SUPPRESSION OF FLAMES WITH VARIABLE Z,,

ae

wip ba,
2 ae ry
E ois i ay
i i}
g 0 a ;
0.05 . *

aoa
0

Qo 0! 02 03 G4 OS 06 07 08

Fic. 4. Flame-suppression effectiveness of 500 ppm
DMMP as an oxidant-side or fuel-side additive versus stoi-
chiometric mixture fraction, Effectiveness is defined as
{ayo — @Vaqo). Flames are methane-nitrogen versus ox-
ygen-nitrogen; ajo = 350 + 10 s~!. Error bars represent
standard deviation of data.

To further clarify the influence of temperature on
this system, a series of tests were performed for a
fixed 7, = 0.055. Tq was manipulated by varying
the reactant stream temperature from 22 to 112
(£1) °C. This resulted in a range of T,q from 2237
to 2283 K. There was a significant influence on the
overall flame strength, as seen in the variation of ayy
from 254 to 375 s~!. However, the effectiveness for
argon at different temperatures varied less than 4%
and showed no trend with T,,. These results con-
firmed that argon’s effectiveness displays no tem-
perature dependence.

Chemically Active Dopant

Having shown that our experimental and normal-
ization techniques produce the expected result of no
temperature or Z,. dependence for an inert sup-
pressant, we next considered the chemically active
agent, DMMP. Figure 4 shows the results from ex-
tinction measurements performed with DMMP for
Z between 0.055 and 0.7. The observed effective-
ness variation with Z,, is somewhat more compli-
cated than for the inert tests, although it still follows
the same general trend. Each data point in Fig. 4
represents the average of eight extinction measure-
ments, with error bars representing the standard de-
viation of the measured values. Differences between
individual data points at a given Z,, show that tests
repeated at identical conditions yielded slightly dif-
ferent results. This variation is attributed largely to
the repeatability of the dopant injection system. The
dopant effectiveness may also be influenced by small
changes in flame temperature resulting from lack of
repeatability of the reactant stream compositions.
‘The resulting overall experimental scatter that we
observed was significantly higher than the inert tests.

2753
ae -
so bid
= 00 3 |
3 3
2 co ‘ g &

Buop 8 © 4
4

E 0 i a4

3 pt tit |

§ 0

Fic. 5. Normalized flame-suppression effectiveness of
DMMP-doped flames versus stoichiometric mixture frac-
tion at fixed aq. Effectiveness is normalized by the quantity
of nonreacting DMMP at the maximum temperature con-
tour in flame, as predicted by a numerical simulation. Error
bars represent standard deviation of data and uncertainty
in calculated loading,

The systematic uncertainty due to the method of ap-
proaching extinction becomes significant for the ox-
idizer-side addition tests due to the high observed
effectiveness. We estimate an overprediction be-
tween 3% and 15% of DMMP effectiveness for this
range of conditions due to this systematic uncer-
tainty. The effect is small for the fuel-side addition
lata.

The normalized effectiveness results for DMMP
(seen in Fig. 5) have some interesting features. First,
it is notable that the DMMP effectiveness per unit
mole fraction is roughly 100 times that of argon, in-
dicating strong chemical suppression by DMMP.

Second, the curves for oxidizer- and fuel-side ad-
dition are offset significantly with respect to one an-
other. This increased oxidizer-side effectiveness has
also been observed for the chemically active agent
CF,Br [1]. The oxidizer-side curve in Fig. 5 displays
an effectiveness roughly twice that of the fuel-side
curve. The slight systematic overprediction of the
oxidizer-side addition effectiveness, described ear-
lier, would reduce the magnitude of this disparity but
would not reconcile the difference in these curves.
It is possible that because the chemically active sup-
pressant works by interfering with the radical chem-
istry rather than as a heat sink within the flame, the
maximum flame temperature contour may not be
the most relevant location for evaluating the mole
fraction of dopant. Rather, the appropriate normal-
ization depends on the quantities present in the
regions of the flame where the most important rad-

chemistry occurs. Because key radical reaction
rates peak on the oxidizer side of the flame, a more
appropriate normalization might involve mole frac-
tions at some location to the oxidizer side of the max-
imum temperature contour. We examined an alter-
native normalization using mole fractions at the


2756

of suppressant concentration at extinction for the cup
burner tests and as the fractional change in laminar flame
speed for the premixed flame experiments. In both cases,
effectiveness was found to have a significant dependence
on temperature, dropping by 15-35% with a rise of 100 K
in adiabatic flame temperature. Although these results are
comparable to our observations for DMMP, based on frac-
tional reduction in extinction strain rate, caution must be
used when making comparisons across three different mea-
sures of effectiveness. There is also a serious difficulty in
consistently evaluating the temperature range over which
the change in effectiveness is observed. In the case of the
cup burner and premixed flames, the adiabatic flame tem-
perature is more representative of the actual flame tem-
perature than in our highly strained flames. We look for-
ward to seeing your results, which may shed light on the
relationship between catalytic recombination mechanisms
and the temperature dependence of suppressant effective-
ness.

