1

Second Joint Meeting of the United States Sections of the Combustion Institute

Thermal Agent Extinguishment of Two Types of Diffusion Flames

William M. Pitts, Rodney A. Bryant, and Jiann C. Yang

Building and Fire Research Laboratory

National Institute of Standards and Technology

Gaithersburg, MD 20899

ABSTRACT

The extinguishment of diffusion flames on two types of burners—an opposed-flow Tsuji burner and an

axisymmetric burner with a surrounding coflow—by thermal agents has been investigated. Thermal agents 

are those that act primarily by extracting heat and diluting the flame. In this study nitrogen, argon, helium, 

and carbon dioxide have been used as thermal agents. Both methane and propane have been used as fuels. 

Extinguishing concentrations are found to depend on the agent, type of burner, and on the fuel used. 

Propane flames are found to be more difficult to extinguish than methane flames on the same burner, while 

the Tsuji-type burner requires much higher concentrations of thermal agents for extinguishment than the

coflow burner. Possible explanations for the observed differences are discussed.

1. INTRODUCTION

The use of halons for fire fighting is being phased out due to their deleterious effects on stratospheric

ozone. This paper describes the findings of a portion of a study designed to characterize and identify

super-effective thermal fire-fighting agents as possible replacements for these effective compounds. The

full report will be available shortly. [1] “Thermal” agents refer to compounds that act simply by extracting 

heat from a flame zone and lowering the temperature to a point where combustion can no longer be

sustained. These types of agents should be contrasted with “chemical” agents that generate active chemical 

species (e.g., the bromine atoms generated by halons) that interfere with the radical chain branching

mechanisms required to sustain combustion.

This work is focused on buoyancy-dominated diffusion flames since most fires fall into this category. 

Some of the earliest work for diffusion flames was associated with the development of the concept of the 

limiting oxygen index (LOI) that is often used to characterize the flammability of a fuel. The LOI is

defined to be the minimum concentration of oxygen in a mixture of air and nitrogen that is just sufficient to 

sustain combustion. Simmons and Wolfhard recorded LOI for a wide range of gaseous and liquid fuels

using a slow flow of fuel from a hemispherical porous burner into an opposed flow of air diluted with

nitrogen. [2] These authors also introduced the concept of a limit flame temperature, which they defined as 

the adiabatic flame temperature calculated for a stoichiometric mixture of fuel and air at the extinguishing 

condition.

Simmons and Wolfhard found that the limit flame temperature varied with fuel. [2] For the C1 to C10

normal alkanes the range was 1636 K to 1791 K. Interestingly, the values for methane and the fuels having 

more than five carbons were all within 20 K of 1791 K. Starting with pentane, the limit flame

temperatures fell rapidly as the number of carbons in the fuel was reduced, reaching a minimum value at 

ethane, before increasing sharply once again for methane. One interpretation of these results is that the

smaller multi carbon hydrocarbons, particularly ethane, are more flammable than methane and their longer 
2

chain analogs. In 1979 Maæek considered these results and provided a convincing argument that, in

general, the flammability of the smaller straight-chain hydrocarbons, including methane, should increase

with decreasing carbon number. [3] This conclusion was based on the behavior of premixed flames and 

consideration of LOIs for the corresponding normal alcohols. On this basis, Maæek concluded that when 

methane is burned as a diffusion flame it has an unexpectedly low flammability.

One of the goals of the overall project on thermal agents was to develop approaches for using detailed

chemical-kinetic models to better understand and predict the effectiveness of thermal agents for

extinguishing buoyancy-dominated flames. Note that we are purposefully distinguishing between the

meanings of flame “extinction” and “extinguishment”. Flame extinction is used to describe a flame that 

goes out due to the local strain rate or other conditions causing the flame to become unstable. Flame 

extinguishment refers to the more specific conditions necessary to put out the buoyancy-dominated flames 

that are expected to be characteristic of most fires.

In order to define an extinguishing condition for the addition of thermal agents to the oxidizer it is

necessary to relate the calculations to an experimental result. For the methane and propane flames of

interest here we wished to use the LOI for this purpose. Somewhat surprisingly, only a limited number of 

LOI values have been published for these flames. 

For methane, Simmons and Wolfhard reported a LOI value of 0.139 using a hemispherical burner located 

in a slow oxidizer counter flow. [2] This corresponds to an added N2 volume fraction of 33.8 %. 

