CPIA PUBLICATION 685

DECEMBER 1998

Hardcopies available from CPIA only. Reproduction

is not authorized except by specific permission.

JANNAF

35TH COMBUSTION SUBCOMMITTEE

AND

17TH PROPULSION SYSTEMS HAZARDS

SUBCOMMITTEE MEETING

JOINT SESSIONS

Raytheon Systems Company

and

Marriott University Park

Tucson, AZ

7 - 11 December 1998

SelVingthe PropulsionCommunity for(Ner FitlyYears

CHEMICAL PROPULSION INFORMATION AGENCY

• THE JOHNS HOPKINS UNIVERSITY.

• WHITING SCHOOL OF ENGINEERING • COLUMBIA, MARYLAND 21044-3204 •

DISTRIBUTION STATEMENT: Approved for public release; distribution is unlimited.

A OTIC-sponsored 000 Information Analysis Center operating under contract SP0700-97-D-4004
-

SPECTROSCOPIC STUDIES OF INmBITED OPPOSED FLOW

PROPANE/AIR FLAMES

RR SKAGGS, RG. DANIEL, A.W. MIZIOLEK, AND K.L. McNESBY

U.S. Army Research Laboratory

Aberdeen Proving Ground, MD 21005

ABSTRACT

Planar Laser Induced Fluorescence (pLIF) is used to measure OH concentration profiles in an atmospheric

pressure, opposed flow, propane (C~)/air flame. Flame inhibiting agents CF~r, N2, Fe(CO)s, C~7H, C~6H2,

CH~(O)(OCH3h, and P~~6 were added to the flame and relative OH concentration profiles were measured as

each flame was extinguished. The OH profiles illustrate that addition of N2, C~6H2, and C~7H, to the flame

produced smaller changes in OH concentrations relative to CF~r implying these agents have chemical inhibition

capacities less than CF~r. However, the addition of CH~(0)(OCH3h and Fe(CO)s to the flame demonstrated

chemical inhibition capabilities greater than CF~r with larger changes in OH concentrations.

INTRODUCTION

Fire protection on military platforms, including ground fighting vehicles, is being challenged by the

impending loss of the ubiquitous fire fighting agent halon 1301 (CF~r) due to environmental concerns related to

the destruction of the stratospheric ozone layer. Replacement:fire extinguishment agents need to be found that will

satisfy numerous criteria including: fast fire suppression, minimum production of toxic gases when used, low

toxicity, compatibility with storage materials and environmental friendliness.

The U.S. Army's search for halon replacement agents has largely involved an empirical approach of

testing and evaluation of commercially available compounds/systems. An alternative approach is to study the

fundamental physical and chemical mechanisms responsible for flame inhibition with the hope that such studies

will uncover differences in the flame inhibition mechanisms which will lead to new chemicals for further

consideration and testing. To this end, we have recently initiated planar laser induced fluorescence (pLIF)

measurements of the OH radical species as flame extinction was approached in a non-premixed, atmospheric

pressure, opposed flow propane/air flame inhibited by halon 1301 [CF;J13r],N2, Fe(CO)s, FM-200 IC~7H], FE-36

[C~6H2], DMMP [CH~(0)(OCH3h], PN [P3N~6]. Presented here are preliminaIy results from this sfudy of

compounds which represent distinctly different chemical families in order to understand the differences between

each agent's inhibition mechanism.

BACKGROUND

Chemical inhibition in a flame arises from the lowering of the radical concentrations due to scavenging

reactions. In general, effective inhibition mechanisms contain two types of reactions: a) radical scavenging

reactions, and b) reactions regenerating inhibitor species that participate in the inhibition cycle. As an example, for

CF~r inhibition a free bromine from decomposed CF~r forms HBr which chemically reacts with a hydrogen

atom and reduces the flame's hydrogen concentration. The consequence of hydrogen recombination is the overall

available radical concentrations (H, 0, OH) and the rate of chain-branching reactions are reduced [1,2,3,4] while

regeneration ofHBr and Br2 occurs carrying on the inhibition cycle.