REFERENCES

1. Lott, J. L., Christian, §.D., Sliepcevich, C.M., and
Tucker, E. E., Fire Technol. 32:260-271 (1996)

2. Fristrom, R. M. and Van Tiggelen, P, in Seventeenth
Symposium (International) on Combustion, The Com-
bustion Institute, Pittsburgh, 1978, pp. 773-785.

Sandra Olson, NASA Lewis Research Center, USA. How
does DMMP toxicity compare to Halon 1301, which was
fairly benign in its unreacted state?

Author's Reply. CF3Br is almost completely inert under
ambient conditions. In contrast, DMMP is classed as an
irritant to the cyes, skin, and respiratory system. There is
also evidence from animal studies that DMMP may be car-
cinogenic and have reproductive effects [1,2]. Our recent
unpublished work indicates that several organic phosphates
and phosphonates perform very similarly to DMMP as
flame suppressants. This finding supports the hypothesis
that it is the phosphorus content of a molecule, not its
structure, that is of primary importance in determining
fire-suppression effectiveness. This suggests the possibility
of identifying a less toxie phosphorus-containing compound
with flame-suppression effectiveness comparable to that of
DMMP.

CHEMICAL FLAME INHIBITION

REFERENCES

1, Bennett, S. R., Bane, J. M., Benford, P.J., and Pyatt,
R.L., “Environmental Hazards of Chemical Agent Si-
mulants,” Chemical Research and Development Cen-
ter, Aberdeen Proving Ground, Technical CRDC-TR-
84055, 1984.

Material Safety Data Sheet—Dimethyl Methylphos-
phonate, MDL Information Systems, Inc., Nashville,
TN, 1998.

we

Dave Zerkle, Los Alamos National Laboratory, USA. The
flame inhibitor is a benign chemical. Where does the phos-
phorus end up after the flame? Comment on the toxicity
of these compounds.

Author's Reply. Thermochemical equilibrium calcula-
tions indicate that the preferred product of phosphorus
combustion is Pgs, or Pgs for rich flames [1]. We have
measured combustion products from DMMP-doped
slightly subatmospheric premixed flames using extractive
sampling followed by chemical derivatization and GC/MS
analysis [2]. These measurements showed significant quan-
tities of orthophosphoric acid [P(O)(OH)s] and methyl-
phosphonic acid [P(CIH3(O)(OH)2}; P20 and P20, are
not likely to be detectable with this technique. All these
phosphorus oxides and acids have significantly higher tox-
icities and corrosivities than the parent compound. Avail-
able health and safety information [3] indicates that health
risks for these compounds are comparable to the products
of combustion of CF3Br (ic., HBr and HF).

REFERENCES

1. Fisher, E. M., “Equilibrium Composition Calculations
for Combustion of Organophosphorus Compounds,”
1995 Fall Technical Meeting of the Eastern States Sec-
tion of the Combustion Institute, Worcester, MA, 1995.

2. Rapp, D.C., Nogueira, M. F. M., Fisher, E.M,, and
Gouldin, F. C., Environ, Eng. Sci. 14:133-140 (1997).

3. Sax, N. I. and Lewis, R. J., Dangerous Properties of In-
dustrial Materials, 7th ed., Van Nostrand Reinhold,
New York, 1989.



2752 CHEMICAL FLAME INHIBITION
Pe y 6
et, aot
= 2250 . a 3
= cy j
g ‘ a4 . &
E220 .
é i. i,
2 : 3
§ nso | ; zg
io 2
3 :
3 8 §
2 a0 jo: ,
0 G1 02 03 04 05 06 0.7 08 oi 02 03 04 OS 06 07 O68
Zz, Zz,

Fic. 1. Adiabatic flame temperature versus stoichiomet-
ric mixture fraction for a methanc-nitrogen versus oxygen—
nitrogen flame with the extinction strain rate held constant
at 350 + 10 s-!, Error bars represent standard deviation
of data.

0.08

0.07

0.06
O05

008

Effectiveness

003

0

O01

8

Fic. 2. Flame-suppression effectiveness of 25,000 ppm
argon as an oxidant-side or fuel-side additive versus stoi-
chiometric mixture fraction. Effectiveness is defined as
{layo ~ 4,)/aqo}. Flames are methane-nitrogen versus ox-
ygen-nitrogen; a4, = 350 + 10 s-'. Error bars represent
standard deviation of data.

the flame at a fixed %,, and manipulating the tem-
perature of the reactant streams. These experiments
were intended to isolate the temperature depen-
dence of the suppression from other effects.