Measurements by Ishizuka and Tsuji in a similar burner yielded a value of 31.9 %. [4] Puri and Seshadri 

reported that a 28.6 % N2 volume fraction was sufficient to extinguish an opposed-jet methane/air diffusion 

flame. [5] In a recent, as yet unpublished, study, researchers at NIST made measurements in a similar low 

strain-rate flame and measured a value of 30.0 % for the maximum N2 volume fraction at extinguishment. 

[6] At the 1999 HOTWC meeting, Ural reported a study of extinguishment of a methane flame in a cup 

burner apparatus. [7] The extinguishing volume fraction of N2 over a relatively wide range of fuel and

oxidizer velocities was 27.1 %.

As for methane, there are only a limited number of LOI values reported for propane/air diffusion flames. 

Simmons and Wolfhard measured a LOI of 0.127 using their porous hemispherical burner, corresponding 

to a nitrogen volume fraction for extinguishment of 39.5 %. [2] Note that that the measured extinguishing 

concentration is roughly 17 % greater than observed for a methane flame. For an opposed-jet diffusion

flame, Puri and Seshadri recorded an extinguishing volume fraction of 34.8 % nitrogen. [5] This value is 

nearly 22 % higher than found for methane by the same authors. Note that for a given fuel, the nitrogen 

extinguishing volume fraction is significantly lower for the opposed-jet burner than for the porous burner.

Several values of nitrogen extinguishing concentration are available for propane flames burning in a cup 

burner configuration. Hamins et al. reported an extinguishing volume fraction of 32.5 %. [8] Babb et al. 

measured a value of 33 %. [9] The results of Ural also yield a value of roughly 33 %. [7] These results 

are all very close together, suggesting that lab-to-lab variations are not responsible for the differences

observed between different burners and fuels. Comparison of the results of Babb et al. for methane and

propane indicates that a propane flame again required a higher concentration for extinguishment (roughly 

22 %). 

The range of extinguishing concentrations for methane and propane flames burning in air for different types 

of burners reported in the various investigations cited above is much larger than can be explained by

uncertainty in the measurements and suggests that different types of diffusion flames yield different results. 

This raises the important question of what type of burner is appropriate for determining a characteristic

extinguishing nitrogen volume fraction (or LOI) for buoyancy-dominated diffusion flames. 
3

Due to the limited data available and the wide range of values reported, it was decided to carry out

additional measurements of extinguishing concentrations for known thermal agents using two diffusion

flame burners available at NIST. The first was a modified version of an apparatus, the Dispersed Liquid 

Agent Fire Suppression Screen (DLAFSS), developed at NIST to screen liquid fire-suppression agents. 

This system incorporates a Tsuji-type opposed-flow cylindrical porous burner. The second was a Santorotype burner (a coflow diffusion flame) previously used at NIST for investigations of diffusion flame

structure. These burners have been used to investigate extinguishment by nitrogen, carbon dioxide, argon, 

and helium.

2. Experimental

A. Dispersed Liquid Agent Fire Suppression Screen

The DLAFSS was designed for screening the effectiveness of liquid fire-fighting agents. It is described in a 

published report. [10] A more detailed description of the apparatus and the experimental procedure is

available in a National Institute of Standards and Technology Internal Report [11]. Figure 1 shows a

schematic for the DLAFSS. Briefly described, the apparatus consists of a small vertical wind tunnel, a

porous cylindrical burner located at the test section, and a small nebulizer located in the settling chamber of 

the tunnel. Even though designed primarily for evaluating liquid fire suppression agents, the system can 

also be used for gaseous agents. [10,11] The injection point for gases is at the bottom of the system as can 

be seen in Figure 1.

The goal of these measurements was to identify extinction conditions for thermal agents added to the

oxidizer flow for porous-burner-stabilized opposed-flow diffusion flames under the relatively low strain

rates characteristic of buoyancy-dominated flames. Typically, the Tsuji burner is operated at considerably 

higher oxidizer flow velocities than required here, and the resulting strain rates at the flame are much

greater than found in buoyancy-dominated flames. In order to generate the required low oxidizer velocities, 

the blower used to induce the flow in the DLAFSS was operated at the lowest setting possible. The thermal 

agents were then added to the induced air flow. Note that the oxidizer flow velocity is expected to vary as 

the agent volume fraction increases.

