SUPPRESSION OF CUP-BURNER DIFFUSION FLAMES nY 

SUPER-EFFECTIVE CHEMICAL INHIBITORS AND INERT COMPOUNDS* 

G.T. Linteris 

National Institute of Standards & Technology 

INTRODUCTION 

The burning velocity and extinction strain rate reduction caused by addition of the iron compounds 

Fe(CO), and ferrocene ((C,H,),Fe, Fec) to premixed Bunsen-type and counterflow diffusion flames has 

been observed [ 1-41. Recent work has extended the work in premixed flames to the new agents, tetramethyltin (TMT, Sn(CH,),) and methylcyclopentadienylmanganese tricarbonyl (MMT, C,H,MnO,) [SI, 

both of which are more effective than CF,Br, but less so than Fe(CO),. No tests with these agents, however, have been done in flames resembling fires. The present work remedies this deficiency by presenting results for addition ofthese highly effective agents to cup-burner flames. Not only do cup burners 

have flame structures that are a reasonable approximation to those in fires, but they are also widely used 

by the fire protection industry as a metric to assess fire suppressant performance. Hence, measurements 

of agent performance in cup burners have clear relevance to their eventual use. 

BACKGROUND 

It is well known that some metallic compounds reduce the burning velocity of premixed flames one or 

two orders of magnitude more than does CF,Br. Previous work has shown this superior performance for 

iron pentacarbonyl Fe(CO),, tetraethyllead (CzH5)4Pb, chromyl chloride CrO,CI,, and ferrocene (C,H,),Fe. 

The overall reaction rate, which is related to the square of the burning velocity [6], is thus greatly reduced 

by these agents under premixed conditions. If means could be found to incorporate such agents in a practical fire suppressant (particularly for unoccupied spaces), very effective agents may be possible. 

One of these agents, Fe(CO),. has recently been studied in some detail. Experiments and modeling have 

quantified its performance and explained its mechanism of inhibition for a variety of conditions [3, 7-91, 

The strong inhibition is believed to occur from a catalytic radical recombination cycle involving iron 

oxides and hydroxides: FeOH + H tj FeO + H,; FeO + H,O tj Fe(OH),; and Fe(OH), + H tj FeOH + 

H,O, which yields the net reaction: H + H tj H,. This mechanism leads to very strong inhibition for 

Fe(CO), mole fractions below about 100 ppm?; however, above this value, the inhihitor begins to lose its 

effectiveness. This is believed to occur from condensation of the active iron-containing intermediates to 

particles [IO]. The reduction in the effectiveness of Fe(CO), as the mole fraction increases was shown to 

be much more dramatic in premixed flames than in counterflow diffusion flames. Any practical fire suppressant using these super-effective agents would require some method to overcome the loss of effectiveness and the high toxicity. 

One approach to overcoming the loss of effectiveness is to combine catalytic agents with inert agents. In 

this case, the overall reaction rate is lowered in part through radical recombination by the catalytic agent, 

and in part through the lower temperature from the added diluent. This approach has been discussed in 

work since the 1950s 19, 11-15], which suggested that combinations of thermally acting and catalytic 

agents might prove beneficial. These predictions have been confirmed in premixed flames with ferrocene 

and CO, as the inhibitors [4] and in premixed and counterflow diffusion flames inhibited by Fe(CO), at 

reduced oxygen mole fraction in the oxidizer (which is equivalent to adding diluent nitrogen) [3]. Moreover, previous tests with Fe(CO), at reduced oxygen mole fraction, and tests with ferrocene blended with 

CO,, show that addition of an inert compound can actually enhance the performance of the catalytic 

* Official contribution of NlST, not subject to copyright in the United States. 

t All reference to ppm in the present paper are on a volume or molar basis, and refer to pL/L. 

Halon Options Technical Working Conference 24-26 April 2001 187 
agent. Nonetheless, it is generally not known a priori if the combination of an inert with the catalytic 

agents will be effective in a particular flame configuration: the lower oxygen mole fraction (or added 

diluent) makes the catalytic effect stronger, while the lower temperature (and consequently longer 

residence in premixed flames) increases the likelihood of condensation [IO]. 