Inert Tests

Argon was introduced as an inert suppressant for
both tests previously described. In these measure-
ments, we expected the inert dopant’s effectiveness
to depend solely on the quantity of dopant present
at the flame. Previous experiments have demon-
strated that, for a range of loadings greater than that
considered in the current study, the effectiveness of
the inert suppressant nitrogen increases linearly with
dopant loading, at fixed Z,, [4,15,22]. Because flame
temperature is reduced by the addition of dopant,

Fic. 3. Normalized flame-suppression effectiveness of
argon-doped flames versus stoichiometric mixture fraction
at fixed ayy. Effectiveness is normalized by the quantity of
argon at the maximum temperature contour in flame,
predicted by a numerical simulation. Error bars represent
standard deviation of data and uncertainty in calculated
loading.

as

this implies temperature invariance of suppression
effectiveness over that loading range. Results from
the first series of tests in which Z,, varied from 0.2
to 0.7 are shown in Fig. 2. The effectiveness of the

n, with both the oxidizer- and fuel-side addition,
is illustrated. Each data point represents the average
of four extinction measurements. The error bars on
the figures represent the standard deviation of the
observed values, which in this case is larger than the
systematic error due to the method of approaching
extinction. As expected, effectiveness with oxidant-
side doping decreases as the flame moves toward the
fuel side (increasing Z,,). The converse is true with
fuel-side doping, Argon has a diffusivity similar to
that of the reactants; thus, the quantity present at
the stoichiometric contour, which determines its ef-
fectiveness, will vary linearly with Z,,.

Figure 3 depicts the argon results, normalized by
the numerically predicted: quantity of argon present
at the maximum temperature contour. The uncer-
tainty in the calculated mole fraction of dopant at
the flame should be dominated by the uncertainty
in the experimental flow rate of the dopant: + 10%
as determined by the manufacturer's stated repeat-
ability of the mass-flow controller. However, the
smooth trends in the data indicate a significantly bet-
ter repeatability. The normalization process reveals
the anticipated behavior for the effectiveness of ar-

n. The effectiveness curves show no trend with Z,,,
indicating that the concomitant changes in T,4 and
flame structure, collectively, have no significant in-
fluence on the argon effectiveness. Other tests, sub-
sequently described, further demonstrate that the
argon effectiveness is independent of Tua In edi
tion, Fig. 3 shows no significant difference, in view
of the experimental uncertainty, between the effec-
tiveness of argon with oxidizer- or fuel-side addition.


structure, which occur at different Z,, values, may
be influencing suppressant effectiveness as well.
Further tests were conducted to clarify this tem-
perature dependence. Due to condensation of the
dopant below 80 °C, we were unable ta use control
of the reactant temperatures to vary T,,q over a sig-
nificant range. Instead, the overall dilution of the
flame, at a fixed value of Z,, = 0.5, was used to vary
Tua. In this experiment, the mass fraction of oxygen
in the oxidizer stream ranged from 0.39 to 0.44, re-
sulting in a T,q ranging from 2132 to 2261 K and an

2754 CHEMICAL FLAME INHIBITION
E 120
E 4 7
Hy ab
z $
5 wo rs
2
3 A
50
& ‘ H
g ot
— 400 +t + ie
é .
& 200
20 NOD SD 22H) 22502300

Adiabatle Flame Temperature (K)

Fic, 6. Normalized effectiveness of DMMP-doped
flames as a function of adiabatic flame temperature. A rep-
resents changing Z,, from 0.1 to 0.7, fixed aq = 350 + 10
s-1,and @ represents a fixed Z,, = 0.5, changing ayy from
294 to 572 s~? by varying mass fraction of oxygen. Error
bars represent standard deviation of data for each set of
data runs.

point of the maximum reaction rate for the chain-
branching reaction 0, + H — OH + O. Because
the concentration gradients of oxidizer- and fuel-side
dopants have opposite directions, this shift in loca-
tion toward the oxidizer side brings the two curves
in Fig. 5 closer together. However, even considering
systematic error, oxidant-side doping remains more
effective.

The disparity between oxidizer- and fuel-side ad-
dition may also be associated with the types and
quantities of radicals and products that are formed
from the parent molecule as it approaches the flame
from different directions and their relevance to the
suppression action [7]. If we assume that the impor-
tant suppression action takes place on the oxidizer
side of the flame, then additives to the fuel stream
must travel through the flame to reach this point.
This may have a significant effect on what phospho-
Tus-containing species are present in the region of
important radical chemistry for different modes of
addition, that is, oxidizer- or fuel-side doping.