The flow rate of agent was controlled using a needle valve, and the volume flow rate was measured using a 

rotameter (Fischer Porter, Model 10A1755*). Even though the agent volume flow rate was known, it was 

found that it was not possible to predict the agent volume fraction using only the assumed airflow rate. 

This observation was attributed to variations in the total flow volume due to changes in the blower

efficiency as agent was added. An alternate approach was adopted to estimate the total flow rate and thus 

the local flow velocity at the flame. The agent concentration was estimated by measuring the oxygen

volume fraction in the oxidizer stream at the center of the settling chamber of the wind tunnel (see Figure

1). This was accomplished using a paramagnetic oxygen meter (Servomex Oxygen Analyzer Model 570A) 

in conjunction with a small sampling pump (Gilian Hi Flow Sampler, Model HFS 513A). The oxygen

analyzer was zeroed and spanned using nitrogen and room air. Since the volume flow rate for the agent 

was known, knowledge of the degree of air dilution allowed the total flow rate to be calculated assuming 

complete mixing.

 

*Certain commercial equipment, instruments, or material are identified in this paper in order to adequately 

specify the experimental procedure. Such identification does not imply recommendation or endorsement by 

the National Institute of Standards and Technology, nor does it imply that the materials or equipment are 

necessarily the best available for the purpose.
4

Figure 1. A schematic of the dispersed liquid agent fire suppression screen (DLAFSS) apparatus is shown.

In a typical DLAFSS experiment, extinction is defined to occur when the flame on the upstream side of the 

burner blows out. For high strain-rate conditions, it is generally found that a flame stabilized in the wake 

of the cylinder continues to burn following blow out of the upstream flame. During the current

experiments, it was observed that when sufficient agent was added to the air flow to induce extinction of 

the stretched flame on the upstream side of the cylinder, the downstream flame was either extinguished at 

the same time, or disappeared when the agent concentration was increased only slightly. Since the

stabilized flame is essentially a buoyancy-dominated flame, this is taken as evidence that the measurements 

are actually recording flame extinguishment values.

Suppression tests were performed by gradually adding the gaseous inhibitor to the oxidizer (air) stream

until blow-off occurred (abrupt transition from envelope to wake flame). The volumetric flow rates of the 

suppressant at blow-off were recorded. The oxygen concentration was then noted for the same flow

conditions.

Test section

Thermal insulation

Contraction section

Transition flange

Settling chamber

Screen + holder

Honeycomb + holder

Diffuser

Transition duct

X-Y traverse

Flexible

aluminum ductwork

Screw

jack

Aluminum plate

Aluminum plate

T-slotted aluminum

structural extrusion

Burner

Droplet generation

device

From blower

Gaseous agent

(optional)
5

Figure 2. A schematic of the modified Santoro burner is shown.

Figure 3. A schematic and photo of the Santoro burner and flow control system are shown.

B. Santoro Burner

An existing coflow burner and computer-controlled gas mixing system were modified to allow

extinguishing concentrations of thermal agents to be determined for buoyancy-dominated laminar diffusion 

flames formed by a flow of gaseous fuel from a circular tube into a slow surrounding coflow of an

air/agent mixture. The burner is based on a design originally developed by Santoro et al. that is commonly 

referred to as a “Santoro burner”. [12] This type of burner has been employed for numerous flame studies 

at NIST. In a particularly relevant study, Smyth and Everest reported volume fractions of CF3Br and CF3I 

in air required to extinguish a propane flame on the Santoro burner. [13]

The actual burner used in this investigation is a modified Santoro burner that was originally employed to 

investigate acoustically forced, time-varying diffusion flames. [14] It has been employed for numerous

studies. [15,16,17,18,19,20,21,22] A drawing of the burner is shown in Figure 2. It consists of an open 

13.7 cm length of tubing with a 1.27 cm outer diameter that serves as the central fuel tube. The tube lip 

thickness is 0.065 cm, yielding an inner diameter of 1.14 cm. One end of the tube is attached to a 