Motivated by these results, tests of ferrocene together with an inert compound generated by a solid 

propellant gas generator (SPGG) have recently been conducted in an enclosure containing a spray flame 

[16]. Unfortunately, the combination did not have the intended high efficiency and failed to extinguish 

the flame. As described above, the combination of inert and catalytic agents has been shown to be effecttive in premixed flames and counterflow diffusion flames. Carefully controlled tests like those in the 

premixed and counterflow diffusion flames have not been performed in cup-burner flames. To explore 

possible reasons for the lack of effectiveness of Fec with the SPGG in suppressing the spray flame tested, 

we performed tests with catalytic metal-based inhibitors and CO, added to the air stream in a cup burner. 

Based on our recently acquired understanding of the action of iron in flames, addition of Fe(CO), alone to 

the air stream of a cup burner would not be expected to be efficient at extinguishing the flame. This is 

because particle formation acts as a sink for the active gas-phase inhibiting species, keeping the maximum inhibiting species mole fractions at their saturation value. That is, adding more and more Fe(CO), 

to the air stream does not increase the gas-phase mole fraction of active inhibiting species; the eventual 

extinction would likely come about through the much less efficient processes involving the condensedphase particles (including surface radical recombination, the particle heat capacity, and particle radiation 

heat losses). However, based on the premixed results with Fec and COT, we would expect that added Fec 

(or Fe(CO),) should significantly reduce the amount of CO, necessary to extinguish the cup-burner flame. 

This approach should be effective for two reasons: (1) based on premixed flame results presented previously, we expect that adding 100 ppm or above of iron should reduce the overall reaction rate by about 

a factor of 4 (i.e., about a factor of 2 reduction in flame speed); and (2) with added inert the effecttiveness 

of the iron in the gas phase should be improved. Hence, we would expect that first adding an effective 

catalytic agent to the air stream would reduce the amount of CO, required to extinguish the flame. 

Two approaches are used in the present work. In the first, catalytic agents (Fe(CO),, TMT, MMT, Br,, 

and CF,Br) are added to the co-flowing air stream at various volume fractions, and the amount of added 

CO, required to extinguish the flame is determined. This is the typical cup-burner flame extinction test. 

In the second approach, the flame is extinguished in the same manner, but at each inhibitor volume fraction up to extinction, the fuel consumption rate of the burner is measured. The fuel consumption rate 

(which is determined by the heat feedback from the flame) scales approximately linearly with total heat 

release. Typically, the fuel consumption rate decreases as inhibitor is added to the air stream, and at 

extinction, the fuel consumption rate approaches zero. In the present tests we measure the fuel consumption rate in the presence of increasing amounts of CO,, with and without added catalytic inhibitor in the 

air stream. An effective fire suppressant would be expected to reduce the fuel consumption rate, much as 

do fire retardants added to materials. Our tests ofthe effect of the inhibitors on the fuel consumption rate 

were performed with heptane and methanol fuels. For the more traditional extinction tests, we present 

some data with heptane and methanol as the fuel, but most were performed using methane. Tests with 

flames of methane are warranted since they have been shown to provide similar inhibitor rankings for 

extinction compared to other larger hydrocarbon fuels [17]. Also, although methane oxidation is atypical 

of that of Iarger hydrocarbons, Babushok and Tsang have recently observed [IS] that for a wide variety 

of hydrocarbons (including methane) the burning velocity is most sensitive to the rates of the same reactions. Since these reactions are the ones most influenced by an inhibitor, the trends in inhibitor effectiveness are the same for most hydrocarbons. 