A third feature of Fig, 5 is that for both the oxi-
dizer- and fuel-side tests, the normalized effective-
ness increases with Z,.. The existence of this slope
implies that the suppressant effectiveness varies with
either temperature or structure of the flame or both.
Studies in a variety of premixed flames have indi-
cated that at high temperatures (above 2350 K)
[20,27], DMMP can become a flame promoter.
Thus, presumably, the suppression effectiveness of
DMMP should decrease with increasing flame tem-
perature. Figure 5 is consistent with this expectation,
showing decreasing suppressant effectiveness with
increasing adiabatic flame temperature (lower Z,.
values). However, it is possible that changes in fame

yy tange of 294-572 s-1. Unfortunately, the varia-
tion in reactant stream compositions increased the
susceptibility of the results to the sensitivity to ran-
dom uncertainties in relative flow rates, producing
considerable scatter. The results from this experi-
ment are shown in Fig, 6, along with the data for
oxidizer-side addition at fixed q0, but varying Z,, val-
ues; all plotted against T,,. Although the large ex-
perimental scatter makes it difficult to draw quan-
titative conclusions, a clear trend of decreasing
effectiveness with increasing 7,4 is observed, com-
parable to that seen in the constant dqy data. How-
ever, these results do not conclusively eliminate the
possibility of flame structure and the detailed chem-
istry of dopant action influencing dopant effective-
ness.

Conclusions

Experimental results indicate that the flame-sup-
pression properties of an inert compound, argon,
added to methane-nitrogen versus oxygen-nitrogen
non-premixed flames can be characterized solely by
the quantity of suppressant present at the flame sur-
face. There was no significant observed dependence
on adiabatic flame temperature or mode of delivery,
that is, oxidizer- or fuel-side doping. There was also
no dependence on the value of the stoichiometric
mixture fraction, excepting that related to its influ-
ence on the quantity of dopant reaching the flame
surface.

Determination of effectiveness for the chemically
active fame-suppressant compound DMMPis more
complex. Even when normalized by the quantity at
the flame, the suppressant is significantly more ef-
fective when added to the oxidizer-side, rather than
fuel-side, of the flame. This disparity in effectiveness
is consistent with other researchers’ observations for
the chemically active flame-suppressant CFsBr. Ex-
periments also indicate that the effectiveness of
DMMP has a strong temperature dependence, with
lower effectiveness at higher adiabatic flame tem-
peratures.

Acknowledgments

This research is part of the Department of Defense's
Next Generation Fire Suppression Technology Program,


“Twenty-Seventh Symposium (International) on Combustion/The Combustion Institute, 1998/pp. 2749-2756

VARIATION OF CHEMICALLY ACTIVE AND INERT FLAME-SUPPRESSION

EFFECTIVENESS WITH STOICHIOMETRIC MIXTURE FRACTION

M. A. MACDONALD, T. M. JAYAWEERA, E. M. FISHER anv F.C. GOULDIN
Sibley School of Mechanical and Aerospace Engineering

Cornell University
Ithaca, NY 14853, USA

The need to find alternative fire suppressants has motivated experiments to determine the mode of
action of possible candidates, An opposed-jet burner was used to characterize the effectiveness of one
possible alternative, dimethyl methylphosphonatc (DMMP). Similar tests were done with an inert com-
pound, argon, for comparison. Flame strength was characterized by the extinction strain rate. Experiments
included both oxidizer-side and fuel-side doping of methane-nitrogen versus oxygen-nitrogen flames of
various compositions. The stoichiometric mixture fraction (Z,,) is varied systematically while holding the
undoped extinction strain-rate constant, by changing the amount of diluent in the reactant flows. This
moves the flame location with respect to the stagnation plane, affecting the fraction of a particular reactant
stream that reaches the flame. Measured effectiveness of fucl-side and oxidant-side doping versus Z,,
reflects this change in quantity of dopant reaching the flame. To account for this dependence on quantity,
effectiveness was normalized by the amount of dopant calculated to reach the maximum temperature
contour of the flame, Argon’s normalized effectiveness was found to be independent of Z,, of adiabatic
flame temperature, and of whether the oxidizer stream or the fuel stream is doped. DMMP’s normalized
effectiveness, however, was observed to be significantly greater when introduced in the oxidizer, rather
than fuel, stream. It also exhibits a marked dependence on adiabatic flame temperature, with lower values

at higher temperatures.