The chemicals Fe(CO)s, D~ ~~. PN investigated in our laboratory flame system were chosen based

on a comprehensive evaluation [5] offire inhibitors that are more effective than CF;J13r.The inhibition mechanisms

for Fe(CO)s, DMMP, and PN are believed to be generally similar to the HBr mechanism. For these postulated

mechanisms, each agent decomposes during combustion into inhibition cycle scavenging species, e.g. FeO, FeOH,

Fe(OH)2 for Fe(CO)s addition, [6] and HOPO and HOP02 for DMMP and PN addition [7]. In the reaction zone of

flames, these scavenging species proceed to behave much like HBr in scavenging hydrogen atoms. FM-200 and

FE-36 were studied here due to their popularity as candidate halon replacement agents. FM-200 and FE-36 are

refrigerants and it is assumed that their primary fire inhibition capabilities are due to their physical properties of

high heat capacities with some chemical reactivity due to CF3radical [8].

Approved for public release, distribution is unlimited.

171
In order to understand a chemical's inhibition mechanism in terms of physical and/or chemical

contributions, both N2 and CF~r are included in this study. That is, N2 represents the upper boundaIy for an

agent's physical influence on flame inhibition since it has no chemical inhibition capabilities. CF~r which has

been shown [9] that at least 20 % of its inhibition potential is caused by its physical properties offers a good

intermediate point with which to compare and contrast the other agents studied.

••• - - - - -. electrical Signal

~m:mmccmccmm Optical Signal

- - - - - - Pulse Signal

ATM-Pressure

wosed Flo

Burner

r;.::L ~---------1•/=_ IIJr----312Filter nm

t

0..,

-.:: .

.§.3

~

w

CylIndrical

~nv8X Lens

j /

~'~J1 UVSheet rn t

/ ~~:Sn1a1 /

Turnmg

Prism

rls

Figure 1: Schematic diagram of the experimental apparatus.

EXPERIMENTAL

OH PLIF imaging measurements were made using the arrangement presented in Figure 1. The opposed

flow burner apparatus is located inside a stainless steel hood to contain any toxic fumes that are exhausted from the

burner. All flames analyzed in this work were studied at atmospheric pressure and consisted of 7.0 Umin synthetic

air (79% N2 + 21% OV flowing from the lower duct, and 5.6 Umin of propane flowing from the upper duct. The

oxidizer and fuel ducts are separated a distance of 1.2 cm and the duct diameter is 2.54 em. Based on the flow

conditions and duct separation, the luminous flame zone is located on the oxidizer side of the stagnation plane. For

all studies presented here, the inhibitor agents are added to the oxidizer flow in gaseous form at room temperature

with the exception of Fe(CO)s which was cooled to 11°C and DMMP which was heated to 70°C. Opposed flow

burners have been used for some time to study the capabilities of an inhibitor agent because a global parameter, the

extinction strain rate [10], can be determined which describes the flame's strength at extinction [11,12,13,14]. The

extinction strain rate is useful because a decreased value demonstrates an inhibitor's efficiency. PLIF

measurements of radical concentrations (0, H, OH) are complimentaIy to the extinction strain rate because the

measurements illustrate an inhibitor's influence on the radical concentration profiles in the flame zone which

indicates if the flame's radical chemistIy is being perturbed by agent addition.

Planar laser induced fluorescence images were measured using a Lambda Physik excimer/dye laser

system. This system consists of a Lambda Physik Compex 102 XeCI excimer laser, a Scanmate 2 dye laser

(Coumarin 153) and a Second Harmonic Generator (SHG). The fundamental output of the dye laser (560 nm

wavelength) was frequency doubled in the SHG unit with a BBO crystal to approximately 281 nm. The UV laser

radiation was tuned to the peak of the R2(9.5) transition at 281.8 om «1,0) A2L+~X2TI) [15,16,17]. The UV light

output of the SHG unit enters an optical train where the beam is turned 900, apertured by a sub mID iris, projected

through a cylindrical plano convex lens to form the UV beam into a vertical sheet. To create a uniform sbeet width,

the sheet is apertured with 0.5 mID vertical slits as it is projected toward the center of the burner. Tbe UV sheet is

apertured just before the burner to produce a vertically uniform intensity that is 1.2 em in height allowing passage

through the entire burner flow field. Laser induced fluorescence from OH passes through a band pass filter

centered at 312 nm with a 11 nm bandwidth and is detected with a Princeton Instruments ICCD camera (Model

120) coupled with a Nikon UV lens located at 90° with respect to the UV sheet. The ICCD camera, which has an I

172
active area of 384 x 576 pixels, bas a field of view with this optical arrangement of approximately 33 cm2 and each

image recorded was acquired with 25 total accumulations on the camera.