188 Halon Options Technical Working Conference 24-26 April 2001 

~~~ ~ ~~ 
EXPERIMENT 

A cup burner [19,20] was used for the tests. The cylindrical glass cup (28 mm outer diameter) was positioned with the top at a height of 20.5 cm inside a 53.3 cm tall, 9.5 cm inner diameter glass chimney. A 

7.5 cm thick bed of glass beads (6 mm dia) at the base of the chimney provided uniform flow in the air 

stream. Fuels were methane, methanol, or heptane. For the methane tests, the fuel cup was filled with 

3 mm dia. glass beads, with two 15.8 mesh/cm stainless steel screens (separated by 1 mm) at the top. For 

the liquid fuels, a dual syringe-pump (Yale Apparatus* model YA-12) system provided fuel at measured 

rates. In the first tests, the fuel consumption rate of the burner was determined by adjusting the pumping 

rate such that the fuel was flush at the edge of the glass fuel cup. This approach, while apparently working well, required long wait periods (several minutes) between changes in the feed rate to ensure that the 

fuel level remained at a constant level. A subsequent method used an overflow system [20]. Gas flows 

were measured with digitally controlled mass flow controllers (Sierra 860) with a quoted repeatability of 

0.2%, which have been calibrated with bubble (Gillian Gilibrator), piston (Bios 20K), and dry (American 

Meter Co. DTM-200A) flow meters so that their uncertainty is 2% of indicated flow. 

The organometallic inhibitors were added to the air stream using multistage saturators in controlled temperature baths. The Fe(CO), was added at volume fractions up to 950 ppm using a two-stage satiirator 

described previously [Y]. The TMT was added using an identical two-stage saturator, with >SO cm' of 

TMT in each stage. The water bath was maintained at (0 ? 0.2) "C or (43.2 k 1 .O) "C with a maximum 

carrier gas flow 0.43 Umin or 0.81 L/min for low and high values of X,,, respectively. For the MMT, the 

saturator had three stages, each a 20 cm long, 2.36 cm inner diameter stainless steel tube, and the entire 

apparatus was submerged in a controlled temperature bath (Neslab). The bath temperature was typically 

(85 f 0.2) "C except the highest concentration, which as at (92.5 It 0.2) "C; the maximum flow of carrier 

gas for this saturator was 1.5 L/min. The mole fraction ofthe organometallic inhibitors in the air stream 

was calculated based on the measured air flow, measured carrier gas flow (N, or CO,), and vapor pressure of the agent at the bath temperature, assuming saturated carrier gas. The parameters in the Antoine 

equation logIO(P)=A-B/(T+C) are (A,B,C): (6.77273,4.0932, 7.2283), (1258.22, 1286.16, 1882), and 

(21 1.587,235.846,200) for Fe(CO), [21], TMT [22], and MMT.? Since the vapor pressure of MMT is 

much lower than that of the other agents, it was necessary to preheat the burner and lines prior to addition 

of this agent. The air was preheated to >SO "C and served to heat the burner convectively to >(35.0 2 0.5) 

"C before and during the tests to reduce the likelihood of MMT condensation. 

For bromine as the inhibitor, all flow tubes downstream of agent addition as well as the burner base were 

replaced with ones made of Teflon to avoid reaction. A computer-controlled syringe pump (Yale Apparatus YA-12) and disposable syringe (10 cc BD lOcc22G1) added the liquid Br,, via a stainless steel 

needle in a viton septum, to the tubing carrying the air and CO,. A 2.1 m length of tubing prior to the 

burner was used to ensure agent mixing with the air, and complete Br, evaporation was observed to occur 

within a tubing length of less than 1 m. 

To determine the extinction condition for pure CO,, the CO, was added to the air flow in increasing volume (in increments of < 1%) until lift-off was observed. The test was repeated at least three times. In the 

presence of a second inhibitor, that agent was first added to the air stream at a constant volume fraction, 

and then the amount of added CO, required for lift-off was determined as in the pure CO, case. The 

amount of CO, required for lift-off is reported as its volume fraction in the burner co-flow stream (i.e., 

the sum of the air, added CO,, and carrier-gas CO,). (Note that in some early tests, to be described in 

* Certain commercial equipment, instruments, or materials are identified in this paper to adequately specify the 

procedure. Such identification does not imply recommendation or endorsement by the NIST, nor does it imply 

that the materials or equipment are necessarily the best available for the intended use. 