Introduction

Restrictions on the production of the fire sup-
pressant CF3Br have spurred interest in finding ef-
fective ozone-friendly replacements for it. Extine-
tion measurements in opposed-jet diffusion flames
[1-6] and laminar and turbulent coflow flames [7,8]
have been used to test the effectiveness of CF;Br
and candidate replacements and to shed light on
their mechanisms of action. Oxidant-side doping of
pure fuel versus pure air flames is the most relevant
configuration for fire-fighting applications. Fuel-side
doping is also of practical interest for compounds
that are used as flame retardants in plastics [9]. Sev-
eral studies [1,2,7] of both fuel-side and oxidant-side
doping of various fuel-inert versus oxygen-inert
flames have found dramatic changes in additive ef-
fectiveness with these changing conditions.

A key factor affecting suppression is the amount
of the additive reaching the non-premixed flame
[1,2,6-8]. When hydrocarbons are burned in air, the
stoichiometric contour, where the flame is assumed
to be, is situated on the oxidant side of the stagnation
plane. Fuel-side additives must diffuse across the
stagnation plane to reach the flame, while air-side
additives are convected through the flame. The re-
sult is that fuel-side additive mole fractions at the

flame are substantially reduced from their fuel-
stream values, while oxidizer-side additive mole frac-
tions are only slightly reduced from their oxidizer-
stream values. The extent of this additive loading
reduction at the flame location varies with the stoi-
chiometric mixture fraction (Z,,), defined as the mass
fraction of material originating from the fuel stream
at the location where the concentrations of reactants
are in stoichiometric proportions. For hydrocarbon-
air flames, Z, is typically <0.1. Considerably differ-
ent values of Z,, can be achieved by varying the com-
positions of the fuel and oxidant streams, either via
dilution or enrichment. For Z,, > 0.5, the flame is
on the fuel side of the stagnation plane, and additive
loading reduction at the flame is more severe for
oxidant-side additives than for fuel-side additives.
In several experimental studies of flame extinction
by CF3Br, the effect of additive loading reduction at
the flame location has been quantified, assuming
unity Lewis number. Results have been mixed. For
methane-air opposed-jet diffusion flames, Trees et
al, [1] found that CF3Br was roughly twice as effec-
tive as an air-side additive than as a fuel-side addi-
tive, when the quantity of additive at the stoichio-
metric contour is compared for equal extinction
strain rates, Work by Masri [7], on the other hand,
found that oxidizer-side and fuel-side addition of

2749


SUPPRESSION OF FLAMES WITH VARIABLE Z,,

funded by the DOD Strategic Environmental Rescarch
and Development Program under DARPA Contract
MDA9Q72-97-M-0013. The authors wish to thank J. Flem-
ing and B. Williams at Naval Research Laboratory; O. Ko-
robeinichev at the Institute of Chemical Kinetics and Com-
bustion, Russian Academy of Sciences, Novosibirsk, and T.
Cool and B. Lippman at Cornell University.

REFERENCES

Trees, D., Grudno, A., and Seshadri, K., Combust. Sct

Technol. 124:311-330 (1997).

Niioka, T., Mitani, T., and Takahashi, M., Combust.

Flame 50:89-97 (1983).

3. Papas, P, Fleming, J.W., and Sheinson, R. S., in

Twenty-Sixth Symposium (International) on Combus-

tion, The Combustion Institute, Pittsburgh, 1996, pp.

1405-1412.

MacDonald, M. A., Jayaweera, T. M., Fisher, E. M.,

and Gouldin, F.C., Combust. Flame 116:166-176

(1999).

Fallon, G.S., Chelliah, 11. K., and Linteris, G.T., in

Twenty-Sixth Symposium (International) on Combus-

tion, The Combustion Institute, Pittsburgh, 1996, pp.

1395-1403.

. Reinelt, D. and Linteris, G. T., in Twenty-Sixth Sym-

postum (International) on Combustion, The Combus-

tion Institute, Pittsburgh, 1996, pp. 1421-1428,

Masri, A.R., Combust. Sci. Technol. 96:189-212

(1994).

Smyth, K. C. and Everest, D. A., in Twenty-Sixth Sym-

posium (International) on Combustion, The Combus-

tion Institute, Pittsburgh, 1996, pp. 1385-1393.

Avondo, G., Vovelle, C., and Delbourgo, R., Combust.

Flame 31:7-16 (1978).

10. Grudno, A. and Seshadri, K., Combust,
112:418-437 (1998).

11. Seshadri, K. and Ilincic, N., Combust, Flame 101:271-
294 (1995).

12. Tapscott, R. E., Moore, T. A., Kaizorman, J. A., Kibert,

C.J, and Tetla, R. A., “Advanced Agent Halon Sub-

stitutes,” presented at 1995 International CFC and

icy

»

a

2

2

©

Flame

20. Hastie, J. W. and Bonne

2

22. Grudno, A. and Seshadri, K., Combust. 8

25. Peters, N., Combust. Set. Technol. 30:
26, Trees, D., Seshadri, K., and Hamins, A., “Experimental

2755

13, Zegers, E. J. P. and Fisher, E. M., Combust. Sei. Tech:
nol. 116-117:69-89 (1996).