Air & Agent Air & Agent

Fuel

1.14 cm

1.27 cm

10.2 cm

0 - 10 SLMN

0 - 10 SLMN

0 - 30 SLMN

0 - 2 SLMN

Mass

Flow

Controllers Pneumatic

Valves

Mixing Chamber

Air

Agent

Fuel

Burner

Screens

P1

P2

0 - 10 SLMN

0 - 10 SLMN

0 - 30 SLMN

0 - 2 SLMN

Mass

Flow

Controllers Pneumatic

Valves

Mixing Chamber

Air

Agent

Fuel

Burner

Screens

0 - 10 SLMN

0 - 10 SLMN

0 - 30 SLMN

0 - 2 SLMN

Mass

Flow

Controllers Pneumatic

Valves

Mixing Chamber

Air

Agent

Fuel

Burner

Screens

P1

P2
6

Table 1. Suppression Results Using the DLAFSS

Agent

Methane Propane

Agent volume fraction 

at blow-off

aT = 2Vo/R

(s-1)

Agent volume fraction 

at blow-off

aT = 2Vo/R

(s-1)

Carbon Dioxide 0.240 ± 0.003* (n = 4) 39 ± 1 0.257 ± 0.008 (n = 3) 45 ± 2

Nitrogen 0.332 ± 0.004 (n = 5) 52 0.390 ± 0.004 (n = 5) 56

Helium 0.207 ± 0.003 (n = 5) 94 0.224 ± 0.003 (n = 5) 95 ± 1

Argon 0.468 ± 0.002 (n = 4) 44 ± 1 0.550 ± 0.005 (n = 5) 44

plenum containing a loud speaker. The speaker, used to acoustically force the fuel flow, was not activated 

during this study. The open end of the fuel tube extends 0.4 cm above a surrounding 10.2 cm diameter 

ceramic honeycomb formed from 0.15 cm square cells and having an overall length of 2.5 cm. After

passing through a chamber filled with glass beads and several screens, a flow of air and agent enters the 

honeycomb to form the surrounding coflow for the flame.

The flow velocities for the fuel and coflow streams were low and resulted in small buoyancy-dominated 

flames that were highly susceptible to room drafts. Screens were placed around the burner to minimize

these disturbances. Flame products were removed from the area by an exhaust cone connected to the room 

exhaust by a flexible hose. The effect of the exhaust cone on the flames was judged to be negligible due to 

its low rate of suction and its location above the burner.

Fuel and the air/agent mixture were delivered to the burner using a system of thermal mass-flow controllers 

(MFC’s). The flow-control system, shown in Figure 3, is a modification of a system developed at NIST 

for a previous investigation as described in detail by Pitts et al. [23] The flow system was software

controlled by a desktop 486-DX personal computer. The air and agent streams were combined upstream of 

the burner in a mixing chamber designed to ensure a homogenous mixture. Pneumatic shut-off valves

prevented unwanted backflow of gases and allowed for easy calibration of each MFC. Modifications for 

the current study included the addition of a 30 SLM (standard liter per minute) mass-flow controller for the 

air flow and moving the 10 SLM and 2 SLM mass-flow controllers to the agent and fuel streams,

respectively. Air was supplied from an in-house high-pressure line, and its relative humidity was a

consistent 4.5 %.

3. Results for Thermal Agent Extinction of Methane and Propane

Diffusion Flames Burning on Tsuji and Santoro Burners

The extinguishment behaviors of flames stabilized on the Tsuji and Santoro burners were investigated using 

the gaseous thermal agents nitrogen, carbon dioxide, argon, and helium. The following two sections

summarize the general observations and the results of extinguishment volume fraction measurements.

A. Tsuji Burner

Multiple extinguishment tests were run for each agent. Table 1 summarizes measured values of

extinguishing volume fraction for the four thermal agents. The repeatability of the measurements is

indicated by the magnitudes of the standard deviation. The actual number of repeated tests is denoted by 

the symbol n. The table also includes values of the stagnation velocity gradient, aT, defined as 2Vo/R, 

* mean ± standard deviation

No. of samples = n
7

where Vo is the velocity based on the total volumetric flow rate of air and gaseous agent at blow-off and R

is the burner radius. aT represents the characteristic stagnation strain rate that is often used for Tsuji

burners and has units of s-1. 

As already discussed, measurements of extinguishing volume fractions in the literature are limited for

opposed-flow porous burners. Simmons and Wolfhard [2] reported values of 34 % and 39.5 % for the

extinguishing volume fractions of methane and propane flames on a hemispherical porous burner, while

Ishizuka and Tsuji [4] found a value of 31.9 % for a methane flame stabilized on a cylindrical porous

burner. These values agree very well with those included in Table 1, (33.2 ± 0.4) % and (39.0 ± 0.4) % for 

methane and propane, respectively. This agreement is found even though two different types of porous

burners were used. Recall that the extinguishing nitrogen volume fractions of Simmons and Wolfhard were 

the highest observed for the various types of diffusion flames discussed earlier. It must be concluded that a 

flame for a given fuel is more stable on an opposed-flow Tsuji-type or hemispherical porous burner than 

for the corresponding flames burning between two axisymmetric opposed-jet flows or in a coflow

configuration. The reasons for these observations are currently unclear.