t Hollrah, U., personal communication, Jan. 2001. 

llalan Options Technical Working Conference 24-26 April 2001 189 
Table 1, N, was used as the carrier gas instead of CO,. In those cases, we included the N, in the CO, 

flow by correcting for differences in specific heat between the two.) The fuel gas is methane (Matheson 

UHF', 99.9%), and the oxidizer is house compressed air (filtered and dried), which is additionally cleaned 

by passing it through an 0.01 pm filter, a carbon filter, and a desiccant bed to remove small aerosols, 

organic vapors, and water vapor. The chemicals used were Fe(C0)s (Aldrich), TMT (Alfa Aesar), MMT 

(Alfa Aesar), CH30H (Aldrich, 99.8%), Br, (Aldrich, 99.5%), CF,Br (Great Lakes), N, (boil-off), and 

CO, (Airgas). 

The uncertainty analysis consists of calculation of individual uncertainty components and root mean 

square summation of components [23]. All uncertainties are reported as expanded uncertainties: X* U, 

where U is ku, and is determined from a combined standard uncertainty (estimated standard deviation) u, 

and a coverage factor k = 2 (level of confidence approximately 95%). Likewise, when reported, the 

relative uncertainty is U/ X. loo%, or ku,/X- 100%. The expanded relative uncertainties for the experimentally determined quantities in this study are as follows: CO, volume fraction, 4%; inhibitor volume 

fraction for organometallics, CF,Br, and Br,: 5%, 2.7%, and 2.0%; or otherwise as noted on the figures. 

RESULTS AND DISCUSSION 

We conducted our first tests in a heptane-fueled cup burner. In the absence of Fe(CO),, the extinction 

condition occurs at a CO, volume fraction of (19.2 k 0.76)% (the value is somewhat low due to the low 

air volumetric flow of 25 Vmin in these tests). With about 450 ppm of Fe(CO), added to the air stream, 

the C02 volume fraction required for extinction is (1 8.3 f 0.74)%, indicating a reduction of only about 

5%. This result is unexpected since adding 150 ppm of Fe(CO), to a premixed methane-air flame reduces 

by a factor of eight the amount of CO, required to halve the burning velocity. 

For a cup-burner flame, addition of Fe(CO), or CO, might not only change the extinction condition, but 

might also change the heat release rate at each agent volume fraction. We measured the fuel consumption 

rate as an approximate measure of the heat release in the flame. Figure 1 shows the heptane consumption 

rate in a cup burner as a function of the CO, mole fraction in the air stream. The upper curve shows the 

results for pure CO,, while the lower curve presents the results for addition of 450 ppm of Fe(CO), to the 

air stream prior to addition of CO, to the air stream. As the figure shows, the presence of Fe(CO), causes 

a 16% to 38% reduction in the fuel consumption rate for CO, mole fractions up to 75% of extinction. 

Unfortunately, the results in Figure 1 are confounded by the sooting tendencies of heptane cup-burner 

flames. Heat transfer to the pool surface is enhanced by soot in the gas phase, as well as by soot on the 

hot chimney walls. With addition of Fe(CO),, soot production was visually observed to be reduced, 

which is consistent with its known effects on the sooting tendencies of flames (Linteris et al. [4] and the 

references therein). 

At higher mole fractions of added CO,, soot formation is visually observed to be suppressed, with and 

without addition of Fe(CO),. We desire to eliminate the complicating effects of soot formation in cupburner flames. To retain a liquid fuel but virtually eliminate sooting, we ran additional tests with methanol as the fuel. The methanol-air flames extinguished at a CO, mole fraction of (26.0 f 1 .I)% without 

Fe(CO),, and (23.9 + I.O)% with 450 ppm of Fe(CO), added to the air stream. As with heptane, the 

reduction in the amount of C02 required for extinction from added Fe(CO), is small, here only about 8%. 

Figure 2 shows the change in the methanol consumption rate with addition of 450 ppm of Fe(CO), to the 

air stream, as a function of the amount of added C02. We see that without the confounding effects of soot 

formation, the fuel consumption rate is reduced by a nearly constant amount. The fuel consumption rate 

is reduced somewhat, but the reduction is not nearly as high as might be expected based the overall reaction rate reduction in premixed methane-air flames. 

190 Halon Options Technical Working Conference 24-26 April 2001 
ow 0 05 010 0 15 0 20 

COz Mole Fraction In Air Stream 