14. Seshadri, K, and Williams, F.A., Int. J. Heat Mass
Transfer 21:251-253 (1978).

15. Puri, I. K. and Seshadri, K., Combust, Flame 65:137—
150 (1986).

16. Reynolds, W.C., “STANJAN,” Stanford University,
Stanford, CA, 1981.

17. Lutz, A. E., Kee, R. J., Grear, J. F,, and Rupley, F. M.,
“OPPDIF: A Fortran Program for Computing Op-
posed-Flow Diffusion Flames,” SAND96-8243, Sandia
National Laboratories, Alburquerque, NM, 1997.

18. Bowman, C. T., Hanson, R. K., Davidson, D. F., Gar
diner Jr., W.C., Lissianski, V., Smith, G. P., Golden,
D.M., Frenklach, M., and Goldenberg, M., “GRI-
Mcch,” Ver 2.11, http/Avww.me.berkeley.cdu/gri
mechy.

19. Twarowski, A., Combust. Flame 105:407-413 (1995).

D.W, “Molecular Chem-

istry of Inhibited Combustion Systems,” National Bu-

reau of Standards, Final NBSIR 80-2169; PB81-

170375, 1980.

Marolewski, T. A. and Weil, E. D., “A Review of Phos-

phate Ester Fire Resistance Mechanisms and Their

Relevance to Fluid Testing,” in Fire Resistance of In-

dustrial Fluids, vol. ASTM STP 1284 (G. E. Totten and

R. Jurgen, eds.), American Society for Testing and Ma-

terials, Philadelphia, 1996, pp. 102-109.

Technol.
112:199-210 (1996).

23. Li, S.C, Libby, P.A., and Williams, F.A., in Twenty-

Fourth Symposium (Iutemational) on Combustion,
The Combustion Institute, Pittsburgh, 1996, pp. 1503-
1512.

24, Kim, J. $. and Williams, F.A., SIAM J. Appl. Math

53:1551-1566 (1993).

17 (1983).

Studies of Diffusion Flame Extinction with Haloge-
nated and Inert Fire Suppressants,” in Halon Replace-
ments; Technology and Sctence, ACS Symposium Se-
ries 611 (A. W, Miziolek and W. Tsang, eds.), American
Chemical Society, Washington, D.C., 1995, pp. 190-
203.

Halon Alternatives Conference, Washington, D.C., 27. Korobeinichey, O. P., In, S. B., and Mokrushin, V. V.,
1995. Combust. Sct. Technol. 116-117:51-67 (1996).
COMMENTS

Yuko Saso, National Research Institute of Fire and Dis-
aster, Japan. 1 am interested in the strong temperature de-
pendence of DMMP effectiveness. Our recent work sug-
gests that the temperature dependence of CF3Br
effectiveness is caused by the inhibitor regeneration cycles,
which are enhanced at lower temperatures.

Recognizing that DMMP oxidation kinetics is not well
known, if you compare the magnitude of temperature de-
pendence of DMMP with that of CFBr, rough estimation

of contribution of catalytic inhibition cycle with DMMP
would be possible

Is the magnitude of the temperature dependence of
DMMP larger than that of CF3Br?

Author's. Reply. The temperature dependence of
CF;Br's effectiveness has been investigated experimentally
in cup burner extinction tests [1] and in laminar flame-
spced experiments [2]. Effectiveness is evaluated in terms


2750

CF3Br had very similar effectiveness in extinguish-
ing turbulent natural gas and air piloted jet flames,
when corrected for additive concentrations at the
flame location. Niioka et al. [2] report comparable
effectiveness for fuel-side and oxidizer-side addition
in extinguishing various ethene-nitrogen versus ox-
ygen-nitrogen opposed-jet diffusion flames, includ-
ing one with Z,, > 0.5. Close examination of the
Niioka data shows that the fuel-side addition is gen-
erally less effective than oxidizer-side addition.