Note that in the case of helium included in Table 1, the values for aT = 2Vo/R are almost a factor of two 

higher than those obtained using nitrogen, carbon dioxide, and argon. Recall that the blower in the

DLAFSS is set to operate at the lowest possible revolution rate. Even though the blower speed is

nominally constant, when the density downstream of the blower is significantly reduced, the blower

generates a higher volume flow rate. The net result is that the characteristic strain rate, aT, at the flame is 

roughly a factor of two higher for the helium case than for the other agents, and it is to be expected that the 

observed extinguishing concentration will be somewhat lower than would be measured for a purely

buoyancy-dominated flame.

B. Santoro Burner

The coflowing gas burner is a deceptively simple flame configuration. During the extinguishment studies 

summarized here a number of complex behaviors were observed that complicated what should have been, 

in principle, a simple measurement of extinguishing concentration. The behaviors are also shown to be fuel 

dependent, being different for methane and propane flames.

Consider first the methane base flame (i.e., fuel and air nominal velocities equal to 7 cm/s) burning in air 

diluted with N2. With only air present, a blue flame was evident slightly below the exit and very close to 

the fuel tube. Further downstream the flame was yellow indicating the presence of soot. As the N2

concentration in the oxidizer was increased, the base of the flame gradually moved downstream and away 

from the burner tube, but the flame still appeared to be attached. At the same time, the fraction of the

flame that appeared yellow decreased. When the N2 volume fraction reached 20.2 %, an up and down

oscillation of the flame base suddenly appeared as indicated in Figure 4. This flame was entirely blue. The 

oscillating flame only existed over a narrow range of N2 volume fraction. Similar oscillations for flames 

near extinction have been reported previously in the literature. [24,25]

When the N2 volume fraction was increased to 20.5 %, the flame detached from the burner and moved

downstream, where it became stable and floated lazily up and down 10 mm to 15 mm above the burner. 

With further increases in N2 concentration, the flame moved further downstream, while becoming shorter 

and broader. These changes in appearance are shown in cartoon form in Figure 5. Similar lifted-flame 

structures have been observed in investigations with different fuels and burner configurations and are

attributed to the presence of triple flames. [26,27,28,29] The presence of lifted flames has also been 
8

Figure 4. Sketch of a methane flame showing the attached flame oscillation and subsequent stabilized 

lifted flame that result from the addition of nitrogen to the air co-flow.

Figure 5. The evolution of the methane diffusion flame structure as additional N2 is added to the co-flow 

air stream is shown.

reported during cup burner experiments using gaseous fuels. [7,8,9] When the N2 volume fraction reached 

21.8 %, the lifted flame could no longer be stabilized, and it simply floated away, i.e., it was blown off.

Increasing N2 dilution of the air resulted in three distinct behaviors—attached flame instability, lift off, and 

blow off. The N2 volume fraction range where these processes occurred was narrow, ranging from 20.5 % 

to 21.9 %. It is difficult to define any of these processes as a true extinguishment, though blow off would 

seem to be the most appropriate. For reasons that will be clear shortly, the onset of the fluctuation

behavior was used for the characteristic extinguishing concentration for methane flames burning in air

diluted with thermal agents.

When CO2, Ar, and He were used as diluents, the response of the baseline methane flame changed from

that observed for N2. For all three agents, the initial effect of increasing their concentration in air was the 

appearance of the attached flame instability. Similar to the results for N2, additional small increases in the