Om 005 010 015 020 025 

CO, Mole Fraclian in Air Stream 

~, 

0 30 

Figure 2. Methanol consumption rate in cupburner flame as a function of added 

CO, mole fraction with and without 

450 ppm Fe(CO), added to the air 

stream. 

Figure 1. Heptane consumption rate in cupburner flame as a function of added 

CO, mole fraction with and without 

450 ppm Fe(CO), added to the air 

stream. 

To further understand the influence of iron in cup-burner flames, we also conducted tests with methane 

fuel. In these tests, Fe(CO), could be added to either the fuel or the air stream, removing the uncertainty 

associated with agent transport to the relevant reaction zone. Table 1 summarizes the results for all of the 

tests. The top line shows that (15.7 f 0.6)% CO, is required to extinguish this flame. The next two lines 

show that addition ofFe(CO), to the air stream at quite high mole fractions (450 ppm and 924 ppm) 

causes only a (9.6 f 0.5)% and (13.5 I 0.8)% reduction in the amount of C02 required for extinction. If 

we add 1% CH, to the air stream to change the flame location and hence the scalar dissipation rate, the 

reduction in the CO, required for extinction with addition of 450 ppm of Fe(CO), is slightly greater, but 

still only about (10.7 f 0.6)%. Likewise, addition of Fe(CO), to the fuel stream at either 450 ppm or 

4500 ppm causes only a (I .3 f O.l)% and (2.6 k 0.2)% reduction in the amount of CO, required. These 

results are in contrast to those for CF,Br with CO, in the cup burner, as shown at the bottom of the table. 

There we see that the addition of CF,Br to either stream at mole fractions, which would approximately 

halve the burning velocity, leads to a reduction of the CO, required for extinction by a factor of two to 

three. Clearly, CF,Br and Fe(CO), behave quite differently in the cup burner with respect to their ability 

to reduce the CO, requirement for extinction. 

The question naturally arises as to whether this is specific to iron, or characteristic of super-effective 

inhibitors in general. To investigate this, we examined the cup-burner extinction data of TMP and 

DMMP from researchers at NMERI [24]. In their tests, they found that about 5% of DMMP was required for extinction, or 5.3% to 6.9% for TMP. It is surprising that so much is required to extinguish 

cup-burner flames considering that in methane-air flames, these phosphorus compounds have been shown 

to be about 17 and 3 times more effective than CF,Br in premixed [25] and diffusion flames [26], respectively. In these tests in cup burners, DMMP and TMP are less effective on a volume basis than CF,Br; on 

a mass basis, they are only about as effective as CO,. Further, at these concentrations, the agent could be 

working through oxygen starvation. For DMMP and TMP, e.g., the stoichiometric mole fraction in air is 

about 4.5 and 5.0%. Hence, the actual mole fractions required for extinguishment are close to those at 

which the agent would consume all of the available oxygen in the air stream. 

IJalon Options Technical Working Conference 24-26 April 2001 19 I 
TABLE 1. EXTINCTION VOLUME FRACTION OF C02 IN METHANE-AIR CUP BURNER 

WITH AND WITHOUT VARIOUS AMOUNTS OF FE(CO), OR CF,BR ADDED TO 

THE FUEL OR AIR STREAM (AIR FLOW =41.6 LMIN). 

Vol% co, %Reduction from 

15.7f0.6 

Catalytic Inhibitor at Extinction Pure CO, 

None 

Fe(CO), 450 ppm In air 14.1 f 0.6 9.6 * 0.5 

450 ppm In air w/ I% CH, 14.0 f 0.6 10.7 f 0.6 

450 ppm In fuel 15.4 f 0.6 1.3 fO.l 

4500 ppm 15.2 f 0.6 2.6 f 0.2 

CF,Br 1.3% In air 4.4 + 0.2 72.0 f 4.1 

11% In fuel 8.7 f 0.3 44.2 f 2.5 

- - - 

924 pprn 13.5 f 0.5 13.5 f 0.s 

The present discussion of the results in Table I may lead one to conclude that the super-effective agent 

Fe(CO), is not effective for suppressing cup-burner flames. If we conduct tests for a continuous range of 

concentrations for added Fe(CO), and CF,Br, however, a more comprehensive picture emerges. Below, 

we present additional detailed data for CO, extinction of methane cup-burner flames for TMT, MMT, 

Br,, and CF,Br. 

Instead of finding the mole fraction of CO, required to extinguish a cup-burner flame with a fixed, constant amount of catalytic agent, we can perform the tests for a range of catalytic inhibitor mole fraction in 

the air stream. Figure 3a shows the results of such tests for five catalytic agents, and Figure 3b shows the 

same data for the region corresponding to the dotted box in the upper left of Figure 3a. 

For extinction of these methane-air flames, pure CO, is required in the air stream at mole fraction of (I 5.7 