RESULTS

The effectiveness of a particular flame inhibitor is typically characterized by its influence on a flame's

propagation chemistry. The most common indicators of the overall reaction rates for premixed and diffusion flame

systems are the burning velocity and extinction strain rate respectively. For premixed flames, the addition of an

inlnoitor decreases the burning velocity. For diffusion flames, the addition of an inhibitor increases the

characteristic chemical reaction time for the same flow time. That is, inhibitor concentration increases in a nonpremixed flame can cause chemical reactions to proceed at times near the characteristic flow time which eventually

can lead to flame extinction. For premixed and non-premixed systems, measurements of radical concentrations (0,

IL OF!) serve as useful indicators of the chemistry being affected by inhibitor addition and are complimentary to

burning velocity and extinction strain rate measurements. OR is monitored in the flames studied here because it is

relatively simple to measure and it is a good indicator of the overall radical pool concentration. even though II, 0,

and OR have been found to not be fully equilibrated in diffusion flames [18].

Figure 2 presents two representative two-dimensional images of OR fluorescence for an uninhibited

propane/air flame and for a propane/air flame to which CF~r was added (1.5 % by volume). Both images, which

are uncorrected for laser energy fluctuations and local quenching rates, illustrate the presence of two luminous

zones as the UV sheet passes through the flame. The lower, thicker zone is the fluorescence from the OR transition

while the upper, thinner zone is the broadband fluorescence due to derivative fuel species such as polycyclic

aromatic hydrocarbons. To construct a spatially resolved OR LIF profile from a OR PLIF image, as shown on the

right hand side of Figure 2, the pixel intensity corresponding to a given height between the fuel and oxidizer ducts

(spatial resolution approximately 0.149 mmlpixel) was summed and averaged over a 1 mm horizontal width. The

two-dimensional images and LIF profiles illustrate that addition of CF~r to the propane flame causes a decrease

in the OR fluorescence signal while the broadband fluorescence appears to increase just slightly. Similar results

have been seen previously for CF~r addition to hydrocarbon diffusion flames [19,20].

5000 ,

4500t

0% CF3Br

D

" ~ 4000 •• ,:; i3500

3000 2500

0

102791345681

DIstance From Fuel Duct. 5000 nvn•

. . .

4500'

1.5 % CF3Br

••

~ 4000 i

£,:; 3500 3000

2500

0

1027968134511

Distance From Fuel Duct. mm

Figure 2: Representative Pill images and the corresponding OH intensity profiles nom an opposed flow propane/air flame

seeded with 0 % (by volume) CF)13r and 1.5 % (by volume) CF)13r. Note the orientation of the PLIF images with respect to the

burner system places the fuel and air ducts at the top and bottom of each image respectively.

173

•••
Obviously the addition of an inhibitor to a flame gives rise to modifications in the flame structure.

Specifically, addition of an inhibitor can change the position and width of the flame's reaction zone. Previons

studies have shown [21,22,23,24,25] that a decrease in the flame's reaction zone width indicates increased

localized strain, which can cause local quenching or flame extinction [26]. The width of the flame's reaction zone

may be characterized by the width of a radical profile [21]. For the analysis of reaction zone modifications and

relative OR concentrations, each OR intensity profile is fit to a gaussian function. A gaussian function determines

the area under the profile curve which provides a general indicator of the entire OR profile for a given flame

condition. Figure 3 plots the results of the measured OR profile areas versus each inhibitor agent's concentration

as the flames were stepped towards extinction.

246 8

Inhibitor Agent Concentration, % volume

Figure 3: Nonnalized OR LIF profile areas versus agent delivery concentrations. The (0) are the N2 data, the ( 0 ) are the FM200 data, the ( (:,.) are the FE-36 data, the ('V) are the PN data, the (0) are the CFJBr data, the (II) is the DMMP data and the

(e) are the Fe(CO)s.