In this study, we systematically investigate the ox-
idizer-side and fuel-side effectiveness of two addi-
tives in extinguishing methane-nitrogen versus oxy-
gen-nitrogen opposed-jet diffusion, flames. One
additive, dimethyl methylphosphonate (DMMP), is
chemically active; the other, argon, is inert. Sup-
pressant effectiveness is defined as the change in ex-
tinction strain rate when the additive is introduced,
normalized by the undoped extinction strain rate,
qo. Previous studies [1,4,10] indicate that for low
loadings of many suppressant agents, the effective-
ness, using this definition, varies linearly with dopant
loading. By varying the dilution level of the fuel and
oxidizer streams, we investigated how the effective-
ness of the two additives changes as a function of the
adiabatic flame temperature, T,4, and Z,,. In the pro-
cess, we also explore the interdependencies of T,,.
and Z,,, along with the aj; parameters that charac-
terize the flame.

Dimethyl methylphosphonate (DMMP) has pre-
viously been shown to be a highly effective flame
suppressant in non-premixed methane-air flames
[4}. Its effectiveness is linear with loading in the air
stream over the range 0-1000 ppm, with more than
a 25% reduction in the extinction strain rate at the
highest loading. This effectiveness is between two
and four times that of CF3Br [3,10,11]. Phosphorus-
containing compounds, such as DMMP, are among
the families of chemicals under consideration as re-

lacements for CF5Br in fire-fighting applications
4,12).

Experimental

Experiments were conducted with an opposed-jet
bummer. Methane (99% pure) diluted with nitrogen
(99.998% pure) was used as the fuel and a mixture
of oxygen (9.994% pure) and nitrogen as the oxi-
dizer. Variable dilution with nitrogen of both reac-
tants allows for non-premixed flames to be stabilized
on either the oxidizer or the fuel side of the stag-
nation plane. The burner was aligned vertically with
the lower tube used as the fuel source and the upper
tube as the oxidizer source. The burner was con-
structed from straight, open glass tubes 30 cm lon,
with an ID of 0.98 cm and a separation distance o}
0.95 cm between opposing nozzles. Annular sheath
flows of nitrogen are provided through 2.22-em-ID

CHEMICAL FLAME INHIBITION

glass tubes. The entire burner is isolated in a glass
enclosure with nitrogen purge. Reactant flow rates
were measured with calibrated mass-flow controllers
that have a manufacturer's stated repeatability of
+0.2% of full scale.

The chemically active flame suppressant used dur-
ing this investigation, DMMP (97% pure), is a liquid
at room temperature with a low vapor pressure. In
order to maintain sufficient concentrations of
DMMP in the vapor phase, the reactant lines were
heated to approximately 100 °C with electrical heat-
ing tapes. DMMP, which does not pyrolize below
910 K [13], is not expected to decompose before it
enters the flame zone. The temperature of the re-
actant streams 10 cm upstream from the exit of the
nozzles was maintained at 100 + 1 °C via active
control of the sheath flow temperature. For all tests
of chemically suppressed flames, a constant loading
of 500 ppm of DMMP was added, via a commer-
cially available syringe pump, to one of the reactant
streams. Argon additive testing was conducted with
a loading of 25,000 ppm. The disparity between
DMMP and argon concentrations is required be-
cause of the strong suppression effect of DMMP.

For each set of measured extinction conditions, a
global extinction strain rate was calculated from re-
actant stream flow rates and nozzle geometry and is
denoted a,. We use equation 1 [14] to evaluate a,

_ Vo Vr (2)"
a = Welt + He (PE q)

L refers to the separation distance between the noz-
zles, V is the stream velocity, and p is the stream
density, with the subscripts O and F referring to ox-
ygen and fuel, respectively. Equation 1 assumes
plug-flow boundary conditions at the nozzle exit
planes.

For a fixed L, extinction is approached by increas-
ing the reactant velocities ‘ant the critical strain rate
is achieved. In most studies [3,15], the velocities of
the fuel and oxidizer streams are increased propor-
tionally such that the flame or stagnation plane po-
sition is maintained near the center of the burner.
Because in our study DMMP is injected with a con-
stant mass flow syringe pump, changing the doped
reactant flow rate would change the dopant concen-
tration, leading to transients in adsorption or de-
sorption of DMMP from the walls of the tubing
downstream of the injection site. Because the time-
scale of these transients is fairly long (on the order
of tens of minutes), accurate determination of the
effectiveness of DMMP using the traditional
method of approaching extinction is impractical. To
circumvent this difficulty, a novel method of per-
forming extinction measurements was developed
and is described in detail elsewhere [4]. With this
method, only the undoped reactant flow rate is al-
tered, resulting in movement of the flame and
nation plane. For the variety of flame conditions



SUPPRESSION OF FLAMES WITH VARIABLE Z,,

relevant to this study, this method yields global ex-
tinction strain rates that agree to within + 2% with
the value found using the traditional method, pro-
vided that the location of the flame and stagnation
plane at extinction is within a specified range. The
deviation of up to 2% varies systematically with
flame position but is difficult to correct for. The con-
sequences of this effect are discussed in more detail
later.