f 0.6)%, whereas CF,Br, a catalytic agent, is required at (2.4 & 0.1)%. As Figure 3a shows, adding CF,Br 

at mole fractions below the extinction value greatly reduces the amount of CO, required for extinction. 

For example, adding half of the extinction value of CF,Br reduces the amount of CO, required by 70%. 

The curvature in the line for CF,Br in Figure 3a indicates that, as described previously [14], the combination of CF,Br and CO, is synergistic; that is, when combined, less of each is required for extinction than 

one would expect based on a linear interpolation of the results for each individually. The curve for Br, 

indicates that it is about twice as effective as CF,Br, confirming the dominance of the Br molecule and 

the lesser hut apparent role of the CF, radical (as has been shown for premixed flames [27]). 

In the sequence CF,Br, TMT, Fe(CO),, and MMT, each is about twice as effective as its predecessor at 

low mole fraction, so that Fe(CO), is about four times as effective as CF,Br. While this performance is 

noteworthy, it is far less than was observed in premixed flames, for which the benefit was one to two 

orders of magnitude for Fe(CO), as compared to CF,Br. Interestingly, TMT, Fe(CO),, and MMT all have 

a decreasing slope as their mole fraction increases. This is similar to their behavior in premixed and 

diffusion flames, leading one to believe that the cause of the loss of effectiveness is the same (condensation of active species). 

In previous work it has been argued that to obtain the good performance by the super-effective agents, it 

might be possible to add small, non-condensing amounts of catalytic agents together with an inert agent. 

We tested this claim by adding a blend of the three catalytic metals to the airKO, stream. In the bottom 

curve of Figure 3b, MMT, Fe(CO),, and TMT are added in linearly increasing amounts, up to 200 ppm, 

420 ppm, and 3 100 ppm, respectively, and the amount of CO, for flame extinction is plotted as a function 

192 Halon Options Technical Working Conference 24-26 April 2001 
6 016 

'S " 

5 a 014 

W 

L 

,o 012 

e '5 0 1 

n 

m < 008 

0 

._ 

.. 006 

LL 

0 a 004 

5 

0 

0" 002 

0 

0 5WO 10000 15000 2owO 25WO 

Catalytic Inhibitor Mole Fraction (ppm) 

(4 

0 1 , __ _- .- -~ , 

0 5w 1wo 

Catalytic Inhibitor Mole Fraction (ppm) 

Figure 3. Mole fraction of CO, required for methane-air cup-burner flame extinction as a function of 

catalytic inhibitor mole fraction, CF,Br, Br, Fe(CO),, TMT, MMT, or a blend of the last 

three (b. has expanded x- and y-axes compared to a.). 

of the MMT mole fraction. As shown, the agents do work together to reduce the amount ofCO, required 

for extinction, and (within the uncertainty of the measurements) the blend does not lose its effectiveness. 

Nonetheless, the performance ofthe blend is only about half what would be expected from sum of the 

individual contribution from each component. Unfortunately, even with these relatively high mole fractions of the inhibitors, we reduce the amount of CO, required by only 25%. 

On a molar basis, the organometallic compounds are effective at reducing the amount of CO, required for 

cup-burner extinction as compared to CF,Br; however, their relative performance is much poorer than 

one would expect based on their behavior in premixed flames. Two possible causes of the loss of inhibition effectiveness are the same as were discussed previously for premixed and counterflow diffusion 

flames, namely (1) saturation of the catalytic cycles and (2) condensation of active gas-phase species. 

They are discussed below. 

The saturation of the catalytic cycles is deemed to be unlikely. This is based on two results shown in Figure 3: those for Br,, and those for the blend of MMT, Fe(CO),, and TMT. If the loss of effectiveness of 

each of the organometallic agents was due to depletion of the radical pool, the region of the flattening behavior in the curve of each would occur near the same value of )(co2. The experiments with Br, were designed to test this. Since Br, does not have a fuel effect (Le., inhibition due to oxygen depletion) as does 

CF,Br, it can more clearly test the role of radical recombination. As Figure 3b shows, the data for Br, are 

very linear in the region where the other curves are starting to flatten out, implying radical saturation is 

not the cause of the loss of effectiveness of the metals. The possibility of radical depletion is further explored in tests with a combination of MMT, Fe(CO),, and TMT. If the agents added alone were losing 