~ 5 mm

~ 15 mm

Attached flame instability

0.202 < XN2 < 0.205

Lifted flame

XN2 > 0.205

~ 5 mm

~ 15 mm

Attached flame instability

0.202 < XN2 < 0.205

Lifted flame

XN2 > 0.205

h

w

l h

w

l

XN2 ~ 0.210 XN2 = 0.218

Blow off

h

w

l h

w

l

XN2 ~ 0.210 XN2 = 0.218

Blow off
9

volume fractions of CO2 and Ar resulted in detachment of the flame from the burner, with stabilization

further downstream. At first, these flames moved further downstream with additional increases in diluent 

concentration, but eventually the flames reached lift-off heights where further increases appeared to have 

little effect. The most likely reason for this observation is that the flames reached a downstream location 

where surrounding room air was able to diffuse through the oxidizer coflow and provide sufficient O2 to 

sustain combustion. For these cases it is impossible to define the agent volume fraction required for flame 

blow off. The behavior of the flame burning in air diluted with He was quite different. After a few

oscillations following the onset of the attached flame instability, the flame was extinguished without further 

increases in helium concentration. Apparently, the onset of the attached flame instability was sufficient to 

extinguish the methane burning in the He/air mixture. Since the appearance of the flame instability was the 

only common behavior for all four of the agents, its onset was used as the characteristic volume fraction for 

extinguishment.

When the baseline fuel and oxidizer volume flow rates for the methane case were used for propane, the

resulting flame length was much greater than observed for methane. This observation is easily understood 

by considering the stoichiometries for complete combustion of the two fuels, 

Methane: 4 2 2O CO2 CH + 2O → 2H +

Propane: 3 8 2 2 2 C H + 5O → 4H O + 3CO ,

which indicate that combustion of a given volume of propane requires 2.5 times more oxidizer than the

same volume of methane. Since the entrainment rate of oxidizer into laminar diffusion flames is roughly

independent of the fuel flow rate, a longer flame length is required for the complete combustion of propane 

with the same fuel flow rate.

In order to have roughly comparable flame lengths for the two fuels, the nominal fuel velocity for propane 

was reduced to 40 % of that used for methane (i.e., a nominal velocity of 2.8 cm/s, volume flow rate of

0.171 SLM). With the oxidizer nominal velocity maintained at 7 cm/s, the propane flame length was

comparable to that for the baseline methane flame. These flow velocities were adopted as the baseline

conditions for the propane flame.

Similar to the baseline methane flame, the propane flame was attached to the fuel tube prior to the addition 

of nitrogen to the coflow. As the nitrogen volume flow rate was increased, the blue region at the base of 

the flame occupied a larger volume, as was also observed for the methane flame. However, unlike the

methane flame, the base of the propane flame broadened substantially as it moved away from the burner

tube with increasing nitrogen addition. When the nitrogen volume fraction reached 26.5 %, the flame base 

detached from the fuel tube and stabilized approximately 5 mm downstream. By the time the nitrogen

volume fraction was increased to 27.6 %, the flame base was approximately 10 mm above the fuel tube, 

and began to oscillate with a combined vertical and radial motion. Further downstream the flame formed a 

necked-down region. With only an additional increase of 0.4 % nitrogen volume fraction, the flame

suddenly extinguished by collapsing at the necked-down region. A cartoon of the nearly extinguished flame 

is shown in Figure 6. The extinguishment of the propane flame following lift off stands in sharp contrast to 

the methane flame that formed a stable lifted flame following detachment from the burner.

Unlike the baseline methane flame, the general response of the propane flame to dilution of the coflow air 

by CO2, Ar, and He was the same as described above for nitrogen dilution. Since an easily characterized
10

Figure 6. A sketch of the time evolution of a nearly extinguished propane flame is shown for the case of 

nitrogen dilution of the co-flow air, t1 < t3.

Table 2. Measured Agent Volume Fractions at Onset of Flame-Base Instability (Osc) and for Flame 

Extinguishment (Ext) of Baseline Methane and Propane Diffusion Coflow Flames 

Methane Propane

Osc Ext Osc Ext

Carbon Dioxide 0.120 

(s=±0.001) -

0.169 

(s=±0.004)

0.176 

(s=±0.001)

Nitrogen 0.202 

(s=±0.002)

0.218 

(s=±0.002)

0.276 

(s=±0.005)

0.280 

(s=±0.008)

Helium 0.200 

(s=±0.001)

0.200 

(s=±0.001)

0.271 

(s=±0.003)

0.273 

(s=±0.002)

Argon 0.290 

(s=±0.001) -

0.356 

(s=±0.002)

0.368 

(s=±0.003)

s = Standard deviation

Number. of samples = 6

extinguishment was observed for each agent using propane, this event was designated as the characteristic 

flame extinguishment behavior.