1.0

ctJ

«~~

e 'i= 0.6 "'0 a. .~:r: 0

Q) 0.4 ctJ

0.2

E 0

z

0 0.0l

FE-36

b;~

10 i

1

j

:j

,

..~

The reported OR profile areas are averaged over three or more separate inhibitor extinction experiments, wbere the

data for each experiment are normalized to the OR profile area measured in the uninhibited flame acquired prior to

each inhibitor extinction experiment to account for changes in burner and camera conditions. The data here

indicate that there are both physical and chemical modes of inhibition being observed for the agents studied N2 has

the least impact on OH with respect to the other agents studied and for the concentration range plotted in Figure 3

the flame was not even extinguished by N2• Similar results are observed for the two fluorinated propanes (FM-200

and FE-36) which show initially small declines in OR, but more rapid decreases as extinction is approached. For

the other agents studied (pN, CF:;Br,DMMP, and Fe(CO)s), the addition of these inhibitors show more dramatic

decreases in the measured OR values. Table I lists the observed inhibitor concentrations in the air stream at

extinction for each agent studied here and their estimated uncertainties.

Table i: inhibitor concentrations ('Yo volume) ana uncertain V (± % volume) at flame extinction

FE-36 CF3Br FM-200 Fe(CO)s DMMP PN N:z

4.4 10.640.50.4 91 35.9

1.56 1.47 0.63 1.88 0.10 0.03 12.3

174
The Table IT width values indicate that the agents, Nz, FE-36, FM-200 do not posses width changes

significantly different than the uninhibited flame. On the contraIy CF~r, PN, DMMP, and Fe(CO)s exhibit width

changes that are equal to or greater than a 20 % decrease from the uninhibited width value. Tbese width decreases

correspond to strain rate values that are at least double the uninhibited flame value. From the strain rate data listed

in Table IT,the inhibitor agents are ranked from lowest to highest strain rate values as: Nz, FE-36, FM-200 having

the lowest strain rate values followed, in increasing strain rate values, by CF~r < PN < DMMP < Fe(CO)s. Tbe

strain rate data trends imply that inhibitor agents with more physical suppression capabilities exhibit less effect on

the flame structure and strain rate than inhibitors with enhanced chemical suppression capabilities.

Table II: Measured OH profile widths (FWHJvf. mm) and corresponding strain rates (see-I; at selected inhibitor

concentrations % volume).

Inhibitor % Volume

OHStrain profileRate,sec-1 width, mm

70.50 1.30076.98 10.64 1.26

. 155.31 4.18 1.0174.69 88 1.2776.80 455 1.26

342.44 0.48 0.76

186.43 5.36 0.95

292.21 0.20 0.81

CONCLUSIONS

The results presented here show for the first time changes in OH profiles as extinction is approached in a

series of inhibited atmospheric pressure, non-premixed, propane/air flames. The OH profilesftom these flames

illustrate that Nz, FE-36, and FM-200, with smaller changes in OH areas relative to CF~r, exhibit chemical

inhibition capacities less than CF~r. On the contraIy, DMMP and Fe(CO)s demonstrate chemical inhibition

capabilities greater than CF~r with their larger changes in OR For the inhibitors studied, agent concentrations at

extinction support these observations with a CF~r concentration of 4.4 % (by volume) compared to Nz with a

concentration of 35.9 % and DMMP and Fe(CO)s each having concentrations less than 1 %. Analysis of the OH

profile widths and their correlation to the flame's strain rate indicate that as the OH profile widths decrease the

strain rates increase. For flames inhibited by Fe(CO)s, DMMP, and PN, the OH profiles width and strain rate

changes are greater than those experienced in the CF~r inhibited flame. Contrariwise, flames inhibited by Nz,

FM-200, and FE-36 do not demonstrate profile width and strain rate changes much different than those observed

for the uninhibited flame.

ACKNOWLEDGMENTS

The authors would like to thank Anthony Hamins (NIS1) for burner fabrication and Valeri Babushok

(NIST) for insightful suggestions and comments concerning experiments and results. This work was supported by

the Next Generation Fire Suppression Technology Program under the auspices of the U.S. Army TACOM (Steve

McCormick). Finally R Skaggs would like to acknowledge financial support from the Army Research LaboratOlY

through an American Society for Engineering Education Postdoctoral Fellowship.