One of the difficulties in conducting experiments
such as these, with oxidizer streams formed by mix-
ing oxygen and nitrogen flows, is the extreme sen-
sitivity of the extinction strain to the mass fraction
of oxygen in the oxidizer stream [15]. This effect
requires very precise metering of reactant flow rates
in order to achieve repeatable results. Even with
mass-flow controllers, an appreciable variability
(+3%) in the global extinction strain has been ob-
served at identical low-rate settings. We have found
a similar sensitivity to dilution of the fuel stream.
Consequently, measurements comparing doped and
undoped flames were conducted by first evaluating
the undoped extinction strain and then adding the
dopant to one reactant stream without altering the
composition of either stream. This technique mini-
mizes the impact of the sensitivity to stream com-
position.

Experiments were performed for stoichiometric
mixture fractions, Z,,, ranging from 0.055 to 0.7. Z,
can be evaluated from reactant compositions in the
nozzles and the stoichiometry of the overall com-
bustion reaction, using Eq. 2:

Ty = oe gy

" Vor
MW ovo\,,
(Tee). * Yo.)

In this equation, Y is mass fraction, MW is molecular
weight, v is the stoichiometric coefficient for com-
plete combustion, and the subscripts + refer to
conditions at the fuel and oxidizer nozzles. Changin
Za by varying the dilution of the oxidizer and uel
streams moves the location of the stoichiometric
contour, and thus the flame, relative to the stagna-
tion plane. Adiabatic flame temperatures for the
various values of Z,, and overall dilution of the flames
considered herein were evaluated for the undoped
case using the STANJAN code [16].

To accurately estimate the quantity of dopant at
the flame surface, numerical calculations using the
OPPDIF code [17] were made with plug-flow ve-
locity boundary conditions, multicomponent diffu-
sion but no thermal diffusion, and detailed chemistry
(excluding phosphorus compounds), using the GRI
mechanism [18] with nitrogen chemistry omitted.
The mole fraction of dopant is evaluated at the max-
imum temperature in the reaction zone, taken to
represent the flame location. The estimated diffusiv-
ity of DMMP is based on the interaction potentials

2751

of similar molecular weight compounds. The nu-
merical calculations are required because this dif-
fusivity is significantly different from the rest of the
reactant stream components, making the effect of
preferential diffusion important [1]

In these calculations, the chemically reactive com-
pound DMMP is treated as inert because complete
chemical kinetic and transport data are not available
for it or its combustion products. This treatment
should give a reasonable estimate of the quantity of
phosphorus present in all chemical forms: In the ap-
proximation of equal diffusivities for all phosphorus

ecies, at a given position the DMMP mole fraction
obtained in calculations treating it as inert is equal
to the sum of the mole fractions of all phosphorus
species present. Furthermore, in studies of other
phosphoras-containing compounds [19,20], it has

.en proposed that it is the phosphorus-containing
radicals formed in the fame that are responsible for
flame suppression. This implies that the form of the
phosphorus compound entering the flame is rela-
tively unimportant [21]. Therefore, modeling
DMMP as an inert compound for the numerical sim-
ulations is a reasonable approximation for estimating
relevant dopant loadings.

Results and Discussion

‘Two types of measurements were performed. The
first test varied Z,, from 0.055 to 0.7. At each Za, @qo
was matched to 350 + 10 s~), by trial-and-error
variation of the dilution of both reactants. This pro-
cess entails changing the overall dilution, a param-
eter that is independent of Z,. Due to the high sen-
sitivity of ayy to reactant composition, it is expected
that this narrow range in allowed ajo results in an
even narrower range of allowed reactant composi-
tions at a given Z,. Experimentally determinin;
those allowed compositions accurately becomes dif-
ficult, however, at high Z,,, where the low methane
flow rates result in a higher percentage uncertainty
in their values.

As Z,, is increased with fixed ajo, Ty declines
steadily, as seen in Fig, 1. Calculations by Grudno et
al. [22] for a fixed Ta, are qualitatively consistent
with this result, finding that @,y increased monoton-
ically as Z, was increased from 0.055 to 0.4. This
variation in dy and T,q with Z,, is attributed to the
changing relationship between the strain rate and
the appropriate timescale for diffusion of reactants.
While the strain rate is always considered to be a
good measure of that timescale, the relationship be-
tween the two depends on 7. [22-25]. Error bars in
Fig. 1 represent standard deviations of the measure-
ments. The large error at high Z,, represents the
methane flow-rate uncertainty previously described.

The second type of test varied only the adiabatic
flame temperature at a fixed Z,,. This was accom-
plished in two ways: changing the overall dilution of