The results of extinguishment studies for baseline methane and propane diffusion flames on a Santoro

burner are summarized in Table 2. Agent volume fractions corresponding to the onset of flame fluctuation 

and flame extinguishment (if observed) are included. Uncertainties represent the standard deviation, s, for 

six repeats of each measurement. Extinguishment was not observed for the methane flame with CO2 and 

Ar. The volume fraction of agent required to induce oscillation is taken as the characteristic extinguishing 

concentration for the methane flame, while the actual concentration at flame extinguishment is used for the 

propane flame. Note that the small differences observed between the two values for a given agent and fuel 

Necked down region

& extinguishment point

Direction of flame

base propagation

a) t=t1 b) t=t2c) t=t3
11

Figure 7. Experimental measurements of extinguishing volume fractions for the indicated thermal agents 

are plotted as a function of a reference concentration for methane flames. Solid symbols

represent current results for Tsuji and Santoro burners, and open symbols are values taken from 

the following references (same order as symbols): Tsuji cylindrical porous burner [4], 

hemispherical porous burner [2], opposed-jet burners [5,6], cup burner [7]. For clarity, the

reference concentration for helium has been increased by 0.5 %.

suggests that only small differences will be introduced by the use of the different definitions for

extinguishment.

4. Discussion

It is desirable to have some means of comparing the various experimental results graphically. For this

purpose we have chosen to plot the results for the various gases as a function of the corresponding

reference volume fraction predicted for extinguishment using detailed chemical-kinetic modeling and the

assumption of a common maximum flame temperature for extinguishment. Details concerning these

calculations will be presented elsewhere. [1]

Figure 7 shows a plot of measured extinguishing volume fractions for thermal agents added to the air flows 

of the Tsuji and Santoro burners used in the current investigation (solid symbols). For comparison

purposes, a number of measurements taken from the literature for a variety of different diffusion flames are 

also included (open symbols). Even though all of the data are recorded for diffusion flames, it is obvious 

that significant variations are observed that seem to be burner dependent. With the exception of helium, the 

highest extinguishing concentrations in the current study were found for the Tsuji burner, i.e., a porous

cylindrical burner in an oxidizer counterflow. For nitrogen the current value is in good agreement with two 

earlier measurements recorded using hemispherical [2] and cylindrical [4] porous burners in counterflow

configurations. Interestingly, the two sets of measurements recorded using opposed-jet diffusion flames

yield extinguishing volume fractions that are significantly lower than those using porous burners.

Figure 7 also includes a cup burner result taken from Ural for nitrogen. [7] The cup burner consists of an 

axisymmetic burner surrounded by an enclosed coflow. It is typically used for liquid fuels, but has been

Reference Volume Fraction

0.20 0.25 0.30 0.35 0.40 0.45

Experimental Extinguishing Volume Fraction

0.10

0.15

0.20

0.25

0.30

0.35

0.40

0.45 Tsuji

Tsuji

Hemishperical

Opposed Jet

Opposed Jet

Cup Burner

Santoro

CO2

N2

He

Ar
12

Figure 8. Experimental extinguishing volume fractions for the indicated thermal agents are plotted as a

function of a reference concentration for methane flames using the Tsuji and Santoro burners

(solid symbols). These results are compared with cup burner measurements using heptane as

fuel (open symbols) taken from the following references (same order as symbols): Hirst [30], 

Sheinson [31], Babb [9], Hamins [8], Moore [32], and Saito [33]. For clarity, the reference

concentration for helium has been increased by 0.5 %.

modified for gaseous fuels. For standard configurations, the “cup” used as the burner has a greater

diameter (roughly a factor of two) than the Santoro burner. The measured extinguishing volume fractions 

fall below those recorded for the opposed-flow burners, but lie well above the corresponding results for the 

Santoro burner.

All of the measurements recorded using the Santoro burner lie well below those recorded with the other

types of burners, with the exception of the helium measurements using the DLAFSS. Recall that due to

experimental problems, the global strain rate was roughly twice as high for helium as for the other agents. 

Even though the relative magnitudes differ significantly, extinguishing volume fractions, for the two

burners used here, again with the exception of helium, vary in very similar manners for the different agents.