176

)

. i

~,!
REFERENCES

1 . Dixon-Lewis, G., Simpson, RI., Sixteenth Symposium (International) on Combustion, The Combustion

Institute, Pittsburgh, 1976, p.llil.

2. Westbrook. C.K Combust. Se1. Technol. 23: 191 (1980).

3 . Westbrook. C.K Nineteenth Symposium (International) on Combustion, The Combustion Institute,

Pittsburgh, 1982, p.127.

4. Westbrook. C.K Combust. Sei. Technol. 34: 201 (1983).

5. Babushok. V., Tsang, W. Chemical and Physical Processes in Combustion: Proceedings of Fall

Technical Meeting of the Eastern States Section of the Combustion Institute, 1997, p.79.

6. Rumminger, MD., Reinelt, D., Babushok. V.I., and Linteris, G.T., Combust. Flame 116: 207 (1999).

7 . MacDonald, M.A Iayaweera, T.M Fisher, E.M, Gouldi.n,F.C. Combust. Flame 116: 166 (1999).

8. Williams, B.A, Fleming, J.W., and Sheinson, RS., Halon Options Technical Working Conference,

Albuquerque, 1997, p.31.

9. Noto, T., Babushok, V., Burgess, D.R, Hamins, A, Tsang, W., Miziolek, A, Twenty-Sixth Symposium

(International) on Combustion, Pittsburgh 1996, p.1377.

10 . Seshadri, K and Williams~F., Int. J. Heat and Mass Tamsftr 21:251 (1978).

11 Potter, AE., Heimel, S., and Butler, IN., Eighth Symposium (International) on Combustion, The

Combustion Institute, Pittsburgh, 1962, p.1027.

12 . Carrier, G.F., Fendell, F.E., and MaIble, F.E., SIAM J.Appl Math. 28: 463-500 (1975)

13 . Linan, A Acta Astronaut 1: 1007 (1974).

14. Williams, F.A Fire Sa! J. 3:163 (1981)

15. Chidsey, I.L., and Crosley, D.R, J. Quant. Spectrosc. Radiat. Transf, 23: 187 (1980)

16 . Dieke, G.H., and Croswhite, H.M., J. Quant. Spectrosc. Radiat. Transf. 2: 97 (1962).

17. Kotlar, A, Private Communication (1998).

18 . Smyth, K.C., Tjossexn,P.I.H., Hamins, A, and Miller, I.H., Combust. Flame 79: 366 (1990).

19. Masri, AR; Dally, B.B., Barlow, RS., and Carter, C.D., Combust. Sei. Technol. 113-114: 17 (1996).

20 . Smyth, K.C., and Everest, D., Twenty-Sixth Symposium (International) on Combustion, The Combustion

Institute, Pittsburgh, 1996, p.1385.

21 . Peters, N. Combust. Sei. Technol. 30: 1 (1983).

22 . Liew, S.K, Bray, KN.C., and Moss, I.B., Combust. Flame 56: 199 (1984).

23 . Haworth, D.C., Drake, MC., and Boot, RI., Combust. Sei. Techno/. 60: 287 (1988).

24 . Roberts, Wm. L., Driscoll, I.F., Drake, MC., and Ratcliffe, I.W., Twenty-Fourth Symposium

(International) on Combustion, The Combustion Institute, Pittsburgh, 1992, p.169.

25 . Miller, I.H., Chemical and Physical Processes in Combustion: Proceedings of Fall Technical Meeting of

the Eastern States Section of the Combustion Institute, 1996, p.l.

26 . Bilger, R W. Twenty-Second Symposium (International) on Combustion, The Combustion Institute,

Pittsburgh 1996, p.1377.

27 . Cup-Burner Flame Extinguishment Concentrations, NMERI Report http://www.nmeri.unm-cget, (1998).

28 . Kaizerman, I.A, and Tapscott, RE. Advanced Streaming Agent Development, Volume III: Phosphorus

CompoundS, New Mexico Engineering Research Institute, Report No. NMERI 96/5/32540, (1996).

177