases on Extinction 

of Laminar Diffusion Flames,” Eighteenth Symposium (International) on Combustion, The 

Combustion Institute, Pittsburgh, PA, pp. 69.5-703, 1981. 

Roberts, A. F. and Quince, B. W., “A Limiting Condition for the Burning of Flammable 

Liquids,” Combustion and Flame 20, 245-25 1 1973. 

Babb, M., Gollahalli, S. R., and Sliepcevich, C. M., “Extinguishment of Liquid Heptane 

and Gaseous Propane Diffusion Flames,” Journal of Propulsion and Power, 15, 260-265, 

1999. 

Hamins. A,, Trees, D., Seshadri, K., and Chelliah, H. K, “Extinction of Nonpremixed 

Flames with Halogenated Fire Suppressants,” Combustion and Flume, 99, 22 1-230, 1994. 

Hamins, A,, Gmurczyk, G., Grosshandler, W., Rehwoldt, R. G., Vazquez, I., Cleary, T., 

Presser, C., and Seshadri, K., “4. Flame Suppression Effectiveness,” in Evaluation of 

Alternative In-Flight Fire Suppressantsfor Full-Scale Testing in Simulated Air-eraft Engine 

Nace//es and Dry Bays, NIST SP 861, National Institute of Standards and Technology, 

Gaithershurg, MD, April 1994. 

Moore, T. A., Weitz, C. A,, and Tapscott, R. E., “An Update of NMERl Cup-Burner Test 

Results,” Proceedings, Halon Options Technical Working Conference, Albuquerque, NM, 

Coward, H. F. and Hartwell, F. J., “Extinction of Methane Flames by Diluent Gases,” 

Journal of the Chemical Society. pp. 1522-1532, 1926. 

3M Corporation; web site http://www.mmm.com/fluids/7 100.html. 

Flynn, R. M. and Thomas, S. D., Fire Extinguishing Process and Composition. US Patent 

5,718,293, Feb. 17, 1998. 

pp. 5.51-564, 1996. 

156 Halon Options Technical Working Conf‘crence 27-29 April I9YY