There are insufficient data in Figure 7 to allow a clear dependence of the extinguishing volume fraction on 

agent to be identified. A much larger number of measurements of extinguishment volume fractions using 

these thermal agents have been reported for heptane flames in cup-burner experiments. Figure 8 compares 

the current results with a number of cup burner results for heptane available in the literature. The results 

for N2 and CO2 tend to fall relatively close together and to lie just below the Tsuji burner results for

methane. Even though the cup burner extinguishing volume fractions are considerably higher for a given 

agent, the relative variations with agent are similar to those for the Santoro burner. It is particularly

noteworthy that both types of burners indicate that the extinguishing effectiveness of nitrogen and helium 

are very similar. As already noted, the results for helium using the Tsuji burner appear to be lower than 

expected. Similarly, there is an indication that the measured value for the Tsuji burner using argon may be 

slightly higher than expected.

Reference Volume Fraction

0.20 0.25 0.30 0.35 0.40 0.45

Experimental Extinguishing Volume Fraction

0.10

0.15

0.20

0.25

0.30

0.35

0.40

0.45 Tsuji

Santoro

Hirst

Sheinson

Babb

Hamins

Moore

Saito

CO2

N2

He

Ar
13

Figure 9. Experimental measurements of extinguishing volume fractions for the indicated thermal agents 

are plotted as a function of a reference concentration for propane flames. Solid symbols

represent current results for Tsuji and Santoro burners, and open symbols are values taken from 

the following references (same order as symbols): hemispherical porous burner [2], opposed-jet 

burners [5,6], and cup burners [7,8,9,32].

The current experimental results for the propane flames are plotted in Figure 9 in a similar manner as the 

methane results in Figure 7. Experimental values for various diffusion flames are also shown. The same 

general behaviors are found as were observed for methane. The porous burners require the highest

concentrations of an agent for extinguishments, with the exception of helium. Opposed-jet burners require 

slightly lower concentrations, and the cup burner requires even less. Values for the Santoro burner are

considerably lower still. There is an indication that the DLAFSS value for argon may be slightly highly 

than expected. The required extinguishing concentrations of helium and nitrogen are very similar for the 

Santoro burner.

Comparison of Figure 7 and Figure 9 suggests that extinguishing concentrations for a given agent are

somewhat lower for methane than propane. This is confirmed by the data shown in Figure 10 that directly 

compares the results for methane and propane flame results. In each case, for a given agent and burner

type, the required agent volume fraction to extinguish the flame is higher for a propane flame than for a

methane flame.

While there are some inconsistencies in the current experimental results, the findings of this study in

conjunction with earlier literature results support the following conclusions:

1. The amount of agent required to extinguish a buoyancy-dominated diffusion flame depends on the 

type of burner employed. The relative ordering of diffusion flame types based on difficulty of

extinguishment is 1) porous burner in a counterflow, 2) opposed-jet, and 3) coflow.

2. Even though both burners use coflows of oxidizer and fuel, it is much easier to extinguish a

Santoro burner than the corresponding cup burner flame. Previous researchers have noted that the 

flame stability in cup burners decreases with the cup diameter, which is consistent with the current 

observations. [34,35]

Reference Volume Fraction

0.25 0.30 0.35 0.40 0.45 0.50 0.55

Experimental Extinguishing Volume Fraction

0.15

0.20

0.25

0.30

0.35

0.40

0.45

0.50

0.55

Tsuji

Hemishperical

Opposed Jet

Opposed Jet

Cup Burner

Cup Burner

Cup Burner

Cup Burner

Santoro

CO2

N2

He

Ar
14

Figure 10. Experimental values from the current study for extinguishing concentrations of the indicated

thermal agents are compared for methane and propane flames. Data are plotted as a function 

of the reference extinguishing concentration for methane. For clarity, the reference

concentration for helium has been increased by 0.5 %.

3. Propane flames require higher concentrations of an agent to induce extinguishment than methane 

flames. This observation is consistent with the discussion of Maæek concerning flame stability

variations with fuel. [3]

4. Even though the amount of a given agent required for extinguishment depends on the fuel, the

relative effectiveness of different agents does not seem to change from fuel to fuel. This is

consistent with recent conclusions reached in the literature. [36,37] 

5. Acknowledgement

This research is part of the Department of Defense’s Next Generation Program Fire Suppression

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

Program under Contract W74RDV73243630.

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5. Puri, I. K. and Seshadri, K., Combust. Flame 65, 137-150 (1986).

Reference Volume Fraction

0.20 0.25 0.30 0.35 0.40 0.45

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Ar
15

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