their effectiveness due to radical depletion, adding a second (or third) catalytic agent to the mix would not 

provide additional inhibition. In the bottom curve of Figure 3b, however, the blend of all three agents 

(added up to (200,420, and 3 100) ppm each, in linearly increasing amounts, and plotted as a function of 

the MMT mole fraction) clearly shows additional inhibition over MMT alone, providing evidence against 

radical saturation. 

Halon Options Technical Working Conference 24-26 April 2001 193 
We speculate that particle formation is the cause of the degraded performance of the metal agents. The 

approximate agent mole fraction for the loss of effectiveness is an order of magnitude higher for TMT 

(4000 ppm) than for the iron or manganese (400 ppm) (as occurs in premixed flames) [5], which is 

consistent with the higher vapor pressure for the tin compounds. A visible outer annulus, apparently 

particles, was observed in all flames with added metals, and the blackbody radiation from that region 

increased with higher agent mole fraction. Finally, as discussed in the literature [IO, 281, the particle 

formation rates appear to be linked to their residence time in the cooler regions of the flame. Since the 

present cup-burner flames have quite different flow fields than either premixed or counterflow diffusion 

flames, it is likely that the condensation behavior in them is different. It is possible that metal compounds 

added to the air stream are sequestered as particles, which are then convected away from the relevant 

radical recombination zones of the flame before they can affect the flame chemistry. 

CONCLUSIONS 

We have presented the first data on the extinction characteristics of highly effective catalytic agents 

added to a cup-burner flame together with CO,. The order of increasing performance is as follows: 

CF,Br, TMT, Fe(CO),, and MMT, with each about twice as effective at low mole fraction as its 

predecessor. The data indicate that while these compounds are very effective compared to CF,Br on a 

molar basis, their relative performance benefit over that agent is far less than in premixed flames. 

Further, they experience a loss in effectiveness at reducing the amount of CO, required for extinction, 

which is reminiscent ofthe behavior in premixed flames. We postulate (but have not demonstrated) that 

the loss in effectiveness is due to condensation of the metal, metal oxides, or metal hydroxides. For 

methanol, the effect of the metal compounds on the fuel consumption rate (caused by the heat feedback to 

the liquid fuel surface) is small. For heptane, the effect is somewhat larger, and this is postulated to be 

due to reduced soot formation. Finally, while we have shown that combinations of organometallic catalytic agents can reduce the amount of CO, required for extinction, addition of 200 ppm of MMT, 420 

ppm of Fe(CO),, and 3 100 ppm of TMT reduced the amount of CO, required for extinction by only 25%. 

This reduction is about half that which would have been expected based on a linear combination of the 

effect from the individual components. In future research, it is suggested to determine if phosphorus 

compounds, which have also been shown to be effective in premixed and counterflow diffusion flames, 

are effective at reducing the amount of CO, required to extinguish a cup-burner flame. It has been postulated that the reason for the lower effectiveness in these diffusion flames is particle formation and the 

subsequent sequestering of the active gas-phase inhibiting compounds prior to their introduction to the 

stabilization regions ofthe flame. It would be helpful to perform measurements of particles in these 

flames via traditional laser scattering and extinction to determine whether this is indeed the reason for 

loss of effectiveness. 

ACKNOWLEDGMENTS 

The author thanks Tania Ricbie for help with the experiments. This research was supported by the 

Department of Defense Next Generation Fire Suppression Technology Program, funded by the DoD 

Strategic Environmental Research and Development Program under contract no. W74RDV83528667, and 

by the Office of Biological and Physical Research, National Aeronautics and Space Administration, 

Washington, DC . 

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I94 Halon Options Technical Working Conference 24-26 April 2001 
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